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This is The GNU C Library Reference Manual, for Version 2.25 of the GNU C Library.


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1 Introduction

The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like. Instead, these facilities are defined in a standard library, which you compile and link with your programs.

The GNU C Library, described in this document, defines all of the library functions that are specified by the ISO C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to GNU systems.

The purpose of this manual is to tell you how to use the facilities of the GNU C Library. We have mentioned which features belong to which standards to help you identify things that are potentially non-portable to other systems. But the emphasis in this manual is not on strict portability.


1.1 Getting Started

This manual is written with the assumption that you are at least somewhat familiar with the C programming language and basic programming concepts. Specifically, familiarity with ISO standard C (see ISO C), rather than “traditional” pre-ISO C dialects, is assumed.

The GNU C Library includes several header files, each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program. For example, the header file stdio.h declares facilities for performing input and output, and the header file string.h declares string processing utilities. The organization of this manual generally follows the same division as the header files.

If you are reading this manual for the first time, you should read all of the introductory material and skim the remaining chapters. There are a lot of functions in the GNU C Library and it’s not realistic to expect that you will be able to remember exactly how to use each and every one of them. It’s more important to become generally familiar with the kinds of facilities that the library provides, so that when you are writing your programs you can recognize when to make use of library functions, and where in this manual you can find more specific information about them.


1.2 Standards and Portability

This section discusses the various standards and other sources that the GNU C Library is based upon. These sources include the ISO C and POSIX standards, and the System V and Berkeley Unix implementations.

The primary focus of this manual is to tell you how to make effective use of the GNU C Library facilities. But if you are concerned about making your programs compatible with these standards, or portable to operating systems other than GNU, this can affect how you use the library. This section gives you an overview of these standards, so that you will know what they are when they are mentioned in other parts of the manual.

See Library Summary, for an alphabetical list of the functions and other symbols provided by the library. This list also states which standards each function or symbol comes from.


1.2.1 ISO C

The GNU C Library is compatible with the C standard adopted by the American National Standards Institute (ANSI): American National Standard X3.159-1989—“ANSI C” and later by the International Standardization Organization (ISO): ISO/IEC 9899:1990, “Programming languages—C”. We here refer to the standard as ISO C since this is the more general standard in respect of ratification. The header files and library facilities that make up the GNU C Library are a superset of those specified by the ISO C standard.

If you are concerned about strict adherence to the ISO C standard, you should use the ‘-ansi’ option when you compile your programs with the GNU C compiler. This tells the compiler to define only ISO standard features from the library header files, unless you explicitly ask for additional features. See Feature Test Macros, for information on how to do this.

Being able to restrict the library to include only ISO C features is important because ISO C puts limitations on what names can be defined by the library implementation, and the GNU extensions don’t fit these limitations. See Reserved Names, for more information about these restrictions.

This manual does not attempt to give you complete details on the differences between ISO C and older dialects. It gives advice on how to write programs to work portably under multiple C dialects, but does not aim for completeness.


1.2.2 POSIX (The Portable Operating System Interface)

The GNU C Library is also compatible with the ISO POSIX family of standards, known more formally as the Portable Operating System Interface for Computer Environments (ISO/IEC 9945). They were also published as ANSI/IEEE Std 1003. POSIX is derived mostly from various versions of the Unix operating system.

The library facilities specified by the POSIX standards are a superset of those required by ISO C; POSIX specifies additional features for ISO C functions, as well as specifying new additional functions. In general, the additional requirements and functionality defined by the POSIX standards are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support which can run in many diverse operating system environments.

The GNU C Library implements all of the functions specified in ISO/IEC 9945-1:1996, the POSIX System Application Program Interface, commonly referred to as POSIX.1. The primary extensions to the ISO C facilities specified by this standard include file system interface primitives (see File System Interface), device-specific terminal control functions (see Low-Level Terminal Interface), and process control functions (see Processes).

Some facilities from ISO/IEC 9945-2:1993, the POSIX Shell and Utilities standard (POSIX.2) are also implemented in the GNU C Library. These include utilities for dealing with regular expressions and other pattern matching facilities (see Pattern Matching).


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1.2.2.1 POSIX Safety Concepts

This manual documents various safety properties of GNU C Library functions, in lines that follow their prototypes and look like:

Preliminary: | MT-Safe | AS-Safe | AC-Safe |

The properties are assessed according to the criteria set forth in the POSIX standard for such safety contexts as Thread-, Async-Signal- and Async-Cancel- -Safety. Intuitive definitions of these properties, attempting to capture the meaning of the standard definitions, follow.

  • MT-Safe or Thread-Safe functions are safe to call in the presence of other threads. MT, in MT-Safe, stands for Multi Thread.

    Being MT-Safe does not imply a function is atomic, nor that it uses any of the memory synchronization mechanisms POSIX exposes to users. It is even possible that calling MT-Safe functions in sequence does not yield an MT-Safe combination. For example, having a thread call two MT-Safe functions one right after the other does not guarantee behavior equivalent to atomic execution of a combination of both functions, since concurrent calls in other threads may interfere in a destructive way.

    Whole-program optimizations that could inline functions across library interfaces may expose unsafe reordering, and so performing inlining across the GNU C Library interface is not recommended. The documented MT-Safety status is not guaranteed under whole-program optimization. However, functions defined in user-visible headers are designed to be safe for inlining.

  • AS-Safe or Async-Signal-Safe functions are safe to call from asynchronous signal handlers. AS, in AS-Safe, stands for Asynchronous Signal.

    Many functions that are AS-Safe may set errno, or modify the floating-point environment, because their doing so does not make them unsuitable for use in signal handlers. However, programs could misbehave should asynchronous signal handlers modify this thread-local state, and the signal handling machinery cannot be counted on to preserve it. Therefore, signal handlers that call functions that may set errno or modify the floating-point environment must save their original values, and restore them before returning.

  • AC-Safe or Async-Cancel-Safe functions are safe to call when asynchronous cancellation is enabled. AC in AC-Safe stands for Asynchronous Cancellation.

    The POSIX standard defines only three functions to be AC-Safe, namely pthread_cancel, pthread_setcancelstate, and pthread_setcanceltype. At present the GNU C Library provides no guarantees beyond these three functions, but does document which functions are presently AC-Safe. This documentation is provided for use by the GNU C Library developers.

    Just like signal handlers, cancellation cleanup routines must configure the floating point environment they require. The routines cannot assume a floating point environment, particularly when asynchronous cancellation is enabled. If the configuration of the floating point environment cannot be performed atomically then it is also possible that the environment encountered is internally inconsistent.

  • MT-Unsafe, AS-Unsafe, AC-Unsafe functions are not safe to call within the safety contexts described above. Calling them within such contexts invokes undefined behavior.

    Functions not explicitly documented as safe in a safety context should be regarded as Unsafe.

  • Preliminary safety properties are documented, indicating these properties may not be counted on in future releases of the GNU C Library.

    Such preliminary properties are the result of an assessment of the properties of our current implementation, rather than of what is mandated and permitted by current and future standards.

    Although we strive to abide by the standards, in some cases our implementation is safe even when the standard does not demand safety, and in other cases our implementation does not meet the standard safety requirements. The latter are most likely bugs; the former, when marked as Preliminary, should not be counted on: future standards may require changes that are not compatible with the additional safety properties afforded by the current implementation.

    Furthermore, the POSIX standard does not offer a detailed definition of safety. We assume that, by “safe to call”, POSIX means that, as long as the program does not invoke undefined behavior, the “safe to call” function behaves as specified, and does not cause other functions to deviate from their specified behavior. We have chosen to use its loose definitions of safety, not because they are the best definitions to use, but because choosing them harmonizes this manual with POSIX.

    Please keep in mind that these are preliminary definitions and annotations, and certain aspects of the definitions are still under discussion and might be subject to clarification or change.

    Over time, we envision evolving the preliminary safety notes into stable commitments, as stable as those of our interfaces. As we do, we will remove the Preliminary keyword from safety notes. As long as the keyword remains, however, they are not to be regarded as a promise of future behavior.

Other keywords that appear in safety notes are defined in subsequent sections.


1.2.2.2 Unsafe Features

Functions that are unsafe to call in certain contexts are annotated with keywords that document their features that make them unsafe to call. AS-Unsafe features in this section indicate the functions are never safe to call when asynchronous signals are enabled. AC-Unsafe features indicate they are never safe to call when asynchronous cancellation is enabled. There are no MT-Unsafe marks in this section.

  • lock

    Functions marked with lock as an AS-Unsafe feature may be interrupted by a signal while holding a non-recursive lock. If the signal handler calls another such function that takes the same lock, the result is a deadlock.

    Functions annotated with lock as an AC-Unsafe feature may, if cancelled asynchronously, fail to release a lock that would have been released if their execution had not been interrupted by asynchronous thread cancellation. Once a lock is left taken, attempts to take that lock will block indefinitely.

  • corrupt

    Functions marked with corrupt as an AS-Unsafe feature may corrupt data structures and misbehave when they interrupt, or are interrupted by, another such function. Unlike functions marked with lock, these take recursive locks to avoid MT-Safety problems, but this is not enough to stop a signal handler from observing a partially-updated data structure. Further corruption may arise from the interrupted function’s failure to notice updates made by signal handlers.

    Functions marked with corrupt as an AC-Unsafe feature may leave data structures in a corrupt, partially updated state. Subsequent uses of the data structure may misbehave.

  • heap

    Functions marked with heap may call heap memory management functions from the malloc/free family of functions and are only as safe as those functions. This note is thus equivalent to:

    | AS-Unsafe lock | AC-Unsafe lock fd mem |

  • dlopen

    Functions marked with dlopen use the dynamic loader to load shared libraries into the current execution image. This involves opening files, mapping them into memory, allocating additional memory, resolving symbols, applying relocations and more, all of this while holding internal dynamic loader locks.

    The locks are enough for these functions to be AS- and AC-Unsafe, but other issues may arise. At present this is a placeholder for all potential safety issues raised by dlopen.

  • plugin

    Functions annotated with plugin may run code from plugins that may be external to the GNU C Library. Such plugin functions are assumed to be MT-Safe, AS-Unsafe and AC-Unsafe. Examples of such plugins are stack unwinding libraries, name service switch (NSS) and character set conversion (iconv) back-ends.

    Although the plugins mentioned as examples are all brought in by means of dlopen, the plugin keyword does not imply any direct involvement of the dynamic loader or the libdl interfaces, those are covered by dlopen. For example, if one function loads a module and finds the addresses of some of its functions, while another just calls those already-resolved functions, the former will be marked with dlopen, whereas the latter will get the plugin. When a single function takes all of these actions, then it gets both marks.

  • i18n

    Functions marked with i18n may call internationalization functions of the gettext family and will be only as safe as those functions. This note is thus equivalent to:

    | MT-Safe env | AS-Unsafe corrupt heap dlopen | AC-Unsafe corrupt |

  • timer

    Functions marked with timer use the alarm function or similar to set a time-out for a system call or a long-running operation. In a multi-threaded program, there is a risk that the time-out signal will be delivered to a different thread, thus failing to interrupt the intended thread. Besides being MT-Unsafe, such functions are always AS-Unsafe, because calling them in signal handlers may interfere with timers set in the interrupted code, and AC-Unsafe, because there is no safe way to guarantee an earlier timer will be reset in case of asynchronous cancellation.


1.2.2.3 Conditionally Safe Features

For some features that make functions unsafe to call in certain contexts, there are known ways to avoid the safety problem other than refraining from calling the function altogether. The keywords that follow refer to such features, and each of their definitions indicate how the whole program needs to be constrained in order to remove the safety problem indicated by the keyword. Only when all the reasons that make a function unsafe are observed and addressed, by applying the documented constraints, does the function become safe to call in a context.

  • init

    Functions marked with init as an MT-Unsafe feature perform MT-Unsafe initialization when they are first called.

    Calling such a function at least once in single-threaded mode removes this specific cause for the function to be regarded as MT-Unsafe. If no other cause for that remains, the function can then be safely called after other threads are started.

    Functions marked with init as an AS- or AC-Unsafe feature use the internal libc_once machinery or similar to initialize internal data structures.

    If a signal handler interrupts such an initializer, and calls any function that also performs libc_once initialization, it will deadlock if the thread library has been loaded.

    Furthermore, if an initializer is partially complete before it is canceled or interrupted by a signal whose handler requires the same initialization, some or all of the initialization may be performed more than once, leaking resources or even resulting in corrupt internal data.

    Applications that need to call functions marked with init as an AS- or AC-Unsafe feature should ensure the initialization is performed before configuring signal handlers or enabling cancellation, so that the AS- and AC-Safety issues related with libc_once do not arise.

  • race

    Functions annotated with race as an MT-Safety issue operate on objects in ways that may cause data races or similar forms of destructive interference out of concurrent execution. In some cases, the objects are passed to the functions by users; in others, they are used by the functions to return values to users; in others, they are not even exposed to users.

    We consider access to objects passed as (indirect) arguments to functions to be data race free. The assurance of data race free objects is the caller’s responsibility. We will not mark a function as MT-Unsafe or AS-Unsafe if it misbehaves when users fail to take the measures required by POSIX to avoid data races when dealing with such objects. As a general rule, if a function is documented as reading from an object passed (by reference) to it, or modifying it, users ought to use memory synchronization primitives to avoid data races just as they would should they perform the accesses themselves rather than by calling the library function. FILE streams are the exception to the general rule, in that POSIX mandates the library to guard against data races in many functions that manipulate objects of this specific opaque type. We regard this as a convenience provided to users, rather than as a general requirement whose expectations should extend to other types.

    In order to remind users that guarding certain arguments is their responsibility, we will annotate functions that take objects of certain types as arguments. We draw the line for objects passed by users as follows: objects whose types are exposed to users, and that users are expected to access directly, such as memory buffers, strings, and various user-visible struct types, do not give reason for functions to be annotated with race. It would be noisy and redundant with the general requirement, and not many would be surprised by the library’s lack of internal guards when accessing objects that can be accessed directly by users.

    As for objects that are opaque or opaque-like, in that they are to be manipulated only by passing them to library functions (e.g., FILE, DIR, obstack, iconv_t), there might be additional expectations as to internal coordination of access by the library. We will annotate, with race followed by a colon and the argument name, functions that take such objects but that do not take care of synchronizing access to them by default. For example, FILE stream unlocked functions will be annotated, but those that perform implicit locking on FILE streams by default will not, even though the implicit locking may be disabled on a per-stream basis.

    In either case, we will not regard as MT-Unsafe functions that may access user-supplied objects in unsafe ways should users fail to ensure the accesses are well defined. The notion prevails that users are expected to safeguard against data races any user-supplied objects that the library accesses on their behalf.

    This user responsibility does not apply, however, to objects controlled by the library itself, such as internal objects and static buffers used to return values from certain calls. When the library doesn’t guard them against concurrent uses, these cases are regarded as MT-Unsafe and AS-Unsafe (although the race mark under AS-Unsafe will be omitted as redundant with the one under MT-Unsafe). As in the case of user-exposed objects, the mark may be followed by a colon and an identifier. The identifier groups all functions that operate on a certain unguarded object; users may avoid the MT-Safety issues related with unguarded concurrent access to such internal objects by creating a non-recursive mutex related with the identifier, and always holding the mutex when calling any function marked as racy on that identifier, as they would have to should the identifier be an object under user control. The non-recursive mutex avoids the MT-Safety issue, but it trades one AS-Safety issue for another, so use in asynchronous signals remains undefined.

    When the identifier relates to a static buffer used to hold return values, the mutex must be held for as long as the buffer remains in use by the caller. Many functions that return pointers to static buffers offer reentrant variants that store return values in caller-supplied buffers instead. In some cases, such as tmpname, the variant is chosen not by calling an alternate entry point, but by passing a non-NULL pointer to the buffer in which the returned values are to be stored. These variants are generally preferable in multi-threaded programs, although some of them are not MT-Safe because of other internal buffers, also documented with race notes.

  • const

    Functions marked with const as an MT-Safety issue non-atomically modify internal objects that are better regarded as constant, because a substantial portion of the GNU C Library accesses them without synchronization. Unlike race, that causes both readers and writers of internal objects to be regarded as MT-Unsafe and AS-Unsafe, this mark is applied to writers only. Writers remain equally MT- and AS-Unsafe to call, but the then-mandatory constness of objects they modify enables readers to be regarded as MT-Safe and AS-Safe (as long as no other reasons for them to be unsafe remain), since the lack of synchronization is not a problem when the objects are effectively constant.

    The identifier that follows the const mark will appear by itself as a safety note in readers. Programs that wish to work around this safety issue, so as to call writers, may use a non-recursve rwlock associated with the identifier, and guard all calls to functions marked with const followed by the identifier with a write lock, and all calls to functions marked with the identifier by itself with a read lock. The non-recursive locking removes the MT-Safety problem, but it trades one AS-Safety problem for another, so use in asynchronous signals remains undefined.

  • sig

    Functions marked with sig as a MT-Safety issue (that implies an identical AS-Safety issue, omitted for brevity) may temporarily install a signal handler for internal purposes, which may interfere with other uses of the signal, identified after a colon.

    This safety problem can be worked around by ensuring that no other uses of the signal will take place for the duration of the call. Holding a non-recursive mutex while calling all functions that use the same temporary signal; blocking that signal before the call and resetting its handler afterwards is recommended.

    There is no safe way to guarantee the original signal handler is restored in case of asynchronous cancellation, therefore so-marked functions are also AC-Unsafe.

    Besides the measures recommended to work around the MT- and AS-Safety problem, in order to avert the cancellation problem, disabling asynchronous cancellation and installing a cleanup handler to restore the signal to the desired state and to release the mutex are recommended.

  • term

    Functions marked with term as an MT-Safety issue may change the terminal settings in the recommended way, namely: call tcgetattr, modify some flags, and then call tcsetattr; this creates a window in which changes made by other threads are lost. Thus, functions marked with term are MT-Unsafe. The same window enables changes made by asynchronous signals to be lost. These functions are also AS-Unsafe, but the corresponding mark is omitted as redundant.

    It is thus advisable for applications using the terminal to avoid concurrent and reentrant interactions with it, by not using it in signal handlers or blocking signals that might use it, and holding a lock while calling these functions and interacting with the terminal. This lock should also be used for mutual exclusion with functions marked with race:tcattr(fd), where fd is a file descriptor for the controlling terminal. The caller may use a single mutex for simplicity, or use one mutex per terminal, even if referenced by different file descriptors.

    Functions marked with term as an AC-Safety issue are supposed to restore terminal settings to their original state, after temporarily changing them, but they may fail to do so if cancelled.

    Besides the measures recommended to work around the MT- and AS-Safety problem, in order to avert the cancellation problem, disabling asynchronous cancellation and installing a cleanup handler to restore the terminal settings to the original state and to release the mutex are recommended.


1.2.2.4 Other Safety Remarks

Additional keywords may be attached to functions, indicating features that do not make a function unsafe to call, but that may need to be taken into account in certain classes of programs:

  • locale

    Functions annotated with locale as an MT-Safety issue read from the locale object without any form of synchronization. Functions annotated with locale called concurrently with locale changes may behave in ways that do not correspond to any of the locales active during their execution, but an unpredictable mix thereof.

    We do not mark these functions as MT- or AS-Unsafe, however, because functions that modify the locale object are marked with const:locale and regarded as unsafe. Being unsafe, the latter are not to be called when multiple threads are running or asynchronous signals are enabled, and so the locale can be considered effectively constant in these contexts, which makes the former safe.

  • env

    Functions marked with env as an MT-Safety issue access the environment with getenv or similar, without any guards to ensure safety in the presence of concurrent modifications.

    We do not mark these functions as MT- or AS-Unsafe, however, because functions that modify the environment are all marked with const:env and regarded as unsafe. Being unsafe, the latter are not to be called when multiple threads are running or asynchronous signals are enabled, and so the environment can be considered effectively constant in these contexts, which makes the former safe.

  • hostid

    The function marked with hostid as an MT-Safety issue reads from the system-wide data structures that hold the “host ID” of the machine. These data structures cannot generally be modified atomically. Since it is expected that the “host ID” will not normally change, the function that reads from it (gethostid) is regarded as safe, whereas the function that modifies it (sethostid) is marked with const:hostid, indicating it may require special care if it is to be called. In this specific case, the special care amounts to system-wide (not merely intra-process) coordination.

  • sigintr

    Functions marked with sigintr as an MT-Safety issue access the _sigintr internal data structure without any guards to ensure safety in the presence of concurrent modifications.

    We do not mark these functions as MT- or AS-Unsafe, however, because functions that modify the this data structure are all marked with const:sigintr and regarded as unsafe. Being unsafe, the latter are not to be called when multiple threads are running or asynchronous signals are enabled, and so the data structure can be considered effectively constant in these contexts, which makes the former safe.

  • fd

    Functions annotated with fd as an AC-Safety issue may leak file descriptors if asynchronous thread cancellation interrupts their execution.

    Functions that allocate or deallocate file descriptors will generally be marked as such. Even if they attempted to protect the file descriptor allocation and deallocation with cleanup regions, allocating a new descriptor and storing its number where the cleanup region could release it cannot be performed as a single atomic operation. Similarly, releasing the descriptor and taking it out of the data structure normally responsible for releasing it cannot be performed atomically. There will always be a window in which the descriptor cannot be released because it was not stored in the cleanup handler argument yet, or it was already taken out before releasing it. It cannot be taken out after release: an open descriptor could mean either that the descriptor still has to be closed, or that it already did so but the descriptor was reallocated by another thread or signal handler.

    Such leaks could be internally avoided, with some performance penalty, by temporarily disabling asynchronous thread cancellation. However, since callers of allocation or deallocation functions would have to do this themselves, to avoid the same sort of leak in their own layer, it makes more sense for the library to assume they are taking care of it than to impose a performance penalty that is redundant when the problem is solved in upper layers, and insufficient when it is not.

    This remark by itself does not cause a function to be regarded as AC-Unsafe. However, cumulative effects of such leaks may pose a problem for some programs. If this is the case, suspending asynchronous cancellation for the duration of calls to such functions is recommended.

  • mem

    Functions annotated with mem as an AC-Safety issue may leak memory if asynchronous thread cancellation interrupts their execution.

    The problem is similar to that of file descriptors: there is no atomic interface to allocate memory and store its address in the argument to a cleanup handler, or to release it and remove its address from that argument, without at least temporarily disabling asynchronous cancellation, which these functions do not do.

    This remark does not by itself cause a function to be regarded as generally AC-Unsafe. However, cumulative effects of such leaks may be severe enough for some programs that disabling asynchronous cancellation for the duration of calls to such functions may be required.

  • cwd

    Functions marked with cwd as an MT-Safety issue may temporarily change the current working directory during their execution, which may cause relative pathnames to be resolved in unexpected ways in other threads or within asynchronous signal or cancellation handlers.

    This is not enough of a reason to mark so-marked functions as MT- or AS-Unsafe, but when this behavior is optional (e.g., nftw with FTW_CHDIR), avoiding the option may be a good alternative to using full pathnames or file descriptor-relative (e.g. openat) system calls.

  • !posix

    This remark, as an MT-, AS- or AC-Safety note to a function, indicates the safety status of the function is known to differ from the specified status in the POSIX standard. For example, POSIX does not require a function to be Safe, but our implementation is, or vice-versa.

    For the time being, the absence of this remark does not imply the safety properties we documented are identical to those mandated by POSIX for the corresponding functions.

  • :identifier

    Annotations may sometimes be followed by identifiers, intended to group several functions that e.g. access the data structures in an unsafe way, as in race and const, or to provide more specific information, such as naming a signal in a function marked with sig. It is envisioned that it may be applied to lock and corrupt as well in the future.

    In most cases, the identifier will name a set of functions, but it may name global objects or function arguments, or identifiable properties or logical components associated with them, with a notation such as e.g. :buf(arg) to denote a buffer associated with the argument arg, or :tcattr(fd) to denote the terminal attributes of a file descriptor fd.

    The most common use for identifiers is to provide logical groups of functions and arguments that need to be protected by the same synchronization primitive in order to ensure safe operation in a given context.

  • /condition

    Some safety annotations may be conditional, in that they only apply if a boolean expression involving arguments, global variables or even the underlying kernel evaluates to true. Such conditions as /hurd or /!linux!bsd indicate the preceding marker only applies when the underlying kernel is the HURD, or when it is neither Linux nor a BSD kernel, respectively. /!ps and /one_per_line indicate the preceding marker only applies when argument ps is NULL, or global variable one_per_line is nonzero.

    When all marks that render a function unsafe are adorned with such conditions, and none of the named conditions hold, then the function can be regarded as safe.


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1.2.3 Berkeley Unix

The GNU C Library defines facilities from some versions of Unix which are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD, and 4.4 BSD Unix systems (also known as Berkeley Unix) and from SunOS (a popular 4.2 BSD derivative that includes some Unix System V functionality). These systems support most of the ISO C and POSIX facilities, and 4.4 BSD and newer releases of SunOS in fact support them all.

The BSD facilities include symbolic links (see Symbolic Links), the select function (see Waiting for I/O), the BSD signal functions (see BSD Signal Handling), and sockets (see Sockets).


1.2.4 SVID (The System V Interface Description)

The System V Interface Description (SVID) is a document describing the AT&T Unix System V operating system. It is to some extent a superset of the POSIX standard (see POSIX).

The GNU C Library defines most of the facilities required by the SVID that are not also required by the ISO C or POSIX standards, for compatibility with System V Unix and other Unix systems (such as SunOS) which include these facilities. However, many of the more obscure and less generally useful facilities required by the SVID are not included. (In fact, Unix System V itself does not provide them all.)

The supported facilities from System V include the methods for inter-process communication and shared memory, the hsearch and drand48 families of functions, fmtmsg and several of the mathematical functions.


1.2.5 XPG (The X/Open Portability Guide)

The X/Open Portability Guide, published by the X/Open Company, Ltd., is a more general standard than POSIX. X/Open owns the Unix copyright and the XPG specifies the requirements for systems which are intended to be a Unix system.

The GNU C Library complies to the X/Open Portability Guide, Issue 4.2, with all extensions common to XSI (X/Open System Interface) compliant systems and also all X/Open UNIX extensions.

The additions on top of POSIX are mainly derived from functionality available in System V and BSD systems. Some of the really bad mistakes in System V systems were corrected, though. Since fulfilling the XPG standard with the Unix extensions is a precondition for getting the Unix brand chances are good that the functionality is available on commercial systems.


1.3 Using the Library

This section describes some of the practical issues involved in using the GNU C Library.


1.3.1 Header Files

Libraries for use by C programs really consist of two parts: header files that define types and macros and declare variables and functions; and the actual library or archive that contains the definitions of the variables and functions.

(Recall that in C, a declaration merely provides information that a function or variable exists and gives its type. For a function declaration, information about the types of its arguments might be provided as well. The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions. A definition, on the other hand, actually allocates storage for a variable or says what a function does.)

In order to use the facilities in the GNU C Library, you should be sure that your program source files include the appropriate header files. This is so that the compiler has declarations of these facilities available and can correctly process references to them. Once your program has been compiled, the linker resolves these references to the actual definitions provided in the archive file.

Header files are included into a program source file by the ‘#include’ preprocessor directive. The C language supports two forms of this directive; the first,

#include "<var>header</var>"

is typically used to include a header file header that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application. By contrast,

#include &lt;file.h&gt;

is typically used to include a header file file.h that contains definitions and declarations for a standard library. This file would normally be installed in a standard place by your system administrator. You should use this second form for the C library header files.

Typically, ‘#include’ directives are placed at the top of the C source file, before any other code. If you begin your source files with some comments explaining what the code in the file does (a good idea), put the ‘#include’ directives immediately afterwards, following the feature test macro definition (see Feature Test Macros).

For more information about the use of header files and ‘#include’ directives, see Header Files in The GNU C Preprocessor Manual.

The GNU C Library provides several header files, each of which contains the type and macro definitions and variable and function declarations for a group of related facilities. This means that your programs may need to include several header files, depending on exactly which facilities you are using.

Some library header files include other library header files automatically. However, as a matter of programming style, you should not rely on this; it is better to explicitly include all the header files required for the library facilities you are using. The GNU C Library header files have been written in such a way that it doesn’t matter if a header file is accidentally included more than once; including a header file a second time has no effect. Likewise, if your program needs to include multiple header files, the order in which they are included doesn’t matter.

Compatibility Note: Inclusion of standard header files in any order and any number of times works in any ISO C implementation. However, this has traditionally not been the case in many older C implementations.

Strictly speaking, you don’t have to include a header file to use a function it declares; you could declare the function explicitly yourself, according to the specifications in this manual. But it is usually better to include the header file because it may define types and macros that are not otherwise available and because it may define more efficient macro replacements for some functions. It is also a sure way to have the correct declaration.


1.3.2 Macro Definitions of Functions

If we describe something as a function in this manual, it may have a macro definition as well. This normally has no effect on how your program runs—the macro definition does the same thing as the function would. In particular, macro equivalents for library functions evaluate arguments exactly once, in the same way that a function call would. The main reason for these macro definitions is that sometimes they can produce an inline expansion that is considerably faster than an actual function call.

Taking the address of a library function works even if it is also defined as a macro. This is because, in this context, the name of the function isn’t followed by the left parenthesis that is syntactically necessary to recognize a macro call.

You might occasionally want to avoid using the macro definition of a function—perhaps to make your program easier to debug. There are two ways you can do this:

  • You can avoid a macro definition in a specific use by enclosing the name of the function in parentheses. This works because the name of the function doesn’t appear in a syntactic context where it is recognizable as a macro call.
  • You can suppress any macro definition for a whole source file by using the ‘#undef’ preprocessor directive, unless otherwise stated explicitly in the description of that facility.

For example, suppose the header file stdlib.h declares a function named abs with

extern int abs (int);

and also provides a macro definition for abs. Then, in:

#include &lt;stdlib.h&gt;
int f (int *i) { return abs (++*i); }

the reference to abs might refer to either a macro or a function. On the other hand, in each of the following examples the reference is to a function and not a macro.

#include &lt;stdlib.h&gt;
int g (int *i) { return (abs) (++*i); }
 
#undef abs
int h (int *i) { return abs (++*i); }

Since macro definitions that double for a function behave in exactly the same way as the actual function version, there is usually no need for any of these methods. In fact, removing macro definitions usually just makes your program slower.


1.3.3 Reserved Names

The names of all library types, macros, variables and functions that come from the ISO C standard are reserved unconditionally; your program may not redefine these names. All other library names are reserved if your program explicitly includes the header file that defines or declares them. There are several reasons for these restrictions:

  • Other people reading your code could get very confused if you were using a function named exit to do something completely different from what the standard exit function does, for example. Preventing this situation helps to make your programs easier to understand and contributes to modularity and maintainability.
  • It avoids the possibility of a user accidentally redefining a library function that is called by other library functions. If redefinition were allowed, those other functions would not work properly.
  • It allows the compiler to do whatever special optimizations it pleases on calls to these functions, without the possibility that they may have been redefined by the user. Some library facilities, such as those for dealing with variadic arguments (see Variadic Functions) and non-local exits (see Non-Local Exits), actually require a considerable amount of cooperation on the part of the C compiler, and with respect to the implementation, it might be easier for the compiler to treat these as built-in parts of the language.

In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (‘_’) and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names. This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs.

Some additional classes of identifier names are reserved for future extensions to the C language or the POSIX.1 environment. While using these names for your own purposes right now might not cause a problem, they do raise the possibility of conflict with future versions of the C or POSIX standards, so you should avoid these names.

  • Names beginning with a capital ‘E’ followed a digit or uppercase letter may be used for additional error code names. See Error Reporting.
  • Names that begin with either ‘is’ or ‘to’ followed by a lowercase letter may be used for additional character testing and conversion functions. See Character Handling.
  • Names that begin with ‘LC_’ followed by an uppercase letter may be used for additional macros specifying locale attributes. See Locales.
  • Names of all existing mathematics functions (see Mathematics) suffixed with ‘f’ or ‘l’ are reserved for corresponding functions that operate on float and long double arguments, respectively.
  • Names that begin with ‘SIG’ followed by an uppercase letter are reserved for additional signal names. See Standard Signals.
  • Names that begin with ‘SIG_’ followed by an uppercase letter are reserved for additional signal actions. See Basic Signal Handling.
  • Names beginning with ‘str’, ‘mem’, or ‘wcs’ followed by a lowercase letter are reserved for additional string and array functions. See String and Array Utilities.
  • Names that end with ‘_t’ are reserved for additional type names.

In addition, some individual header files reserve names beyond those that they actually define. You only need to worry about these restrictions if your program includes that particular header file.

  • The header file dirent.h reserves names prefixed with ‘d_’.
  • The header file fcntl.h reserves names prefixed with ‘l_’, ‘F_’, ‘O_’, and ‘S_’.
  • The header file grp.h reserves names prefixed with ‘gr_’.
  • The header file limits.h reserves names suffixed with ‘_MAX’.
  • The header file pwd.h reserves names prefixed with ‘pw_’.
  • The header file signal.h reserves names prefixed with ‘sa_’ and ‘SA_’.
  • The header file sys/stat.h reserves names prefixed with ‘st_’ and ‘S_’.
  • The header file sys/times.h reserves names prefixed with ‘tms_’.
  • The header file termios.h reserves names prefixed with ‘c_’, ‘V’, ‘I’, ‘O’, and ‘TC’; and names prefixed with ‘B’ followed by a digit.

1.3.4 Feature Test Macros

The exact set of features available when you compile a source file is controlled by which feature test macros you define.

If you compile your programs using ‘gcc -ansi’, you get only the ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. See GNU CC Command Options in The GNU CC Manual, for more information about GCC options.

You should define these macros by using ‘#define’ preprocessor directives at the top of your source code files. These directives must come before any #include of a system header file. It is best to make them the very first thing in the file, preceded only by comments. You could also use the ‘-D’ option to GCC, but it’s better if you make the source files indicate their own meaning in a self-contained way.

This system exists to allow the library to conform to multiple standards. Although the different standards are often described as supersets of each other, they are usually incompatible because larger standards require functions with names that smaller ones reserve to the user program. This is not mere pedantry — it has been a problem in practice. For instance, some non-GNU programs define functions named getline that have nothing to do with this library’s getline. They would not be compilable if all features were enabled indiscriminately.

This should not be used to verify that a program conforms to a limited standard. It is insufficient for this purpose, as it will not protect you from including header files outside the standard, or relying on semantics undefined within the standard.

Macro: _POSIX_SOURCE

If you define this macro, then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is available, as well as all of the ISO C facilities.

The state of _POSIX_SOURCE is irrelevant if you define the macro _POSIX_C_SOURCE to a positive integer.

Macro: _POSIX_C_SOURCE

Define this macro to a positive integer to control which POSIX functionality is made available. The greater the value of this macro, the more functionality is made available.

If you define this macro to a value greater than or equal to 1, then the functionality from the 1990 edition of the POSIX.1 standard (IEEE Standard 1003.1-1990) is made available.

If you define this macro to a value greater than or equal to 2, then the functionality from the 1992 edition of the POSIX.2 standard (IEEE Standard 1003.2-1992) is made available.

If you define this macro to a value greater than or equal to 199309L, then the functionality from the 1993 edition of the POSIX.1b standard (IEEE Standard 1003.1b-1993) is made available.

Greater values for _POSIX_C_SOURCE will enable future extensions. The POSIX standards process will define these values as necessary, and the GNU C Library should support them some time after they become standardized. The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that if you define _POSIX_C_SOURCE to a value greater than or equal to 199506L, then the functionality from the 1996 edition is made available.

Macro: _XOPEN_SOURCE
Macro: _XOPEN_SOURCE_EXTENDED

If you define this macro, functionality described in the X/Open Portability Guide is included. This is a superset of the POSIX.1 and POSIX.2 functionality and in fact _POSIX_SOURCE and _POSIX_C_SOURCE are automatically defined.

As the unification of all Unices, functionality only available in BSD and SVID is also included.

If the macro _XOPEN_SOURCE_EXTENDED is also defined, even more functionality is available. The extra functions will make all functions available which are necessary for the X/Open Unix brand.

If the macro _XOPEN_SOURCE has the value 500 this includes all functionality described so far plus some new definitions from the Single Unix Specification, version 2.

Macro: _LARGEFILE_SOURCE

If this macro is defined some extra functions are available which rectify a few shortcomings in all previous standards. Specifically, the functions fseeko and ftello are available. Without these functions the difference between the ISO C interface (fseek, ftell) and the low-level POSIX interface (lseek) would lead to problems.

This macro was introduced as part of the Large File Support extension (LFS).

Macro: _LARGEFILE64_SOURCE

If you define this macro an additional set of functions is made available which enables 32 bit systems to use files of sizes beyond the usual limit of 2GB. This interface is not available if the system does not support files that large. On systems where the natural file size limit is greater than 2GB (i.e., on 64 bit systems) the new functions are identical to the replaced functions.

The new functionality is made available by a new set of types and functions which replace the existing ones. The names of these new objects contain 64 to indicate the intention, e.g., off_t vs. off64_t and fseeko vs. fseeko64.

This macro was introduced as part of the Large File Support extension (LFS). It is a transition interface for the period when 64 bit offsets are not generally used (see _FILE_OFFSET_BITS).

Macro: _FILE_OFFSET_BITS

This macro determines which file system interface shall be used, one replacing the other. Whereas _LARGEFILE64_SOURCE makes the 64 bit interface available as an additional interface, _FILE_OFFSET_BITS allows the 64 bit interface to replace the old interface.

If _FILE_OFFSET_BITS is undefined, or if it is defined to the value 32, nothing changes. The 32 bit interface is used and types like off_t have a size of 32 bits on 32 bit systems.

If the macro is defined to the value 64, the large file interface replaces the old interface. I.e., the functions are not made available under different names (as they are with _LARGEFILE64_SOURCE). Instead the old function names now reference the new functions, e.g., a call to fseeko now indeed calls fseeko64.

This macro should only be selected if the system provides mechanisms for handling large files. On 64 bit systems this macro has no effect since the *64 functions are identical to the normal functions.

This macro was introduced as part of the Large File Support extension (LFS).

Macro: _ISOC99_SOURCE

Until the revised ISO C standard is widely adopted the new features are not automatically enabled. The GNU C Library nevertheless has a complete implementation of the new standard and to enable the new features the macro _ISOC99_SOURCE should be defined.

Macro: __STDC_WANT_LIB_EXT2__

If you define this macro to the value 1, features from ISO/IEC TR 24731-2:2010 (Dynamic Allocation Functions) are enabled. Only some of the features from this TR are supported by the GNU C Library.

Macro: __STDC_WANT_IEC_60559_BFP_EXT__

If you define this macro, features from ISO/IEC TS 18661-1:2014 (Floating-point extensions for C: Binary floating-point arithmetic) are enabled. Only some of the features from this TS are supported by the GNU C Library.

Macro: __STDC_WANT_IEC_60559_FUNCS_EXT__

If you define this macro, features from ISO/IEC TS 18661-4:2015 (Floating-point extensions for C: Supplementary functions) are enabled. Only some of the features from this TS are supported by the GNU C Library.

Macro: _GNU_SOURCE

If you define this macro, everything is included: ISO C89, ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions. In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence.

Macro: _DEFAULT_SOURCE

If you define this macro, most features are included apart from X/Open, LFS and GNU extensions: the effect is to enable features from the 2008 edition of POSIX, as well as certain BSD and SVID features without a separate feature test macro to control them. Defining this macro, on its own and without using compiler options such as -ansi or -std=c99, has the same effect as not defining any feature test macros; defining it together with other feature test macros, or when options such as -ansi are used, enables those features even when the other options would otherwise cause them to be disabled.

Macro: _REENTRANT
Macro: _THREAD_SAFE

These macros are obsolete. They have the same effect as defining _POSIX_C_SOURCE with the value 199506L.

Some very old C libraries required one of these macros to be defined for basic functionality (e.g. getchar) to be thread-safe.

We recommend you use _GNU_SOURCE in new programs. If you don’t specify the ‘-ansi’ option to GCC, or other conformance options such as -std=c99, and don’t define any of these macros explicitly, the effect is the same as defining _DEFAULT_SOURCE to 1.

When you define a feature test macro to request a larger class of features, it is harmless to define in addition a feature test macro for a subset of those features. For example, if you define _POSIX_C_SOURCE, then defining _POSIX_SOURCE as well has no effect. Likewise, if you define _GNU_SOURCE, then defining either _POSIX_SOURCE or _POSIX_C_SOURCE as well has no effect.


1.4 Roadmap to the Manual

Here is an overview of the contents of the remaining chapters of this manual.

  • Error Reporting, describes how errors detected by the library are reported.
  • Memory, describes the GNU C Library’s facilities for managing and using virtual and real memory, including dynamic allocation of virtual memory. If you do not know in advance how much memory your program needs, you can allocate it dynamically instead, and manipulate it via pointers.
  • Character Handling, contains information about character classification functions (such as isspace) and functions for performing case conversion.
  • String and Array Utilities, has descriptions of functions for manipulating strings (null-terminated character arrays) and general byte arrays, including operations such as copying and comparison.
  • Character Set Handling, contains information about manipulating characters and strings using character sets larger than will fit in the usual char data type.
  • Locales, describes how selecting a particular country or language affects the behavior of the library. For example, the locale affects collation sequences for strings and how monetary values are formatted.
  • Searching and Sorting, contains information about functions for searching and sorting arrays. You can use these functions on any kind of array by providing an appropriate comparison function.
  • Pattern Matching, presents functions for matching regular expressions and shell file name patterns, and for expanding words as the shell does.
  • I/O Overview, gives an overall look at the input and output facilities in the library, and contains information about basic concepts such as file names.
  • I/O on Streams, describes I/O operations involving streams (or FILE&nbsp;* objects). These are the normal C library functions from stdio.h.
  • Low-Level I/O, contains information about I/O operations on file descriptors. File descriptors are a lower-level mechanism specific to the Unix family of operating systems.
  • File System Interface, has descriptions of operations on entire files, such as functions for deleting and renaming them and for creating new directories. This chapter also contains information about how you can access the attributes of a file, such as its owner and file protection modes.
  • Pipes and FIFOs, contains information about simple interprocess communication mechanisms. Pipes allow communication between two related processes (such as between a parent and child), while FIFOs allow communication between processes sharing a common file system on the same machine.
  • Sockets, describes a more complicated interprocess communication mechanism that allows processes running on different machines to communicate over a network. This chapter also contains information about Internet host addressing and how to use the system network databases.
  • Low-Level Terminal Interface, describes how you can change the attributes of a terminal device. If you want to disable echo of characters typed by the user, for example, read this chapter.
  • Mathematics, contains information about the math library functions. These include things like random-number generators and remainder functions on integers as well as the usual trigonometric and exponential functions on floating-point numbers.
  • Low-Level Arithmetic Functions, describes functions for simple arithmetic, analysis of floating-point values, and reading numbers from strings.
  • Date and Time, describes functions for measuring both calendar time and CPU time, as well as functions for setting alarms and timers.
  • Non-Local Exits, contains descriptions of the setjmp and longjmp functions. These functions provide a facility for goto-like jumps which can jump from one function to another.
  • Signal Handling, tells you all about signals—what they are, how to establish a handler that is called when a particular kind of signal is delivered, and how to prevent signals from arriving during critical sections of your program.
  • Program Basics, tells how your programs can access their command-line arguments and environment variables.
  • Processes, contains information about how to start new processes and run programs.
  • Job Control, describes functions for manipulating process groups and the controlling terminal. This material is probably only of interest if you are writing a shell or other program which handles job control specially.
  • Name Service Switch, describes the services which are available for looking up names in the system databases, how to determine which service is used for which database, and how these services are implemented so that contributors can design their own services.
  • User Database, and Group Database, tell you how to access the system user and group databases.
  • System Management, describes functions for controlling and getting information about the hardware and software configuration your program is executing under.
  • System Configuration, tells you how you can get information about various operating system limits. Most of these parameters are provided for compatibility with POSIX.
  • Language Features, contains information about library support for standard parts of the C language, including things like the sizeof operator and the symbolic constant NULL, how to write functions accepting variable numbers of arguments, and constants describing the ranges and other properties of the numerical types. There is also a simple debugging mechanism which allows you to put assertions in your code, and have diagnostic messages printed if the tests fail.
  • Library Summary, gives a summary of all the functions, variables, and macros in the library, with complete data types and function prototypes, and says what standard or system each is derived from.
  • Installation, explains how to build and install the GNU C Library on your system, and how to report any bugs you might find.
  • Maintenance, explains how to add new functions or port the library to a new system.

If you already know the name of the facility you are interested in, you can look it up in Library Summary. This gives you a summary of its syntax and a pointer to where you can find a more detailed description. This appendix is particularly useful if you just want to verify the order and type of arguments to a function, for example. It also tells you what standard or system each function, variable, or macro is derived from.


Next: , Previous: , Up: Top   [Contents][Index]

2 Error Reporting

Many functions in the GNU C Library detect and report error conditions, and sometimes your programs need to check for these error conditions. For example, when you open an input file, you should verify that the file was actually opened correctly, and print an error message or take other appropriate action if the call to the library function failed.

This chapter describes how the error reporting facility works. Your program should include the header file errno.h to use this facility.


2.1 Checking for Errors

Most library functions return a special value to indicate that they have failed. The special value is typically -1, a null pointer, or a constant such as EOF that is defined for that purpose. But this return value tells you only that an error has occurred. To find out what kind of error it was, you need to look at the error code stored in the variable errno. This variable is declared in the header file errno.h.

Variable: volatile int errno

The variable errno contains the system error number. You can change the value of errno.

Since errno is declared volatile, it might be changed asynchronously by a signal handler; see Defining Handlers. However, a properly written signal handler saves and restores the value of errno, so you generally do not need to worry about this possibility except when writing signal handlers.

The initial value of errno at program startup is zero. Many library functions are guaranteed to set it to certain nonzero values when they encounter certain kinds of errors. These error conditions are listed for each function. These functions do not change errno when they succeed; thus, the value of errno after a successful call is not necessarily zero, and you should not use errno to determine whether a call failed. The proper way to do that is documented for each function. If the call failed, you can examine errno.

Many library functions can set errno to a nonzero value as a result of calling other library functions which might fail. You should assume that any library function might alter errno when the function returns an error.

Portability Note: ISO C specifies errno as a “modifiable lvalue” rather than as a variable, permitting it to be implemented as a macro. For example, its expansion might involve a function call, like <span class="nolinebreak">*__errno_location</span>&nbsp;(). In fact, that is what it is on GNU/Linux and GNU/Hurd systems. The GNU C Library, on each system, does whatever is right for the particular system.

There are a few library functions, like sqrt and atan, that return a perfectly legitimate value in case of an error, but also set errno. For these functions, if you want to check to see whether an error occurred, the recommended method is to set errno to zero before calling the function, and then check its value afterward.

All the error codes have symbolic names; they are macros defined in errno.h. The names start with ‘E’ and an upper-case letter or digit; you should consider names of this form to be reserved names. See Reserved Names.

The error code values are all positive integers and are all distinct, with one exception: EWOULDBLOCK and EAGAIN are the same. Since the values are distinct, you can use them as labels in a switch statement; just don’t use both EWOULDBLOCK and EAGAIN. Your program should not make any other assumptions about the specific values of these symbolic constants.

The value of errno doesn’t necessarily have to correspond to any of these macros, since some library functions might return other error codes of their own for other situations. The only values that are guaranteed to be meaningful for a particular library function are the ones that this manual lists for that function.

Except on GNU/Hurd systems, almost any system call can return EFAULT if it is given an invalid pointer as an argument. Since this could only happen as a result of a bug in your program, and since it will not happen on GNU/Hurd systems, we have saved space by not mentioning EFAULT in the descriptions of individual functions.

In some Unix systems, many system calls can also return EFAULT if given as an argument a pointer into the stack, and the kernel for some obscure reason fails in its attempt to extend the stack. If this ever happens, you should probably try using statically or dynamically allocated memory instead of stack memory on that system.


2.2 Error Codes

The error code macros are defined in the header file errno.h. All of them expand into integer constant values. Some of these error codes can’t occur on GNU systems, but they can occur using the GNU C Library on other systems.

Macro: int EPERM

Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation.

Macro: int ENOENT

No such file or directory. This is a “file doesn’t exist” error for ordinary files that are referenced in contexts where they are expected to already exist.

Macro: int ESRCH

No process matches the specified process ID.

Macro: int EINTR

Interrupted function call; an asynchronous signal occurred and prevented completion of the call. When this happens, you should try the call again.

You can choose to have functions resume after a signal that is handled, rather than failing with EINTR; see Interrupted Primitives.

Macro: int EIO

Input/output error; usually used for physical read or write errors.

Macro: int ENXIO

No such device or address. The system tried to use the device represented by a file you specified, and it couldn’t find the device. This can mean that the device file was installed incorrectly, or that the physical device is missing or not correctly attached to the computer.

Macro: int E2BIG

Argument list too long; used when the arguments passed to a new program being executed with one of the exec functions (see Executing a File) occupy too much memory space. This condition never arises on GNU/Hurd systems.

Macro: int ENOEXEC

Invalid executable file format. This condition is detected by the exec functions; see Executing a File.

Macro: int EBADF

Bad file descriptor; for example, I/O on a descriptor that has been closed or reading from a descriptor open only for writing (or vice versa).

Macro: int ECHILD

There are no child processes. This error happens on operations that are supposed to manipulate child processes, when there aren’t any processes to manipulate.

Macro: int EDEADLK

Deadlock avoided; allocating a system resource would have resulted in a deadlock situation. The system does not guarantee that it will notice all such situations. This error means you got lucky and the system noticed; it might just hang. See File Locks, for an example.

Macro: int ENOMEM

No memory available. The system cannot allocate more virtual memory because its capacity is full.

Macro: int EACCES

Permission denied; the file permissions do not allow the attempted operation.

Macro: int EFAULT

Bad address; an invalid pointer was detected. On GNU/Hurd systems, this error never happens; you get a signal instead.

Macro: int ENOTBLK

A file that isn’t a block special file was given in a situation that requires one. For example, trying to mount an ordinary file as a file system in Unix gives this error.

Macro: int EBUSY

Resource busy; a system resource that can’t be shared is already in use. For example, if you try to delete a file that is the root of a currently mounted filesystem, you get this error.

Macro: int EEXIST

File exists; an existing file was specified in a context where it only makes sense to specify a new file.

Macro: int EXDEV

An attempt to make an improper link across file systems was detected. This happens not only when you use link (see Hard Links) but also when you rename a file with rename (see Renaming Files).

Macro: int ENODEV

The wrong type of device was given to a function that expects a particular sort of device.

Macro: int ENOTDIR

A file that isn’t a directory was specified when a directory is required.

Macro: int EISDIR

File is a directory; you cannot open a directory for writing, or create or remove hard links to it.

Macro: int EINVAL

Invalid argument. This is used to indicate various kinds of problems with passing the wrong argument to a library function.

Macro: int EMFILE

The current process has too many files open and can’t open any more. Duplicate descriptors do count toward this limit.

In BSD and GNU, the number of open files is controlled by a resource limit that can usually be increased. If you get this error, you might want to increase the RLIMIT_NOFILE limit or make it unlimited; see Limits on Resources.

Macro: int ENFILE

There are too many distinct file openings in the entire system. Note that any number of linked channels count as just one file opening; see Linked Channels. This error never occurs on GNU/Hurd systems.

Macro: int ENOTTY

Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file.

Macro: int ETXTBSY

An attempt to execute a file that is currently open for writing, or write to a file that is currently being executed. Often using a debugger to run a program is considered having it open for writing and will cause this error. (The name stands for “text file busy”.) This is not an error on GNU/Hurd systems; the text is copied as necessary.

Macro: int EFBIG

File too big; the size of a file would be larger than allowed by the system.

Macro: int ENOSPC

No space left on device; write operation on a file failed because the disk is full.

Macro: int ESPIPE

Invalid seek operation (such as on a pipe).

Macro: int EROFS

An attempt was made to modify something on a read-only file system.

Macro: int EMLINK

Too many links; the link count of a single file would become too large. rename can cause this error if the file being renamed already has as many links as it can take (see Renaming Files).

Macro: int EPIPE

Broken pipe; there is no process reading from the other end of a pipe. Every library function that returns this error code also generates a SIGPIPE signal; this signal terminates the program if not handled or blocked. Thus, your program will never actually see EPIPE unless it has handled or blocked SIGPIPE.

Macro: int EDOM

Domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined.

Macro: int ERANGE

Range error; used by mathematical functions when the result value is not representable because of overflow or underflow.

Macro: int EAGAIN

Resource temporarily unavailable; the call might work if you try again later. The macro EWOULDBLOCK is another name for EAGAIN; they are always the same in the GNU C Library.

This error can happen in a few different situations:

  • An operation that would block was attempted on an object that has non-blocking mode selected. Trying the same operation again will block until some external condition makes it possible to read, write, or connect (whatever the operation). You can use select to find out when the operation will be possible; see Waiting for I/O.

    Portability Note: In many older Unix systems, this condition was indicated by EWOULDBLOCK, which was a distinct error code different from EAGAIN. To make your program portable, you should check for both codes and treat them the same.

  • A temporary resource shortage made an operation impossible. fork can return this error. It indicates that the shortage is expected to pass, so your program can try the call again later and it may succeed. It is probably a good idea to delay for a few seconds before trying it again, to allow time for other processes to release scarce resources. Such shortages are usually fairly serious and affect the whole system, so usually an interactive program should report the error to the user and return to its command loop.
Macro: int EWOULDBLOCK

In the GNU C Library, this is another name for EAGAIN (above). The values are always the same, on every operating system.

C libraries in many older Unix systems have EWOULDBLOCK as a separate error code.

Macro: int EINPROGRESS

An operation that cannot complete immediately was initiated on an object that has non-blocking mode selected. Some functions that must always block (such as connect; see Connecting) never return EAGAIN. Instead, they return EINPROGRESS to indicate that the operation has begun and will take some time. Attempts to manipulate the object before the call completes return EALREADY. You can use the select function to find out when the pending operation has completed; see Waiting for I/O.

Macro: int EALREADY

An operation is already in progress on an object that has non-blocking mode selected.

Macro: int ENOTSOCK

A file that isn’t a socket was specified when a socket is required.

Macro: int EMSGSIZE

The size of a message sent on a socket was larger than the supported maximum size.

Macro: int EPROTOTYPE

The socket type does not support the requested communications protocol.

Macro: int ENOPROTOOPT

You specified a socket option that doesn’t make sense for the particular protocol being used by the socket. See Socket Options.

Macro: int EPROTONOSUPPORT

The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). See Creating a Socket.

Macro: int ESOCKTNOSUPPORT

The socket type is not supported.

Macro: int EOPNOTSUPP

The operation you requested is not supported. Some socket functions don’t make sense for all types of sockets, and others may not be implemented for all communications protocols. On GNU/Hurd systems, this error can happen for many calls when the object does not support the particular operation; it is a generic indication that the server knows nothing to do for that call.

Macro: int EPFNOSUPPORT

The socket communications protocol family you requested is not supported.

Macro: int EAFNOSUPPORT

The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket. See Sockets.

Macro: int EADDRINUSE

The requested socket address is already in use. See Socket Addresses.

Macro: int EADDRNOTAVAIL

The requested socket address is not available; for example, you tried to give a socket a name that doesn’t match the local host name. See Socket Addresses.

Macro: int ENETDOWN

A socket operation failed because the network was down.

Macro: int ENETUNREACH

A socket operation failed because the subnet containing the remote host was unreachable.

Macro: int ENETRESET

A network connection was reset because the remote host crashed.

Macro: int ECONNABORTED

A network connection was aborted locally.

Macro: int ECONNRESET

A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting or an unrecoverable protocol violation.

Macro: int ENOBUFS

The kernel’s buffers for I/O operations are all in use. In GNU, this error is always synonymous with ENOMEM; you may get one or the other from network operations.

Macro: int EISCONN

You tried to connect a socket that is already connected. See Connecting.

Macro: int ENOTCONN

The socket is not connected to anything. You get this error when you try to transmit data over a socket, without first specifying a destination for the data. For a connectionless socket (for datagram protocols, such as UDP), you get EDESTADDRREQ instead.

Macro: int EDESTADDRREQ

No default destination address was set for the socket. You get this error when you try to transmit data over a connectionless socket, without first specifying a destination for the data with connect.

Macro: int ESHUTDOWN

The socket has already been shut down.

Macro: int ETOOMANYREFS

???

Macro: int ETIMEDOUT

A socket operation with a specified timeout received no response during the timeout period.

Macro: int ECONNREFUSED

A remote host refused to allow the network connection (typically because it is not running the requested service).

Macro: int ELOOP

Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links.

Macro: int ENAMETOOLONG

Filename too long (longer than PATH_MAX; see Limits for Files) or host name too long (in gethostname or sethostname; see Host Identification).

Macro: int EHOSTDOWN

The remote host for a requested network connection is down.

Macro: int EHOSTUNREACH

The remote host for a requested network connection is not reachable.

Macro: int ENOTEMPTY

Directory not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory.

Macro: int EPROCLIM

This means that the per-user limit on new process would be exceeded by an attempted fork. See Limits on Resources, for details on the RLIMIT_NPROC limit.

Macro: int EUSERS

The file quota system is confused because there are too many users.

Macro: int EDQUOT

The user’s disk quota was exceeded.

Macro: int ESTALE

Stale file handle. This indicates an internal confusion in the file system which is due to file system rearrangements on the server host for NFS file systems or corruption in other file systems. Repairing this condition usually requires unmounting, possibly repairing and remounting the file system.

Macro: int EREMOTE

An attempt was made to NFS-mount a remote file system with a file name that already specifies an NFS-mounted file. (This is an error on some operating systems, but we expect it to work properly on GNU/Hurd systems, making this error code impossible.)

Macro: int EBADRPC

???

Macro: int ERPCMISMATCH

???

Macro: int EPROGUNAVAIL

???

Macro: int EPROGMISMATCH

???

Macro: int EPROCUNAVAIL

???

Macro: int ENOLCK

No locks available. This is used by the file locking facilities; see File Locks. This error is never generated by GNU/Hurd systems, but it can result from an operation to an NFS server running another operating system.

Macro: int EFTYPE

Inappropriate file type or format. The file was the wrong type for the operation, or a data file had the wrong format.

On some systems chmod returns this error if you try to set the sticky bit on a non-directory file; see Setting Permissions.

Macro: int EAUTH

???

Macro: int ENEEDAUTH

???

Macro: int ENOSYS

Function not implemented. This indicates that the function called is not implemented at all, either in the C library itself or in the operating system. When you get this error, you can be sure that this particular function will always fail with ENOSYS unless you install a new version of the C library or the operating system.

Macro: int ENOTSUP

Not supported. A function returns this error when certain parameter values are valid, but the functionality they request is not available. This can mean that the function does not implement a particular command or option value or flag bit at all. For functions that operate on some object given in a parameter, such as a file descriptor or a port, it might instead mean that only that specific object (file descriptor, port, etc.) is unable to support the other parameters given; different file descriptors might support different ranges of parameter values.

If the entire function is not available at all in the implementation, it returns ENOSYS instead.

Macro: int EILSEQ

While decoding a multibyte character the function came along an invalid or an incomplete sequence of bytes or the given wide character is invalid.

Macro: int EBACKGROUND

On GNU/Hurd systems, servers supporting the term protocol return this error for certain operations when the caller is not in the foreground process group of the terminal. Users do not usually see this error because functions such as read and write translate it into a SIGTTIN or SIGTTOU signal. See Job Control, for information on process groups and these signals.

Macro: int EDIED

On GNU/Hurd systems, opening a file returns this error when the file is translated by a program and the translator program dies while starting up, before it has connected to the file.

Macro: int ED

The experienced user will know what is wrong.

Macro: int EGREGIOUS

You did what?

Macro: int EIEIO

Go home and have a glass of warm, dairy-fresh milk.

Macro: int EGRATUITOUS

This error code has no purpose.

Macro: int EBADMSG
Macro: int EIDRM
Macro: int EMULTIHOP
Macro: int ENODATA
Macro: int ENOLINK
Macro: int ENOMSG
Macro: int ENOSR
Macro: int ENOSTR
Macro: int EOVERFLOW
Macro: int EPROTO
Macro: int ETIME
Macro: int ECANCELED

Operation canceled; an asynchronous operation was canceled before it completed. See Asynchronous I/O. When you call aio_cancel, the normal result is for the operations affected to complete with this error; see Cancel AIO Operations.

The following error codes are defined by the Linux/i386 kernel. They are not yet documented.

Macro: int ERESTART
Macro: int ECHRNG
Macro: int EL2NSYNC
Macro: int EL3HLT
Macro: int EL3RST
Macro: int ELNRNG
Macro: int EUNATCH
Macro: int ENOCSI
Macro: int EL2HLT
Macro: int EBADE
Macro: int EBADR
Macro: int EXFULL
Macro: int ENOANO
Macro: int EBADRQC
Macro: int EBADSLT
Macro: int EDEADLOCK
Macro: int EBFONT
Macro: int ENONET
Macro: int ENOPKG
Macro: int EADV
Macro: int ESRMNT
Macro: int ECOMM
Macro: int EDOTDOT
Macro: int ENOTUNIQ
Macro: int EBADFD
Macro: int EREMCHG
Macro: int ELIBACC
Macro: int ELIBBAD
Macro: int ELIBSCN
Macro: int ELIBMAX
Macro: int ELIBEXEC
Macro: int ESTRPIPE
Macro: int EUCLEAN
Macro: int ENOTNAM
Macro: int ENAVAIL
Macro: int EISNAM
Macro: int EREMOTEIO
Macro: int ENOMEDIUM
Macro: int EMEDIUMTYPE
Macro: int ENOKEY
Macro: int EKEYEXPIRED
Macro: int EKEYREVOKED
Macro: int EKEYREJECTED
Macro: int EOWNERDEAD
Macro: int ENOTRECOVERABLE
Macro: int ERFKILL
Macro: int EHWPOISON

Previous: , Up: Error Reporting   [Contents][Index]

2.3 Error Messages

The library has functions and variables designed to make it easy for your program to report informative error messages in the customary format about the failure of a library call. The functions strerror and perror give you the standard error message for a given error code; the variable <span class="nolinebreak">program_invocation_short_name</span> gives you convenient access to the name of the program that encountered the error.

Function: char * strerror (int errnum)

Preliminary: | MT-Unsafe race:strerror | AS-Unsafe heap i18n | AC-Unsafe mem | See POSIX Safety Concepts.

The strerror function maps the error code (see Checking for Errors) specified by the errnum argument to a descriptive error message string. The return value is a pointer to this string.

The value errnum normally comes from the variable errno.

You should not modify the string returned by strerror. Also, if you make subsequent calls to strerror, the string might be overwritten. (But it’s guaranteed that no library function ever calls strerror behind your back.)

The function strerror is declared in string.h.

Function: char * strerror_r (int errnum, char *buf, size_t n)

Preliminary: | MT-Safe | AS-Unsafe i18n | AC-Unsafe | See POSIX Safety Concepts.

The strerror_r function works like strerror but instead of returning the error message in a statically allocated buffer shared by all threads in the process, it returns a private copy for the thread. This might be either some permanent global data or a message string in the user supplied buffer starting at buf with the length of n bytes.

At most n characters are written (including the NUL byte) so it is up to the user to select a buffer large enough.

This function should always be used in multi-threaded programs since there is no way to guarantee the string returned by strerror really belongs to the last call of the current thread.

The function strerror_r is a GNU extension and it is declared in string.h.

Function: void perror (const char *message)

Preliminary: | MT-Safe race:stderr | AS-Unsafe corrupt i18n heap lock | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

This function prints an error message to the stream stderr; see Standard Streams. The orientation of stderr is not changed.

If you call perror with a message that is either a null pointer or an empty string, perror just prints the error message corresponding to errno, adding a trailing newline.

If you supply a non-null message argument, then perror prefixes its output with this string. It adds a colon and a space character to separate the message from the error string corresponding to errno.

The function perror is declared in stdio.h.

strerror and perror produce the exact same message for any given error code; the precise text varies from system to system. With the GNU C Library, the messages are fairly short; there are no multi-line messages or embedded newlines. Each error message begins with a capital letter and does not include any terminating punctuation.

Many programs that don’t read input from the terminal are designed to exit if any system call fails. By convention, the error message from such a program should start with the program’s name, sans directories. You can find that name in the variable program_invocation_short_name; the full file name is stored the variable program_invocation_name.

Variable: char * program_invocation_name

This variable’s value is the name that was used to invoke the program running in the current process. It is the same as argv[0]. Note that this is not necessarily a useful file name; often it contains no directory names. See Program Arguments.

This variable is a GNU extension and is declared in errno.h.

Variable: char * program_invocation_short_name

This variable’s value is the name that was used to invoke the program running in the current process, with directory names removed. (That is to say, it is the same as program_invocation_name minus everything up to the last slash, if any.)

This variable is a GNU extension and is declared in errno.h.

The library initialization code sets up both of these variables before calling main.

Portability Note: If you want your program to work with non-GNU libraries, you must save the value of argv[0] in main, and then strip off the directory names yourself. We added these extensions to make it possible to write self-contained error-reporting subroutines that require no explicit cooperation from main.

Here is an example showing how to handle failure to open a file correctly. The function open_sesame tries to open the named file for reading and returns a stream if successful. The fopen library function returns a null pointer if it couldn’t open the file for some reason. In that situation, open_sesame constructs an appropriate error message using the strerror function, and terminates the program. If we were going to make some other library calls before passing the error code to strerror, we’d have to save it in a local variable instead, because those other library functions might overwrite errno in the meantime.

#define _GNU_SOURCE
 
#include &lt;errno.h&gt;
#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;
 
FILE *
open_sesame (char *name)
{
  FILE *stream;
 
  errno = 0;
  stream = fopen (name, "r");
  if (stream == NULL)
    {
      fprintf (stderr, "%s: Couldn't open file %s; %s\n",
               program_invocation_short_name, name, strerror (errno));
      exit (EXIT_FAILURE);
    }
  else
    return stream;
}

Using perror has the advantage that the function is portable and available on all systems implementing ISO C. But often the text perror generates is not what is wanted and there is no way to extend or change what perror does. The GNU coding standard, for instance, requires error messages to be preceded by the program name and programs which read some input files should provide information about the input file name and the line number in case an error is encountered while reading the file. For these occasions there are two functions available which are widely used throughout the GNU project. These functions are declared in error.h.

Function: void error (int status, int errnum, const char *format, …)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Safe | See POSIX Safety Concepts.

The error function can be used to report general problems during program execution. The format argument is a format string just like those given to the printf family of functions. The arguments required for the format can follow the format parameter. Just like perror, error also can report an error code in textual form. But unlike perror the error value is explicitly passed to the function in the errnum parameter. This eliminates the problem mentioned above that the error reporting function must be called immediately after the function causing the error since otherwise errno might have a different value.

error prints first the program name. If the application defined a global variable error_print_progname and points it to a function this function will be called to print the program name. Otherwise the string from the global variable program_name is used. The program name is followed by a colon and a space which in turn is followed by the output produced by the format string. If the errnum parameter is non-zero the format string output is followed by a colon and a space, followed by the error message for the error code errnum. In any case is the output terminated with a newline.

The output is directed to the stderr stream. If the stderr wasn’t oriented before the call it will be narrow-oriented afterwards.

The function will return unless the status parameter has a non-zero value. In this case the function will call exit with the status value for its parameter and therefore never return. If error returns, the global variable error_message_count is incremented by one to keep track of the number of errors reported.

Function: void error_at_line (int status, int errnum, const char *fname, unsigned int lineno, const char *format, …)

Preliminary: | MT-Unsafe race:error_at_line/error_one_per_line locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt/error_one_per_line | See POSIX Safety Concepts.

The error_at_line function is very similar to the error function. The only differences are the additional parameters fname and lineno. The handling of the other parameters is identical to that of error except that between the program name and the string generated by the format string additional text is inserted.

Directly following the program name a colon, followed by the file name pointed to by fname, another colon, and the value of lineno is printed.

This additional output of course is meant to be used to locate an error in an input file (like a programming language source code file etc).

If the global variable error_one_per_line is set to a non-zero value error_at_line will avoid printing consecutive messages for the same file and line. Repetition which are not directly following each other are not caught.

Just like error this function only returns if status is zero. Otherwise exit is called with the non-zero value. If error returns, the global variable error_message_count is incremented by one to keep track of the number of errors reported.

As mentioned above, the error and error_at_line functions can be customized by defining a variable named error_print_progname.

Variable: void (*error_print_progname) (void)

If the error_print_progname variable is defined to a non-zero value the function pointed to is called by error or error_at_line. It is expected to print the program name or do something similarly useful.

The function is expected to print to the stderr stream and must be able to handle whatever orientation the stream has.

The variable is global and shared by all threads.

Variable: unsigned int error_message_count

The error_message_count variable is incremented whenever one of the functions error or error_at_line returns. The variable is global and shared by all threads.

Variable: int error_one_per_line

The error_one_per_line variable influences only error_at_line. Normally the error_at_line function creates output for every invocation. If error_one_per_line is set to a non-zero value error_at_line keeps track of the last file name and line number for which an error was reported and avoids directly following messages for the same file and line. This variable is global and shared by all threads.

A program which read some input file and reports errors in it could look like this:

{
  char *line = NULL;
  size_t len = 0;
  unsigned int lineno = 0;
 
  error_message_count = 0;
  while (! feof_unlocked (fp))
    {
      ssize_t n = getline (&amp;line, &amp;len, fp);
      if (n &lt;= 0)
        /* <span class="roman">End of file or error.</span>  */
        break;
      ++lineno;
 
      /* <span class="roman">Process the line.</span>  */
      …
 
      if (<span class="roman">Detect error in line</span>)
        error_at_line (0, errval, filename, lineno,
                       "some error text %s", some_variable);
    }
 
  if (error_message_count != 0)
    error (EXIT_FAILURE, 0, "%u errors found", error_message_count);
}

error and error_at_line are clearly the functions of choice and enable the programmer to write applications which follow the GNU coding standard. The GNU C Library additionally contains functions which are used in BSD for the same purpose. These functions are declared in err.h. It is generally advised to not use these functions. They are included only for compatibility.

Function: void warn (const char *format, …)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.

The warn function is roughly equivalent to a call like

  error (0, errno, format, <span class="roman">the parameters</span>)

except that the global variables error respects and modifies are not used.

Function: void vwarn (const char *format, va_list ap)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.

The vwarn function is just like warn except that the parameters for the handling of the format string format are passed in as a value of type va_list.

Function: void warnx (const char *format, …)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.

The warnx function is roughly equivalent to a call like

  error (0, 0, format, <span class="roman">the parameters</span>)

except that the global variables error respects and modifies are not used. The difference to warn is that no error number string is printed.

Function: void vwarnx (const char *format, va_list ap)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.

The vwarnx function is just like warnx except that the parameters for the handling of the format string format are passed in as a value of type va_list.

Function: void err (int status, const char *format, …)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.

The err function is roughly equivalent to a call like

  error (status, errno, format, <span class="roman">the parameters</span>)

except that the global variables error respects and modifies are not used and that the program is exited even if status is zero.

Function: void verr (int status, const char *format, va_list ap)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.

The verr function is just like err except that the parameters for the handling of the format string format are passed in as a value of type va_list.

Function: void errx (int status, const char *format, …)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.

The errx function is roughly equivalent to a call like

  error (status, 0, format, <span class="roman">the parameters</span>)

except that the global variables error respects and modifies are not used and that the program is exited even if status is zero. The difference to err is that no error number string is printed.

Function: void verrx (int status, const char *format, va_list ap)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.

The verrx function is just like errx except that the parameters for the handling of the format string format are passed in as a value of type va_list.


Next: , Previous: , Up: Top   [Contents][Index]

3 Virtual Memory Allocation And Paging

This chapter describes how processes manage and use memory in a system that uses the GNU C Library.

The GNU C Library has several functions for dynamically allocating virtual memory in various ways. They vary in generality and in efficiency. The library also provides functions for controlling paging and allocation of real memory.

Memory mapped I/O is not discussed in this chapter. See Memory-mapped I/O.


3.1 Process Memory Concepts

One of the most basic resources a process has available to it is memory. There are a lot of different ways systems organize memory, but in a typical one, each process has one linear virtual address space, with addresses running from zero to some huge maximum. It need not be contiguous; i.e., not all of these addresses actually can be used to store data.

The virtual memory is divided into pages (4 kilobytes is typical). Backing each page of virtual memory is a page of real memory (called a frame) or some secondary storage, usually disk space. The disk space might be swap space or just some ordinary disk file. Actually, a page of all zeroes sometimes has nothing at all backing it – there’s just a flag saying it is all zeroes.

The same frame of real memory or backing store can back multiple virtual pages belonging to multiple processes. This is normally the case, for example, with virtual memory occupied by GNU C Library code. The same real memory frame containing the printf function backs a virtual memory page in each of the existing processes that has a printf call in its program.

In order for a program to access any part of a virtual page, the page must at that moment be backed by (“connected to”) a real frame. But because there is usually a lot more virtual memory than real memory, the pages must move back and forth between real memory and backing store regularly, coming into real memory when a process needs to access them and then retreating to backing store when not needed anymore. This movement is called paging.

When a program attempts to access a page which is not at that moment backed by real memory, this is known as a page fault. When a page fault occurs, the kernel suspends the process, places the page into a real page frame (this is called “paging in” or “faulting in”), then resumes the process so that from the process’ point of view, the page was in real memory all along. In fact, to the process, all pages always seem to be in real memory. Except for one thing: the elapsed execution time of an instruction that would normally be a few nanoseconds is suddenly much, much, longer (because the kernel normally has to do I/O to complete the page-in). For programs sensitive to that, the functions described in Locking Pages can control it.

Within each virtual address space, a process has to keep track of what is at which addresses, and that process is called memory allocation. Allocation usually brings to mind meting out scarce resources, but in the case of virtual memory, that’s not a major goal, because there is generally much more of it than anyone needs. Memory allocation within a process is mainly just a matter of making sure that the same byte of memory isn’t used to store two different things.

Processes allocate memory in two major ways: by exec and programmatically. Actually, forking is a third way, but it’s not very interesting. See Creating a Process.

Exec is the operation of creating a virtual address space for a process, loading its basic program into it, and executing the program. It is done by the “exec” family of functions (e.g. execl). The operation takes a program file (an executable), it allocates space to load all the data in the executable, loads it, and transfers control to it. That data is most notably the instructions of the program (the text), but also literals and constants in the program and even some variables: C variables with the static storage class (see Memory Allocation and C).

Once that program begins to execute, it uses programmatic allocation to gain additional memory. In a C program with the GNU C Library, there are two kinds of programmatic allocation: automatic and dynamic. See Memory Allocation and C.

Memory-mapped I/O is another form of dynamic virtual memory allocation. Mapping memory to a file means declaring that the contents of certain range of a process’ addresses shall be identical to the contents of a specified regular file. The system makes the virtual memory initially contain the contents of the file, and if you modify the memory, the system writes the same modification to the file. Note that due to the magic of virtual memory and page faults, there is no reason for the system to do I/O to read the file, or allocate real memory for its contents, until the program accesses the virtual memory. See Memory-mapped I/O.

Just as it programmatically allocates memory, the program can programmatically deallocate (free) it. You can’t free the memory that was allocated by exec. When the program exits or execs, you might say that all its memory gets freed, but since in both cases the address space ceases to exist, the point is really moot. See Program Termination.

A process’ virtual address space is divided into segments. A segment is a contiguous range of virtual addresses. Three important segments are:

  • The text segment contains a program’s instructions and literals and static constants. It is allocated by exec and stays the same size for the life of the virtual address space.
  • The data segment is working storage for the program. It can be preallocated and preloaded by exec and the process can extend or shrink it by calling functions as described in See Resizing the Data Segment. Its lower end is fixed.
  • The stack segment contains a program stack. It grows as the stack grows, but doesn’t shrink when the stack shrinks.

3.2 Allocating Storage For Program Data

This section covers how ordinary programs manage storage for their data, including the famous malloc function and some fancier facilities special to the GNU C Library and GNU Compiler.


3.2.1 Memory Allocation in C Programs

The C language supports two kinds of memory allocation through the variables in C programs:

  • Static allocation is what happens when you declare a static or global variable. Each static or global variable defines one block of space, of a fixed size. The space is allocated once, when your program is started (part of the exec operation), and is never freed.
  • Automatic allocation happens when you declare an automatic variable, such as a function argument or a local variable. The space for an automatic variable is allocated when the compound statement containing the declaration is entered, and is freed when that compound statement is exited.

    In GNU C, the size of the automatic storage can be an expression that varies. In other C implementations, it must be a constant.

A third important kind of memory allocation, dynamic allocation, is not supported by C variables but is available via GNU C Library functions.

3.2.1.1 Dynamic Memory Allocation

Dynamic memory allocation is a technique in which programs determine as they are running where to store some information. You need dynamic allocation when the amount of memory you need, or how long you continue to need it, depends on factors that are not known before the program runs.

For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the memory dynamically and make it dynamically larger as you read more of the line.

Or, you may need a block for each record or each definition in the input data; since you can’t know in advance how many there will be, you must allocate a new block for each record or definition as you read it.

When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly. You call a function or macro when you want to allocate space, and specify the size with an argument. If you want to free the space, you do so by calling another function or macro. You can do these things whenever you want, as often as you want.

Dynamic allocation is not supported by C variables; there is no storage class “dynamic”, and there can never be a C variable whose value is stored in dynamically allocated space. The only way to get dynamically allocated memory is via a system call (which is generally via a GNU C Library function call), and the only way to refer to dynamically allocated space is through a pointer. Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers generally use dynamic allocation only when neither static nor automatic allocation will serve.

For example, if you want to allocate dynamically some space to hold a struct foobar, you cannot declare a variable of type

struct
foobar
whose contents are the dynamically allocated space. But you can declare a variable of pointer type struct foobar * and assign it the address of the space. Then you can use the operators ‘*’ and ‘->’ on this pointer variable to refer to the contents of the space:

{
  struct foobar *ptr
     = (struct foobar *) malloc (sizeof (struct foobar));
  ptr-&gt;name = x;
  ptr-&gt;next = current_foobar;
  current_foobar = ptr;
}

3.2.2 The GNU Allocator

The malloc implementation in the GNU C Library is derived from ptmalloc (pthreads malloc), which in turn is derived from dlmalloc (Doug Lea malloc). This malloc may allocate memory in two different ways depending on their size and certain parameters that may be controlled by users. The most common way is to allocate portions of memory (called chunks) from a large contiguous area of memory and manage these areas to optimize their use and reduce wastage in the form of unusable chunks. Traditionally the system heap was set up to be the one large memory area but the GNU C Library malloc implementation maintains multiple such areas to optimize their use in multi-threaded applications. Each such area is internally referred to as an arena.

As opposed to other versions, the malloc in the GNU C Library does not round up chunk sizes to powers of two, neither for large nor for small sizes. Neighboring chunks can be coalesced on a free no matter what their size is. This makes the implementation suitable for all kinds of allocation patterns without generally incurring high memory waste through fragmentation. The presence of multiple arenas allows multiple threads to allocate memory simultaneously in separate arenas, thus improving performance.

The other way of memory allocation is for very large blocks, i.e. much larger than a page. These requests are allocated with mmap (anonymous or via /dev/zero; see Memory-mapped I/O)). This has the great advantage that these chunks are returned to the system immediately when they are freed. Therefore, it cannot happen that a large chunk becomes “locked” in between smaller ones and even after calling free wastes memory. The size threshold for mmap to be used is dynamic and gets adjusted according to allocation patterns of the program. mallopt can be used to statically adjust the threshold using M_MMAP_THRESHOLD and the use of mmap can be disabled completely with M_MMAP_MAX; see Malloc Tunable Parameters.

A more detailed technical description of the GNU Allocator is maintained in the GNU C Library wiki. See https://sourceware.org/glibc/wiki/MallocInternals.


3.2.3 Unconstrained Allocation

The most general dynamic allocation facility is malloc. It allows you to allocate blocks of memory of any size at any time, make them bigger or smaller at any time, and free the blocks individually at any time (or never).


3.2.3.1 Basic Memory Allocation

To allocate a block of memory, call malloc. The prototype for this function is in stdlib.h.

Function: void * malloc (size_t size)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

This function returns a pointer to a newly allocated block size bytes long, or a null pointer if the block could not be allocated.

The contents of the block are undefined; you must initialize it yourself (or use calloc instead; see Allocating Cleared Space). Normally you would cast the value as a pointer to the kind of object that you want to store in the block. Here we show an example of doing so, and of initializing the space with zeros using the library function memset (see Copying Strings and Arrays):

struct foo *ptr;
…
ptr = (struct foo *) malloc (sizeof (struct foo));
if (ptr == 0) abort ();
memset (ptr, 0, sizeof (struct foo));

You can store the result of malloc into any pointer variable without a cast, because ISO C automatically converts the type void * to another type of pointer when necessary. But the cast is necessary in contexts other than assignment operators or if you might want your code to run in traditional C.

Remember that when allocating space for a string, the argument to malloc must be one plus the length of the string. This is because a string is terminated with a null character that doesn’t count in the “length” of the string but does need space. For example:

char *ptr;
…
ptr = (char *) malloc (length + 1);

See Representation of Strings, for more information about this.


3.2.3.2 Examples of malloc

If no more space is available, malloc returns a null pointer. You should check the value of every call to malloc. It is useful to write a subroutine that calls malloc and reports an error if the value is a null pointer, returning only if the value is nonzero. This function is conventionally called xmalloc. Here it is:

void *
xmalloc (size_t size)
{
  void *value = malloc (size);
  if (value == 0)
    fatal ("virtual memory exhausted");
  return value;
}

Here is a real example of using malloc (by way of xmalloc). The function savestring will copy a sequence of characters into a newly allocated null-terminated string:

char *
savestring (const char *ptr, size_t len)
{
  char *value = (char *) xmalloc (len + 1);
  value[len] = '\0';
  return (char *) memcpy (value, ptr, len);
}

The block that malloc gives you is guaranteed to be aligned so that it can hold any type of data. On GNU systems, the address is always a multiple of eight on 32-bit systems, and a multiple of 16 on 64-bit systems. Only rarely is any higher boundary (such as a page boundary) necessary; for those cases, use aligned_alloc or posix_memalign (see Aligned Memory Blocks).

Note that the memory located after the end of the block is likely to be in use for something else; perhaps a block already allocated by another call to malloc. If you attempt to treat the block as longer than you asked for it to be, you are liable to destroy the data that malloc uses to keep track of its blocks, or you may destroy the contents of another block. If you have already allocated a block and discover you want it to be bigger, use realloc (see Changing Block Size).


3.2.3.3 Freeing Memory Allocated with malloc

When you no longer need a block that you got with malloc, use the function free to make the block available to be allocated again. The prototype for this function is in stdlib.h.

Function: void free (void *ptr)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

The free function deallocates the block of memory pointed at by ptr.

Function: void cfree (void *ptr)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

This function does the same thing as free. It’s provided for backward compatibility with SunOS; you should use free instead.

Freeing a block alters the contents of the block. Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it. Copy whatever you need out of the block before freeing it! Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to:

struct chain
  {
    struct chain *next;
    char *name;
  }
 
void
free_chain (struct chain *chain)
{
  while (chain != 0)
    {
      struct chain *next = chain-&gt;next;
      free (chain-&gt;name);
      free (chain);
      chain = next;
    }
}

Occasionally, free can actually return memory to the operating system and make the process smaller. Usually, all it can do is allow a later call to malloc to reuse the space. In the meantime, the space remains in your program as part of a free-list used internally by malloc.

There is no point in freeing blocks at the end of a program, because all of the program’s space is given back to the system when the process terminates.


3.2.3.4 Changing the Size of a Block

Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block. For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.

You can make the block longer by calling realloc. This function is declared in stdlib.h.

Function: void * realloc (void *ptr, size_t newsize)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

The realloc function changes the size of the block whose address is ptr to be newsize.

Since the space after the end of the block may be in use, realloc may find it necessary to copy the block to a new address where more free space is available. The value of realloc is the new address of the block. If the block needs to be moved, realloc copies the old contents.

If you pass a null pointer for ptr, realloc behaves just like ‘malloc (newsize)’. This can be convenient, but beware that older implementations (before ISO C) may not support this behavior, and will probably crash when realloc is passed a null pointer.

Like malloc, realloc may return a null pointer if no memory space is available to make the block bigger. When this happens, the original block is untouched; it has not been modified or relocated.

In most cases it makes no difference what happens to the original block when realloc fails, because the application program cannot continue when it is out of memory, and the only thing to do is to give a fatal error message. Often it is convenient to write and use a subroutine, conventionally called xrealloc, that takes care of the error message as xmalloc does for malloc:

void *
xrealloc (void *ptr, size_t size)
{
  void *value = realloc (ptr, size);
  if (value == 0)
    fatal ("Virtual memory exhausted");
  return value;
}

You can also use realloc to make a block smaller. The reason you would do this is to avoid tying up a lot of memory space when only a little is needed. In several allocation implementations, making a block smaller sometimes necessitates copying it, so it can fail if no other space is available.

If the new size you specify is the same as the old size, realloc is guaranteed to change nothing and return the same address that you gave.


3.2.3.5 Allocating Cleared Space

The function calloc allocates memory and clears it to zero. It is declared in stdlib.h.

Function: void * calloc (size_t count, size_t eltsize)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

This function allocates a block long enough to contain a vector of count elements, each of size eltsize. Its contents are cleared to zero before calloc returns.

You could define calloc as follows:

void *
calloc (size_t count, size_t eltsize)
{
  size_t size = count * eltsize;
  void *value = malloc (size);
  if (value != 0)
    memset (value, 0, size);
  return value;
}

But in general, it is not guaranteed that calloc calls malloc internally. Therefore, if an application provides its own malloc/realloc/free outside the C library, it should always define calloc, too.


3.2.3.6 Allocating Aligned Memory Blocks

The address of a block returned by malloc or realloc in GNU systems is always a multiple of eight (or sixteen on 64-bit systems). If you need a block whose address is a multiple of a higher power of two than that, use aligned_alloc or posix_memalign. aligned_alloc and posix_memalign are declared in stdlib.h.

Function: void * aligned_alloc (size_t alignment, size_t size)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

The aligned_alloc function allocates a block of size bytes whose address is a multiple of alignment. The alignment must be a power of two and size must be a multiple of alignment.

The aligned_alloc function returns a null pointer on error and sets errno to one of the following values:

ENOMEM

There was insufficient memory available to satisfy the request.

EINVAL

alignment is not a power of two.

This function was introduced in ISO C11 and hence may have better portability to modern non-POSIX systems than posix_memalign.

Function: void * memalign (size_t boundary, size_t size)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

The memalign function allocates a block of size bytes whose address is a multiple of boundary. The boundary must be a power of two! The function memalign works by allocating a somewhat larger block, and then returning an address within the block that is on the specified boundary.

The memalign function returns a null pointer on error and sets errno to one of the following values:

ENOMEM

There was insufficient memory available to satisfy the request.

EINVAL

boundary is not a power of two.

The memalign function is obsolete and aligned_alloc or posix_memalign should be used instead.

Function: int posix_memalign (void **memptr, size_t alignment, size_t size)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

The posix_memalign function is similar to the memalign function in that it returns a buffer of size bytes aligned to a multiple of alignment. But it adds one requirement to the parameter alignment: the value must be a power of two multiple of sizeof (void *).

If the function succeeds in allocation memory a pointer to the allocated memory is returned in *<var>memptr</var> and the return value is zero. Otherwise the function returns an error value indicating the problem. The possible error values returned are:

ENOMEM

There was insufficient memory available to satisfy the request.

EINVAL

alignment is not a power of two multiple of sizeof (void *).

This function was introduced in POSIX 1003.1d. Although this function is superseded by aligned_alloc, it is more portable to older POSIX systems that do not support ISO C11.

Function: void * valloc (size_t size)

Preliminary: | MT-Unsafe init | AS-Unsafe init lock | AC-Unsafe init lock fd mem | See POSIX Safety Concepts.

Using valloc is like using memalign and passing the page size as the value of the first argument. It is implemented like this:

void *
valloc (size_t size)
{
  return memalign (getpagesize (), size);
}

Query Memory Parameters for more information about the memory subsystem.

The valloc function is obsolete and aligned_alloc or posix_memalign should be used instead.


3.2.3.7 Malloc Tunable Parameters

You can adjust some parameters for dynamic memory allocation with the mallopt function. This function is the general SVID/XPG interface, defined in malloc.h.

Function: int mallopt (int param, int value)

Preliminary: | MT-Unsafe init const:mallopt | AS-Unsafe init lock | AC-Unsafe init lock | See POSIX Safety Concepts.

When calling mallopt, the param argument specifies the parameter to be set, and value the new value to be set. Possible choices for param, as defined in malloc.h, are:

M_MMAP_MAX

The maximum number of chunks to allocate with mmap. Setting this to zero disables all use of mmap.

The default value of this parameter is 65536.

This parameter can also be set for the process at startup by setting the environment variable MALLOC_MMAP_MAX_ to the desired value.

M_MMAP_THRESHOLD

All chunks larger than this value are allocated outside the normal heap, using the mmap system call. This way it is guaranteed that the memory for these chunks can be returned to the system on free. Note that requests smaller than this threshold might still be allocated via mmap.

If this parameter is not set, the default value is set as 128 KiB and the threshold is adjusted dynamically to suit the allocation patterns of the program. If the parameter is set, the dynamic adjustment is disabled and the value is set statically to the input value.

This parameter can also be set for the process at startup by setting the environment variable MALLOC_MMAP_THRESHOLD_ to the desired value.

M_PERTURB

If non-zero, memory blocks are filled with values depending on some low order bits of this parameter when they are allocated (except when allocated by calloc) and freed. This can be used to debug the use of uninitialized or freed heap memory. Note that this option does not guarantee that the freed block will have any specific values. It only guarantees that the content the block had before it was freed will be overwritten.

The default value of this parameter is  .

This parameter can also be set for the process at startup by setting the environment variable MALLOC_MMAP_PERTURB_ to the desired value.

M_TOP_PAD

This parameter determines the amount of extra memory to obtain from the system when an arena needs to be extended. It also specifies the number of bytes to retain when shrinking an arena. This provides the necessary hysteresis in heap size such that excessive amounts of system calls can be avoided.

The default value of this parameter is  .

This parameter can also be set for the process at startup by setting the environment variable MALLOC_TOP_PAD_ to the desired value.

M_TRIM_THRESHOLD

This is the minimum size (in bytes) of the top-most, releasable chunk that will trigger a system call in order to return memory to the system.

If this parameter is not set, the default value is set as 128 KiB and the threshold is adjusted dynamically to suit the allocation patterns of the program. If the parameter is set, the dynamic adjustment is disabled and the value is set statically to the provided input.

This parameter can also be set for the process at startup by setting the environment variable MALLOC_TRIM_THRESHOLD_ to the desired value.

M_ARENA_TEST

This parameter specifies the number of arenas that can be created before the test on the limit to the number of arenas is conducted. The value is ignored if M_ARENA_MAX is set.

The default value of this parameter is 2 on 32-bit systems and 8 on 64-bit systems.

This parameter can also be set for the process at startup by setting the environment variable MALLOC_ARENA_TEST to the desired value.

M_ARENA_MAX

This parameter sets the number of arenas to use regardless of the number of cores in the system.

The default value of this tunable is  , meaning that the limit on the number of arenas is determined by the number of CPU cores online. For 32-bit systems the limit is twice the number of cores online and on 64-bit systems, it is eight times the number of cores online. Note that the default value is not derived from the default value of M_ARENA_TEST and is computed independently.

This parameter can also be set for the process at startup by setting the environment variable MALLOC_ARENA_MAX to the desired value.


3.2.3.8 Heap Consistency Checking

You can ask malloc to check the consistency of dynamic memory by using the mcheck function. This function is a GNU extension, declared in mcheck.h.

Function: int mcheck (void (*abortfn) (enum mcheck_status status))

Preliminary: | MT-Unsafe race:mcheck const:malloc_hooks | AS-Unsafe corrupt | AC-Unsafe corrupt | See POSIX Safety Concepts.

Calling mcheck tells malloc to perform occasional consistency checks. These will catch things such as writing past the end of a block that was allocated with malloc.

The abortfn argument is the function to call when an inconsistency is found. If you supply a null pointer, then mcheck uses a default function which prints a message and calls abort (see Aborting a Program). The function you supply is called with one argument, which says what sort of inconsistency was detected; its type is described below.

It is too late to begin allocation checking once you have allocated anything with malloc. So mcheck does nothing in that case. The function returns -1 if you call it too late, and   otherwise (when it is successful).

The easiest way to arrange to call mcheck early enough is to use the option ‘-lmcheck’ when you link your program; then you don’t need to modify your program source at all. Alternatively you might use a debugger to insert a call to mcheck whenever the program is started, for example these gdb commands will automatically call mcheck whenever the program starts:

(gdb) break main
Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10
(gdb) command 1
Type commands for when breakpoint 1 is hit, one per line.
End with a line saying just "end".
&gt;call mcheck(0)
&gt;continue
&gt;end
(gdb) …

This will however only work if no initialization function of any object involved calls any of the malloc functions since mcheck must be called before the first such function.

Function: enum mcheck_status mprobe (void *pointer)

Preliminary: | MT-Unsafe race:mcheck const:malloc_hooks | AS-Unsafe corrupt | AC-Unsafe corrupt | See POSIX Safety Concepts.

The mprobe function lets you explicitly check for inconsistencies in a particular allocated block. You must have already called mcheck at the beginning of the program, to do its occasional checks; calling mprobe requests an additional consistency check to be done at the time of the call.

The argument pointer must be a pointer returned by malloc or realloc. mprobe returns a value that says what inconsistency, if any, was found. The values are described below.

Data Type: enum mcheck_status

This enumerated type describes what kind of inconsistency was detected in an allocated block, if any. Here are the possible values:

MCHECK_DISABLED

mcheck was not called before the first allocation. No consistency checking can be done.

MCHECK_OK

No inconsistency detected.

MCHECK_HEAD

The data immediately before the block was modified. This commonly happens when an array index or pointer is decremented too far.

MCHECK_TAIL

The data immediately after the block was modified. This commonly happens when an array index or pointer is incremented too far.

MCHECK_FREE

The block was already freed.

Another possibility to check for and guard against bugs in the use of malloc, realloc and free is to set the environment variable MALLOC_CHECK_. When MALLOC_CHECK_ is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of free with the same argument, or overruns of a single byte (off-by-one bugs). Not all such errors can be protected against, however, and memory leaks can result. If MALLOC_CHECK_ is set to  , any detected heap corruption is silently ignored; if set to 1, a diagnostic is printed on stderr; if set to 2, abort is called immediately. This can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down.

There is one problem with MALLOC_CHECK_: in SUID or SGID binaries it could possibly be exploited since diverging from the normal programs behavior it now writes something to the standard error descriptor. Therefore the use of MALLOC_CHECK_ is disabled by default for SUID and SGID binaries. It can be enabled again by the system administrator by adding a file /etc/suid-debug (the content is not important it could be empty).

So, what’s the difference between using MALLOC_CHECK_ and linking with ‘-lmcheck’? MALLOC_CHECK_ is orthogonal with respect to ‘-lmcheck’. ‘-lmcheck’ has been added for backward compatibility. Both MALLOC_CHECK_ and ‘-lmcheck’ should uncover the same bugs - but using MALLOC_CHECK_ you don’t need to recompile your application.


3.2.3.9 Memory Allocation Hooks

The GNU C Library lets you modify the behavior of malloc, realloc, and free by specifying appropriate hook functions. You can use these hooks to help you debug programs that use dynamic memory allocation, for example.

The hook variables are declared in malloc.h.

Variable: __malloc_hook

The value of this variable is a pointer to the function that malloc uses whenever it is called. You should define this function to look like malloc; that is, like:

void *<var>function</var> (size_t <var>size</var>, const void *<var>caller</var>)

The value of caller is the return address found on the stack when the malloc function was called. This value allows you to trace the memory consumption of the program.

Variable: __realloc_hook

The value of this variable is a pointer to function that realloc uses whenever it is called. You should define this function to look like realloc; that is, like:

void *<var>function</var> (void *<var>ptr</var>, size_t <var>size</var>, const void *<var>caller</var>)

The value of caller is the return address found on the stack when the realloc function was called. This value allows you to trace the memory consumption of the program.

Variable: __free_hook

The value of this variable is a pointer to function that free uses whenever it is called. You should define this function to look like free; that is, like:

void <var>function</var> (void *<var>ptr</var>, const void *<var>caller</var>)

The value of caller is the return address found on the stack when the free function was called. This value allows you to trace the memory consumption of the program.

Variable: __memalign_hook

The value of this variable is a pointer to function that aligned_alloc, memalign, posix_memalign and valloc use whenever they are called. You should define this function to look like aligned_alloc; that is, like:

void *<var>function</var> (size_t <var>alignment</var>, size_t <var>size</var>, const void *<var>caller</var>)

The value of caller is the return address found on the stack when the aligned_alloc, memalign, posix_memalign or valloc functions are called. This value allows you to trace the memory consumption of the program.

You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first! Otherwise, your program will get stuck in an infinite recursion. Before calling the function recursively, one should make sure to restore all the hooks to their previous value. When coming back from the recursive call, all the hooks should be resaved since a hook might modify itself.

An issue to look out for is the time at which the malloc hook functions can be safely installed. If the hook functions call the malloc-related functions recursively, it is necessary that malloc has already properly initialized itself at the time when __malloc_hook etc. is assigned to. On the other hand, if the hook functions provide a complete malloc implementation of their own, it is vital that the hooks are assigned to before the very first malloc call has completed, because otherwise a chunk obtained from the ordinary, un-hooked malloc may later be handed to __free_hook, for example.

Here is an example showing how to use __malloc_hook and __free_hook properly. It installs a function that prints out information every time malloc or free is called. We just assume here that realloc and memalign are not used in our program.

/* Prototypes for __malloc_hook, __free_hook */
#include &lt;malloc.h&gt;
 
/* Prototypes for our hooks.  */
static void my_init_hook (void);
static void *my_malloc_hook (size_t, const void *);
static void my_free_hook (void*, const void *);
 
static void
my_init (void)
{
  old_malloc_hook = __malloc_hook;
  old_free_hook = __free_hook;
  __malloc_hook = my_malloc_hook;
  __free_hook = my_free_hook;
}
 
static void *
my_malloc_hook (size_t size, const void *caller)
{
  void *result;
  /* Restore all old hooks */
  __malloc_hook = old_malloc_hook;
  __free_hook = old_free_hook;
  /* Call recursively */
  result = malloc (size);
  /* Save underlying hooks */
  old_malloc_hook = __malloc_hook;
  old_free_hook = __free_hook;
  /* <span class="roman"><code>printf</code> might call <code>malloc</code>, so protect it too.</span> */
  printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
  /* Restore our own hooks */
  __malloc_hook = my_malloc_hook;
  __free_hook = my_free_hook;
  return result;
}
 
static void
my_free_hook (void *ptr, const void *caller)
{
  /* Restore all old hooks */
  __malloc_hook = old_malloc_hook;
  __free_hook = old_free_hook;
  /* Call recursively */
  free (ptr);
  /* Save underlying hooks */
  old_malloc_hook = __malloc_hook;
  old_free_hook = __free_hook;
  /* <span class="roman"><code>printf</code> might call <code>free</code>, so protect it too.</span> */
  printf ("freed pointer %p\n", ptr);
  /* Restore our own hooks */
  __malloc_hook = my_malloc_hook;
  __free_hook = my_free_hook;
}
 
main ()
{
  my_init ();
  …
}

The mcheck function (see Heap Consistency Checking) works by installing such hooks.


3.2.3.10 Statistics for Memory Allocation with malloc

You can get information about dynamic memory allocation by calling the mallinfo function. This function and its associated data type are declared in malloc.h; they are an extension of the standard SVID/XPG version.

Data Type: struct mallinfo

This structure type is used to return information about the dynamic memory allocator. It contains the following members:

int arena

This is the total size of memory allocated with sbrk by malloc, in bytes.

int ordblks

This is the number of chunks not in use. (The memory allocator internally gets chunks of memory from the operating system, and then carves them up to satisfy individual malloc requests; see The GNU Allocator.)

int smblks

This field is unused.

int hblks

This is the total number of chunks allocated with mmap.

int hblkhd

This is the total size of memory allocated with mmap, in bytes.

int usmblks

This field is unused and always 0.

int fsmblks

This field is unused.

int uordblks

This is the total size of memory occupied by chunks handed out by malloc.

int fordblks

This is the total size of memory occupied by free (not in use) chunks.

int keepcost

This is the size of the top-most releasable chunk that normally borders the end of the heap (i.e., the high end of the virtual address space’s data segment).

Function: struct mallinfo mallinfo (void)

Preliminary: | MT-Unsafe init const:mallopt | AS-Unsafe init lock | AC-Unsafe init lock | See POSIX Safety Concepts.

This function returns information about the current dynamic memory usage in a structure of type struct mallinfo.


3.2.3.11 Summary of malloc-Related Functions

Here is a summary of the functions that work with malloc:

void *malloc (size_t <var>size</var>)

Allocate a block of size bytes. See Basic Allocation.

void free (void *<var>addr</var>)

Free a block previously allocated by malloc. See Freeing after Malloc.

void *realloc (void *<var>addr</var>, size_t <var>size</var>)

Make a block previously allocated by malloc larger or smaller, possibly by copying it to a new location. See Changing Block Size.

void *calloc (size_t <var>count</var>, size_t <var>eltsize</var>)

Allocate a block of count * eltsize bytes using malloc, and set its contents to zero. See Allocating Cleared Space.

void *valloc (size_t <var>size</var>)

Allocate a block of size bytes, starting on a page boundary. See Aligned Memory Blocks.

void *aligned_alloc (size_t <var>size</var>, size_t <var>alignment</var>)

Allocate a block of size bytes, starting on an address that is a multiple of alignment. See Aligned Memory Blocks.

int posix_memalign (void **<var>memptr</var>, size_t <var>alignment</var>, size_t <var>size</var>)

Allocate a block of size bytes, starting on an address that is a multiple of alignment. See Aligned Memory Blocks.

void *memalign (size_t <var>size</var>, size_t <var>boundary</var>)

Allocate a block of size bytes, starting on an address that is a multiple of boundary. See Aligned Memory Blocks.

int mallopt (int <var>param</var>, int <var>value</var>)

Adjust a tunable parameter. See Malloc Tunable Parameters.

int mcheck (void (*<var>abortfn</var>) (void))

Tell malloc to perform occasional consistency checks on dynamically allocated memory, and to call abortfn when an inconsistency is found. See Heap Consistency Checking.

void *(*__malloc_hook) (size_t <var>size</var>, const void *<var>caller</var>)

A pointer to a function that malloc uses whenever it is called.

void *(*__realloc_hook) (void *<var>ptr</var>, size_t <var>size</var>, const void *<var>caller</var>)

A pointer to a function that realloc uses whenever it is called.

void (*__free_hook) (void *<var>ptr</var>, const void *<var>caller</var>)

A pointer to a function that free uses whenever it is called.

void (*__memalign_hook) (size_t <var>size</var>, size_t <var>alignment</var>, const void *<var>caller</var>)

A pointer to a function that aligned_alloc, memalign, posix_memalign and valloc use whenever they are called.

struct mallinfo mallinfo (void)

Return information about the current dynamic memory usage. See Statistics of Malloc.


3.2.4 Allocation Debugging

A complicated task when programming with languages which do not use garbage collected dynamic memory allocation is to find memory leaks. Long running programs must ensure that dynamically allocated objects are freed at the end of their lifetime. If this does not happen the system runs out of memory, sooner or later.

The malloc implementation in the GNU C Library provides some simple means to detect such leaks and obtain some information to find the location. To do this the application must be started in a special mode which is enabled by an environment variable. There are no speed penalties for the program if the debugging mode is not enabled.


3.2.4.1 How to install the tracing functionality

Function: void mtrace (void)

Preliminary: | MT-Unsafe env race:mtrace const:malloc_hooks init | AS-Unsafe init heap corrupt lock | AC-Unsafe init corrupt lock fd mem | See POSIX Safety Concepts.

When the mtrace function is called it looks for an environment variable named MALLOC_TRACE. This variable is supposed to contain a valid file name. The user must have write access. If the file already exists it is truncated. If the environment variable is not set or it does not name a valid file which can be opened for writing nothing is done. The behavior of malloc etc. is not changed. For obvious reasons this also happens if the application is installed with the SUID or SGID bit set.

If the named file is successfully opened, mtrace installs special handlers for the functions malloc, realloc, and free (see Hooks for Malloc). From then on, all uses of these functions are traced and protocolled into the file. There is now of course a speed penalty for all calls to the traced functions so tracing should not be enabled during normal use.

This function is a GNU extension and generally not available on other systems. The prototype can be found in mcheck.h.

Function: void muntrace (void)

Preliminary: | MT-Unsafe race:mtrace const:malloc_hooks locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt mem lock fd | See POSIX Safety Concepts.

The muntrace function can be called after mtrace was used to enable tracing the malloc calls. If no (successful) call of mtrace was made muntrace does nothing.

Otherwise it deinstalls the handlers for malloc, realloc, and free and then closes the protocol file. No calls are protocolled anymore and the program runs again at full speed.

This function is a GNU extension and generally not available on other systems. The prototype can be found in mcheck.h.


3.2.4.2 Example program excerpts

Even though the tracing functionality does not influence the runtime behavior of the program it is not a good idea to call mtrace in all programs. Just imagine that you debug a program using mtrace and all other programs used in the debugging session also trace their malloc calls. The output file would be the same for all programs and thus is unusable. Therefore one should call mtrace only if compiled for debugging. A program could therefore start like this:

#include &lt;mcheck.h&gt;
 
int
main (int argc, char *argv[])
{
#ifdef DEBUGGING
  mtrace ();
#endif
  …
}

This is all that is needed if you want to trace the calls during the whole runtime of the program. Alternatively you can stop the tracing at any time with a call to muntrace. It is even possible to restart the tracing again with a new call to mtrace. But this can cause unreliable results since there may be calls of the functions which are not called. Please note that not only the application uses the traced functions, also libraries (including the C library itself) use these functions.

This last point is also why it is not a good idea to call muntrace before the program terminates. The libraries are informed about the termination of the program only after the program returns from main or calls exit and so cannot free the memory they use before this time.

So the best thing one can do is to call mtrace as the very first function in the program and never call muntrace. So the program traces almost all uses of the malloc functions (except those calls which are executed by constructors of the program or used libraries).


3.2.4.3 Some more or less clever ideas

You know the situation. The program is prepared for debugging and in all debugging sessions it runs well. But once it is started without debugging the error shows up. A typical example is a memory leak that becomes visible only when we turn off the debugging. If you foresee such situations you can still win. Simply use something equivalent to the following little program:

#include &lt;mcheck.h&gt;
#include &lt;signal.h&gt;
 
static void
enable (int sig)
{
  mtrace ();
  signal (SIGUSR1, enable);
}
 
static void
disable (int sig)
{
  muntrace ();
  signal (SIGUSR2, disable);
}
 
int
main (int argc, char *argv[])
{
  …
 
  signal (SIGUSR1, enable);
  signal (SIGUSR2, disable);
 
  …
}

I.e., the user can start the memory debugger any time s/he wants if the program was started with MALLOC_TRACE set in the environment. The output will of course not show the allocations which happened before the first signal but if there is a memory leak this will show up nevertheless.


3.2.4.4 Interpreting the traces

If you take a look at the output it will look similar to this:

= Start
&nbsp;[0x8048209] - 0x8064cc8
&nbsp;[0x8048209] - 0x8064ce0
&nbsp;[0x8048209] - 0x8064cf8
&nbsp;[0x80481eb] + 0x8064c48 0x14
&nbsp;[0x80481eb] + 0x8064c60 0x14
&nbsp;[0x80481eb] + 0x8064c78 0x14
&nbsp;[0x80481eb] + 0x8064c90 0x14
= End

What this all means is not really important since the trace file is not meant to be read by a human. Therefore no attention is given to readability. Instead there is a program which comes with the GNU C Library which interprets the traces and outputs a summary in an user-friendly way. The program is called mtrace (it is in fact a Perl script) and it takes one or two arguments. In any case the name of the file with the trace output must be specified. If an optional argument precedes the name of the trace file this must be the name of the program which generated the trace.

drepper$ mtrace tst-mtrace log
No memory leaks.

In this case the program tst-mtrace was run and it produced a trace file log. The message printed by mtrace shows there are no problems with the code, all allocated memory was freed afterwards.

If we call mtrace on the example trace given above we would get a different outout:

drepper$ mtrace errlog
- 0x08064cc8 Free 2 was never alloc'd 0x8048209
- 0x08064ce0 Free 3 was never alloc'd 0x8048209
- 0x08064cf8 Free 4 was never alloc'd 0x8048209
 
Memory not freed:
-----------------
   Address     Size     Caller
0x08064c48     0x14  at 0x80481eb
0x08064c60     0x14  at 0x80481eb
0x08064c78     0x14  at 0x80481eb
0x08064c90     0x14  at 0x80481eb

We have called mtrace with only one argument and so the script has no chance to find out what is meant with the addresses given in the trace. We can do better:

drepper$ mtrace tst errlog
- 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39
- 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39
- 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39
 
Memory not freed:
-----------------
   Address     Size     Caller
0x08064c48     0x14  at /home/drepper/tst.c:33
0x08064c60     0x14  at /home/drepper/tst.c:33
0x08064c78     0x14  at /home/drepper/tst.c:33
0x08064c90     0x14  at /home/drepper/tst.c:33

Suddenly the output makes much more sense and the user can see immediately where the function calls causing the trouble can be found.

Interpreting this output is not complicated. There are at most two different situations being detected. First, free was called for pointers which were never returned by one of the allocation functions. This is usually a very bad problem and what this looks like is shown in the first three lines of the output. Situations like this are quite rare and if they appear they show up very drastically: the program normally crashes.

The other situation which is much harder to detect are memory leaks. As you can see in the output the mtrace function collects all this information and so can say that the program calls an allocation function from line 33 in the source file /home/drepper/tst-mtrace.c four times without freeing this memory before the program terminates. Whether this is a real problem remains to be investigated.


3.2.5 Obstacks

An obstack is a pool of memory containing a stack of objects. You can create any number of separate obstacks, and then allocate objects in specified obstacks. Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other.

Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size. They are implemented with macros, so allocation is usually very fast as long as the objects are usually small. And the only space overhead per object is the padding needed to start each object on a suitable boundary.


3.2.5.1 Creating Obstacks

The utilities for manipulating obstacks are declared in the header file obstack.h.

Data Type: struct obstack

An obstack is represented by a data structure of type

struct
obstack
. This structure has a small fixed size; it records the status of the obstack and how to find the space in which objects are allocated. It does not contain any of the objects themselves. You should not try to access the contents of the structure directly; use only the functions described in this chapter.

You can declare variables of type struct obstack and use them as obstacks, or you can allocate obstacks dynamically like any other kind of object. Dynamic allocation of obstacks allows your program to have a variable number of different stacks. (You can even allocate an obstack structure in another obstack, but this is rarely useful.)

All the functions that work with obstacks require you to specify which obstack to use. You do this with a pointer of type

struct obstack
*
. In the following, we often say “an obstack” when strictly speaking the object at hand is such a pointer.

The objects in the obstack are packed into large blocks called chunks. The struct obstack structure points to a chain of the chunks currently in use.

The obstack library obtains a new chunk whenever you allocate an object that won’t fit in the previous chunk. Since the obstack library manages chunks automatically, you don’t need to pay much attention to them, but you do need to supply a function which the obstack library should use to get a chunk. Usually you supply a function which uses malloc directly or indirectly. You must also supply a function to free a chunk. These matters are described in the following section.


3.2.5.2 Preparing for Using Obstacks

Each source file in which you plan to use the obstack functions must include the header file obstack.h, like this:

#include &lt;obstack.h&gt;

Also, if the source file uses the macro obstack_init, it must declare or define two functions or macros that will be called by the obstack library. One, obstack_chunk_alloc, is used to allocate the chunks of memory into which objects are packed. The other, obstack_chunk_free, is used to return chunks when the objects in them are freed. These macros should appear before any use of obstacks in the source file.

Usually these are defined to use malloc via the intermediary xmalloc (see Unconstrained Allocation). This is done with the following pair of macro definitions:

#define obstack_chunk_alloc xmalloc
#define obstack_chunk_free free

Though the memory you get using obstacks really comes from malloc, using obstacks is faster because malloc is called less often, for larger blocks of memory. See Obstack Chunks, for full details.

At run time, before the program can use a struct obstack object as an obstack, it must initialize the obstack by calling obstack_init.

Function: int obstack_init (struct obstack *obstack-ptr)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Safe mem | See POSIX Safety Concepts.

Initialize obstack obstack-ptr for allocation of objects. This function calls the obstack’s obstack_chunk_alloc function. If allocation of memory fails, the function pointed to by obstack_alloc_failed_handler is called. The obstack_init function always returns 1 (Compatibility notice: Former versions of obstack returned 0 if allocation failed).

Here are two examples of how to allocate the space for an obstack and initialize it. First, an obstack that is a static variable:

static struct obstack myobstack;
…
obstack_init (&amp;myobstack);

Second, an obstack that is itself dynamically allocated:

struct obstack *myobstack_ptr
  = (struct obstack *) xmalloc (sizeof (struct obstack));
 
obstack_init (myobstack_ptr);
Variable: obstack_alloc_failed_handler

The value of this variable is a pointer to a function that obstack uses when obstack_chunk_alloc fails to allocate memory. The default action is to print a message and abort. You should supply a function that either calls exit (see Program Termination) or longjmp (see Non-Local Exits) and doesn’t return.

void my_obstack_alloc_failed (void)
…
obstack_alloc_failed_handler = &amp;my_obstack_alloc_failed;

3.2.5.3 Allocation in an Obstack

The most direct way to allocate an object in an obstack is with obstack_alloc, which is invoked almost like malloc.

Function: void * obstack_alloc (struct obstack *obstack-ptr, int size)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

This allocates an uninitialized block of size bytes in an obstack and returns its address. Here obstack-ptr specifies which obstack to allocate the block in; it is the address of the struct obstack object which represents the obstack. Each obstack function or macro requires you to specify an obstack-ptr as the first argument.

This function calls the obstack’s obstack_chunk_alloc function if it needs to allocate a new chunk of memory; it calls obstack_alloc_failed_handler if allocation of memory by obstack_chunk_alloc failed.

For example, here is a function that allocates a copy of a string str in a specific obstack, which is in the variable string_obstack:

struct obstack string_obstack;
 
char *
copystring (char *string)
{
  size_t len = strlen (string) + 1;
  char *s = (char *) obstack_alloc (&amp;string_obstack, len);
  memcpy (s, string, len);
  return s;
}

To allocate a block with specified contents, use the function obstack_copy, declared like this:

Function: void * obstack_copy (struct obstack *obstack-ptr, void *address, int size)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

This allocates a block and initializes it by copying size bytes of data starting at address. It calls obstack_alloc_failed_handler if allocation of memory by obstack_chunk_alloc failed.

Function: void * obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

Like obstack_copy, but appends an extra byte containing a null character. This extra byte is not counted in the argument size.

The obstack_copy0 function is convenient for copying a sequence of characters into an obstack as a null-terminated string. Here is an example of its use:

char *
obstack_savestring (char *addr, int size)
{
  return obstack_copy0 (&amp;myobstack, addr, size);
}

Contrast this with the previous example of savestring using malloc (see Basic Allocation).


3.2.5.4 Freeing Objects in an Obstack

To free an object allocated in an obstack, use the function obstack_free. Since the obstack is a stack of objects, freeing one object automatically frees all other objects allocated more recently in the same obstack.

Function: void obstack_free (struct obstack *obstack-ptr, void *object)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt | See POSIX Safety Concepts.

If object is a null pointer, everything allocated in the obstack is freed. Otherwise, object must be the address of an object allocated in the obstack. Then object is freed, along with everything allocated in obstack-ptr since object.

Note that if object is a null pointer, the result is an uninitialized obstack. To free all memory in an obstack but leave it valid for further allocation, call obstack_free with the address of the first object allocated on the obstack:

obstack_free (obstack_ptr, first_object_allocated_ptr);

Recall that the objects in an obstack are grouped into chunks. When all the objects in a chunk become free, the obstack library automatically frees the chunk (see Preparing for Obstacks). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.


3.2.5.5 Obstack Functions and Macros

The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler. The obstack facility works with all C compilers, including both ISO C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C.

If you are using an old-fashioned non-ISO C compiler, all the obstack “functions” are actually defined only as macros. You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address).

Calling the macros requires a special precaution: namely, the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once. For example, if you write this:

obstack_alloc (get_obstack (), 4);

you will find that get_obstack may be called several times. If you use *obstack_list_ptr++ as the obstack pointer argument, you will get very strange results since the incrementation may occur several times.

In ISO C, each function has both a macro definition and a function definition. The function definition is used if you take the address of the function without calling it. An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here:

char *x;
void *(*funcp) ();
/* <span class="roman">Use the macro</span>.  */
x = (char *) obstack_alloc (obptr, size);
/* <span class="roman">Call the function</span>.  */
x = (char *) (obstack_alloc) (obptr, size);
/* <span class="roman">Take the address of the function</span>.  */
funcp = obstack_alloc;

This is the same situation that exists in ISO C for the standard library functions. See Macro Definitions.

Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ISO C.

If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once.


3.2.5.6 Growing Objects

Because memory in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object. With this technique, you do not need to know how much data you will put in the object until you come to the end of it. We call this the technique of growing objects. The special functions for adding data to the growing object are described in this section.

You don’t need to do anything special when you start to grow an object. Using one of the functions to add data to the object automatically starts it. However, it is necessary to say explicitly when the object is finished. This is done with the function obstack_finish.

The actual address of the object thus built up is not known until the object is finished. Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk.

While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object. If you try to do so, the space already added to the growing object will become part of the other object.

Function: void obstack_blank (struct obstack *obstack-ptr, int size)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

The most basic function for adding to a growing object is obstack_blank, which adds space without initializing it.

Function: void obstack_grow (struct obstack *obstack-ptr, void *data, int size)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

To add a block of initialized space, use obstack_grow, which is the growing-object analogue of obstack_copy. It adds size bytes of data to the growing object, copying the contents from data.

Function: void obstack_grow0 (struct obstack *obstack-ptr, void *data, int size)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

This is the growing-object analogue of obstack_copy0. It adds size bytes copied from data, followed by an additional null character.

Function: void obstack_1grow (struct obstack *obstack-ptr, char c)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

To add one character at a time, use the function obstack_1grow. It adds a single byte containing c to the growing object.

Function: void obstack_ptr_grow (struct obstack *obstack-ptr, void *data)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

Adding the value of a pointer one can use the function obstack_ptr_grow. It adds sizeof (void *) bytes containing the value of data.

Function: void obstack_int_grow (struct obstack *obstack-ptr, int data)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

A single value of type int can be added by using the obstack_int_grow function. It adds sizeof (int) bytes to the growing object and initializes them with the value of data.

Function: void * obstack_finish (struct obstack *obstack-ptr)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt | See POSIX Safety Concepts.

When you are finished growing the object, use the function obstack_finish to close it off and return its final address.

Once you have finished the object, the obstack is available for ordinary allocation or for growing another object.

This function can return a null pointer under the same conditions as obstack_alloc (see Allocation in an Obstack).

When you build an object by growing it, you will probably need to know afterward how long it became. You need not keep track of this as you grow the object, because you can find out the length from the obstack just before finishing the object with the function obstack_object_size, declared as follows:

Function: int obstack_object_size (struct obstack *obstack-ptr)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function returns the current size of the growing object, in bytes. Remember to call this function before finishing the object. After it is finished, obstack_object_size will return zero.

If you have started growing an object and wish to cancel it, you should finish it and then free it, like this:

obstack_free (obstack_ptr, obstack_finish (obstack_ptr));

This has no effect if no object was growing.

You can use obstack_blank with a negative size argument to make the current object smaller. Just don’t try to shrink it beyond zero length—there’s no telling what will happen if you do that.


3.2.5.7 Extra Fast Growing Objects

The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk. If you are frequently constructing objects in small steps of growth, this overhead can be significant.

You can reduce the overhead by using special “fast growth” functions that grow the object without checking. In order to have a robust program, you must do the checking yourself. If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do. But if you can arrange to check less often, or check more efficiently, then you make the program faster.

The function obstack_room returns the amount of room available in the current chunk. It is declared as follows:

Function: int obstack_room (struct obstack *obstack-ptr)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This returns the number of bytes that can be added safely to the current growing object (or to an object about to be started) in obstack obstack-ptr using the fast growth functions.

While you know there is room, you can use these fast growth functions for adding data to a growing object:

Function: void obstack_1grow_fast (struct obstack *obstack-ptr, char c)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

The function obstack_1grow_fast adds one byte containing the character c to the growing object in obstack obstack-ptr.

Function: void obstack_ptr_grow_fast (struct obstack *obstack-ptr, void *data)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function obstack_ptr_grow_fast adds sizeof (void *) bytes containing the value of data to the growing object in obstack obstack-ptr.

Function: void obstack_int_grow_fast (struct obstack *obstack-ptr, int data)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function obstack_int_grow_fast adds sizeof (int) bytes containing the value of data to the growing object in obstack obstack-ptr.

Function: void obstack_blank_fast (struct obstack *obstack-ptr, int size)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function obstack_blank_fast adds size bytes to the growing object in obstack obstack-ptr without initializing them.

When you check for space using obstack_room and there is not enough room for what you want to add, the fast growth functions are not safe. In this case, simply use the corresponding ordinary growth function instead. Very soon this will copy the object to a new chunk; then there will be lots of room available again.

So, each time you use an ordinary growth function, check afterward for sufficient space using obstack_room. Once the object is copied to a new chunk, there will be plenty of space again, so the program will start using the fast growth functions again.

Here is an example:

void
add_string (struct obstack *obstack, const char *ptr, int len)
{
  while (len &gt; 0)
    {
      int room = obstack_room (obstack);
      if (room == 0)
        {
          /* <span class="roman">Not enough room.  Add one character slowly,</span>
             <span class="roman">which may copy to a new chunk and make room.</span>  */
          obstack_1grow (obstack, *ptr++);
          len--;
        }
      else
        {
          if (room &gt; len)
            room = len;
          /* <span class="roman">Add fast as much as we have room for.</span> */
          len -= room;
          while (room-- &gt; 0)
            obstack_1grow_fast (obstack, *ptr++);
        }
    }
}

3.2.5.8 Status of an Obstack

Here are functions that provide information on the current status of allocation in an obstack. You can use them to learn about an object while still growing it.

Function: void * obstack_base (struct obstack *obstack-ptr)

Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Safe | See POSIX Safety Concepts.

This function returns the tentative address of the beginning of the currently growing object in obstack-ptr. If you finish the object immediately, it will have that address. If you make it larger first, it may outgrow the current chunk—then its address will change!

If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk).

Function: void * obstack_next_free (struct obstack *obstack-ptr)

Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Safe | See POSIX Safety Concepts.

This function returns the address of the first free byte in the current chunk of obstack obstack-ptr. This is the end of the currently growing object. If no object is growing, obstack_next_free returns the same value as obstack_base.

Function: int obstack_object_size (struct obstack *obstack-ptr)

Preliminary: | MT-Safe race:obstack-ptr | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function returns the size in bytes of the currently growing object. This is equivalent to

obstack_next_free (<var>obstack-ptr</var>) - obstack_base (<var>obstack-ptr</var>)

3.2.5.9 Alignment of Data in Obstacks

Each obstack has an alignment boundary; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary. By default, this boundary is aligned so that the object can hold any type of data.

To access an obstack’s alignment boundary, use the macro obstack_alignment_mask, whose function prototype looks like this:

Macro: int obstack_alignment_mask (struct obstack *obstack-ptr)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The value is a bit mask; a bit that is 1 indicates that the corresponding bit in the address of an object should be 0. The mask value should be one less than a power of 2; the effect is that all object addresses are multiples of that power of 2. The default value of the mask is a value that allows aligned objects to hold any type of data: for example, if its value is 3, any type of data can be stored at locations whose addresses are multiples of 4. A mask value of 0 means an object can start on any multiple of 1 (that is, no alignment is required).

The expansion of the macro obstack_alignment_mask is an lvalue, so you can alter the mask by assignment. For example, this statement:

obstack_alignment_mask (obstack_ptr) = 0;

has the effect of turning off alignment processing in the specified obstack.

Note that a change in alignment mask does not take effect until after the next time an object is allocated or finished in the obstack. If you are not growing an object, you can make the new alignment mask take effect immediately by calling obstack_finish. This will finish a zero-length object and then do proper alignment for the next object.


3.2.5.10 Obstack Chunks

Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests. Chunks are normally 4096 bytes long unless you specify a different chunk size. The chunk size includes 8 bytes of overhead that are not actually used for storing objects. Regardless of the specified size, longer chunks will be allocated when necessary for long objects.

The obstack library allocates chunks by calling the function obstack_chunk_alloc, which you must define. When a chunk is no longer needed because you have freed all the objects in it, the obstack library frees the chunk by calling obstack_chunk_free, which you must also define.

These two must be defined (as macros) or declared (as functions) in each source file that uses obstack_init (see Creating Obstacks). Most often they are defined as macros like this:

#define obstack_chunk_alloc malloc
#define obstack_chunk_free free

Note that these are simple macros (no arguments). Macro definitions with arguments will not work! It is necessary that obstack_chunk_alloc or obstack_chunk_free, alone, expand into a function name if it is not itself a function name.

If you allocate chunks with malloc, the chunk size should be a power of 2. The default chunk size, 4096, was chosen because it is long enough to satisfy many typical requests on the obstack yet short enough not to waste too much memory in the portion of the last chunk not yet used.

Macro: int obstack_chunk_size (struct obstack *obstack-ptr)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This returns the chunk size of the given obstack.

Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value. Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future. It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size. Here is how to do so cleanly:

if (obstack_chunk_size (obstack_ptr) &lt; <var>new-chunk-size</var>)
  obstack_chunk_size (obstack_ptr) = <var>new-chunk-size</var>;

Previous: , Up: Obstacks   [Contents][Index]

3.2.5.11 Summary of Obstack Functions

Here is a summary of all the functions associated with obstacks. Each takes the address of an obstack (struct obstack *) as its first argument.

void obstack_init (struct obstack *<var>obstack-ptr</var>)

Initialize use of an obstack. See Creating Obstacks.

void *obstack_alloc (struct obstack *<var>obstack-ptr</var>, int <var>size</var>)

Allocate an object of size uninitialized bytes. See Allocation in an Obstack.

void *obstack_copy (struct obstack *<var>obstack-ptr</var>, void *<var>address</var>, int <var>size</var>)

Allocate an object of size bytes, with contents copied from address. See Allocation in an Obstack.

void *obstack_copy0 (struct obstack *<var>obstack-ptr</var>, void *<var>address</var>, int <var>size</var>)

Allocate an object of size+1 bytes, with size of them copied from address, followed by a null character at the end. See Allocation in an Obstack.

void obstack_free (struct obstack *<var>obstack-ptr</var>, void *<var>object</var>)

Free object (and everything allocated in the specified obstack more recently than object). See Freeing Obstack Objects.

void obstack_blank (struct obstack *<var>obstack-ptr</var>, int <var>size</var>)

Add size uninitialized bytes to a growing object. See Growing Objects.

void obstack_grow (struct obstack *<var>obstack-ptr</var>, void *<var>address</var>, int <var>size</var>)

Add size bytes, copied from address, to a growing object. See Growing Objects.

void obstack_grow0 (struct obstack *<var>obstack-ptr</var>, void *<var>address</var>, int <var>size</var>)

Add size bytes, copied from address, to a growing object, and then add another byte containing a null character. See Growing Objects.

void obstack_1grow (struct obstack *<var>obstack-ptr</var>, char <var>data-char</var>)

Add one byte containing data-char to a growing object. See Growing Objects.

void *obstack_finish (struct obstack *<var>obstack-ptr</var>)

Finalize the object that is growing and return its permanent address. See Growing Objects.

int obstack_object_size (struct obstack *<var>obstack-ptr</var>)

Get the current size of the currently growing object. See Growing Objects.

void obstack_blank_fast (struct obstack *<var>obstack-ptr</var>, int <var>size</var>)

Add size uninitialized bytes to a growing object without checking that there is enough room. See Extra Fast Growing.

void obstack_1grow_fast (struct obstack *<var>obstack-ptr</var>, char <var>data-char</var>)

Add one byte containing data-char to a growing object without checking that there is enough room. See Extra Fast Growing.

int obstack_room (struct obstack *<var>obstack-ptr</var>)

Get the amount of room now available for growing the current object. See Extra Fast Growing.

int obstack_alignment_mask (struct obstack *<var>obstack-ptr</var>)

The mask used for aligning the beginning of an object. This is an lvalue. See Obstacks Data Alignment.

int obstack_chunk_size (struct obstack *<var>obstack-ptr</var>)

The size for allocating chunks. This is an lvalue. See Obstack Chunks.

void *obstack_base (struct obstack *<var>obstack-ptr</var>)

Tentative starting address of the currently growing object. See Status of an Obstack.

void *obstack_next_free (struct obstack *<var>obstack-ptr</var>)

Address just after the end of the currently growing object. See Status of an Obstack.


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3.2.6 Automatic Storage with Variable Size

The function alloca supports a kind of half-dynamic allocation in which blocks are allocated dynamically but freed automatically.

Allocating a block with alloca is an explicit action; you can allocate as many blocks as you wish, and compute the size at run time. But all the blocks are freed when you exit the function that alloca was called from, just as if they were automatic variables declared in that function. There is no way to free the space explicitly.

The prototype for alloca is in stdlib.h. This function is a BSD extension.

Function: void * alloca (size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The return value of alloca is the address of a block of size bytes of memory, allocated in the stack frame of the calling function.

Do not use alloca inside the arguments of a function call—you will get unpredictable results, because the stack space for the alloca would appear on the stack in the middle of the space for the function arguments. An example of what to avoid is

foo (x,
alloca (4), y)
.


3.2.6.1 alloca Example

As an example of the use of alloca, here is a function that opens a file name made from concatenating two argument strings, and returns a file descriptor or minus one signifying failure:

int
open2 (char *str1, char *str2, int flags, int mode)
{
  char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
  stpcpy (stpcpy (name, str1), str2);
  return open (name, flags, mode);
}

Here is how you would get the same results with malloc and free:

int
open2 (char *str1, char *str2, int flags, int mode)
{
  char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
  int desc;
  if (name == 0)
    fatal ("virtual memory exceeded");
  stpcpy (stpcpy (name, str1), str2);
  desc = open (name, flags, mode);
  free (name);
  return desc;
}

As you can see, it is simpler with alloca. But alloca has other, more important advantages, and some disadvantages.


3.2.6.2 Advantages of alloca

Here are the reasons why alloca may be preferable to malloc:

  • Using alloca wastes very little space and is very fast. (It is open-coded by the GNU C compiler.)
  • Since alloca does not have separate pools for different sizes of blocks, space used for any size block can be reused for any other size. alloca does not cause memory fragmentation.
  • Nonlocal exits done with longjmp (see Non-Local Exits) automatically free the space allocated with alloca when they exit through the function that called alloca. This is the most important reason to use alloca.

    To illustrate this, suppose you have a function open_or_report_error which returns a descriptor, like open, if it succeeds, but does not return to its caller if it fails. If the file cannot be opened, it prints an error message and jumps out to the command level of your program using longjmp. Let’s change open2 (see Alloca Example) to use this subroutine:

    int
    open2 (char *str1, char *str2, int flags, int mode)
    {
      char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
      stpcpy (stpcpy (name, str1), str2);
      return open_or_report_error (name, flags, mode);
    }

    Because of the way alloca works, the memory it allocates is freed even when an error occurs, with no special effort required.

    By contrast, the previous definition of open2 (which uses malloc and free) would develop a memory leak if it were changed in this way. Even if you are willing to make more changes to fix it, there is no easy way to do so.


3.2.6.3 Disadvantages of alloca

These are the disadvantages of alloca in comparison with malloc:

  • If you try to allocate more memory than the machine can provide, you don’t get a clean error message. Instead you get a fatal signal like the one you would get from an infinite recursion; probably a segmentation violation (see Program Error Signals).
  • Some non-GNU systems fail to support alloca, so it is less portable. However, a slower emulation of alloca written in C is available for use on systems with this deficiency.

3.2.6.4 GNU C Variable-Size Arrays

In GNU C, you can replace most uses of alloca with an array of variable size. Here is how open2 would look then:

int open2 (char *str1, char *str2, int flags, int mode)
{
  char name[strlen (str1) + strlen (str2) + 1];
  stpcpy (stpcpy (name, str1), str2);
  return open (name, flags, mode);
}

But alloca is not always equivalent to a variable-sized array, for several reasons:

  • A variable size array’s space is freed at the end of the scope of the name of the array. The space allocated with alloca remains until the end of the function.
  • It is possible to use alloca within a loop, allocating an additional block on each iteration. This is impossible with variable-sized arrays.

NB: If you mix use of alloca and variable-sized arrays within one function, exiting a scope in which a variable-sized array was declared frees all blocks allocated with alloca during the execution of that scope.


Next: , Previous: , Up: Memory   [Contents][Index]

3.3 Resizing the Data Segment

The symbols in this section are declared in unistd.h.

You will not normally use the functions in this section, because the functions described in Memory Allocation are easier to use. Those are interfaces to a GNU C Library memory allocator that uses the functions below itself. The functions below are simple interfaces to system calls.

Function: int brk (void *addr)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

brk sets the high end of the calling process’ data segment to addr.

The address of the end of a segment is defined to be the address of the last byte in the segment plus 1.

The function has no effect if addr is lower than the low end of the data segment. (This is considered success, by the way.)

The function fails if it would cause the data segment to overlap another segment or exceed the process’ data storage limit (see Limits on Resources).

The function is named for a common historical case where data storage and the stack are in the same segment. Data storage allocation grows upward from the bottom of the segment while the stack grows downward toward it from the top of the segment and the curtain between them is called the break.

The return value is zero on success. On failure, the return value is -1 and errno is set accordingly. The following errno values are specific to this function:

ENOMEM

The request would cause the data segment to overlap another segment or exceed the process’ data storage limit.

Function: void *sbrk (ptrdiff_t delta)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is the same as brk except that you specify the new end of the data segment as an offset delta from the current end and on success the return value is the address of the resulting end of the data segment instead of zero.

This means you can use ‘sbrk(0)’ to find out what the current end of the data segment is.


3.4 Locking Pages

You can tell the system to associate a particular virtual memory page with a real page frame and keep it that way — i.e., cause the page to be paged in if it isn’t already and mark it so it will never be paged out and consequently will never cause a page fault. This is called locking a page.

The functions in this chapter lock and unlock the calling process’ pages.


3.4.1 Why Lock Pages

Because page faults cause paged out pages to be paged in transparently, a process rarely needs to be concerned about locking pages. However, there are two reasons people sometimes are:

  • Speed. A page fault is transparent only insofar as the process is not sensitive to how long it takes to do a simple memory access. Time-critical processes, especially realtime processes, may not be able to wait or may not be able to tolerate variance in execution speed.

    A process that needs to lock pages for this reason probably also needs priority among other processes for use of the CPU. See Priority.

    In some cases, the programmer knows better than the system’s demand paging allocator which pages should remain in real memory to optimize system performance. In this case, locking pages can help.

  • Privacy. If you keep secrets in virtual memory and that virtual memory gets paged out, that increases the chance that the secrets will get out. If a password gets written out to disk swap space, for example, it might still be there long after virtual and real memory have been wiped clean.

Be aware that when you lock a page, that’s one fewer page frame that can be used to back other virtual memory (by the same or other processes), which can mean more page faults, which means the system runs more slowly. In fact, if you lock enough memory, some programs may not be able to run at all for lack of real memory.


3.4.2 Locked Memory Details

A memory lock is associated with a virtual page, not a real frame. The paging rule is: If a frame backs at least one locked page, don’t page it out.

Memory locks do not stack. I.e., you can’t lock a particular page twice so that it has to be unlocked twice before it is truly unlocked. It is either locked or it isn’t.

A memory lock persists until the process that owns the memory explicitly unlocks it. (But process termination and exec cause the virtual memory to cease to exist, which you might say means it isn’t locked any more).

Memory locks are not inherited by child processes. (But note that on a modern Unix system, immediately after a fork, the parent’s and the child’s virtual address space are backed by the same real page frames, so the child enjoys the parent’s locks). See Creating a Process.

Because of its ability to impact other processes, only the superuser can lock a page. Any process can unlock its own page.

The system sets limits on the amount of memory a process can have locked and the amount of real memory it can have dedicated to it. See Limits on Resources.

In Linux, locked pages aren’t as locked as you might think. Two virtual pages that are not shared memory can nonetheless be backed by the same real frame. The kernel does this in the name of efficiency when it knows both virtual pages contain identical data, and does it even if one or both of the virtual pages are locked.

But when a process modifies one of those pages, the kernel must get it a separate frame and fill it with the page’s data. This is known as a copy-on-write page fault. It takes a small amount of time and in a pathological case, getting that frame may require I/O.

To make sure this doesn’t happen to your program, don’t just lock the pages. Write to them as well, unless you know you won’t write to them ever. And to make sure you have pre-allocated frames for your stack, enter a scope that declares a C automatic variable larger than the maximum stack size you will need, set it to something, then return from its scope.


3.4.3 Functions To Lock And Unlock Pages

The symbols in this section are declared in sys/mman.h. These functions are defined by POSIX.1b, but their availability depends on your kernel. If your kernel doesn’t allow these functions, they exist but always fail. They are available with a Linux kernel.

Portability Note: POSIX.1b requires that when the mlock and munlock functions are available, the file unistd.h define the macro _POSIX_MEMLOCK_RANGE and the file limits.h define the macro PAGESIZE to be the size of a memory page in bytes. It requires that when the mlockall and munlockall functions are available, the unistd.h file define the macro _POSIX_MEMLOCK. The GNU C Library conforms to this requirement.

Function: int mlock (const void *addr, size_t len)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

mlock locks a range of the calling process’ virtual pages.

The range of memory starts at address addr and is len bytes long. Actually, since you must lock whole pages, it is the range of pages that include any part of the specified range.

When the function returns successfully, each of those pages is backed by (connected to) a real frame (is resident) and is marked to stay that way. This means the function may cause page-ins and have to wait for them.

When the function fails, it does not affect the lock status of any pages.

The return value is zero if the function succeeds. Otherwise, it is -1 and errno is set accordingly. errno values specific to this function are:

ENOMEM
  • At least some of the specified address range does not exist in the calling process’ virtual address space.
  • The locking would cause the process to exceed its locked page limit.
EPERM

The calling process is not superuser.

EINVAL

len is not positive.

ENOSYS

The kernel does not provide mlock capability.

You can lock all a process’ memory with mlockall. You unlock memory with munlock or munlockall.

To avoid all page faults in a C program, you have to use mlockall, because some of the memory a program uses is hidden from the C code, e.g. the stack and automatic variables, and you wouldn’t know what address to tell mlock.

Function: int munlock (const void *addr, size_t len)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

munlock unlocks a range of the calling process’ virtual pages.

munlock is the inverse of mlock and functions completely analogously to mlock, except that there is no EPERM failure.

Function: int mlockall (int flags)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

mlockall locks all the pages in a process’ virtual memory address space, and/or any that are added to it in the future. This includes the pages of the code, data and stack segment, as well as shared libraries, user space kernel data, shared memory, and memory mapped files.

flags is a string of single bit flags represented by the following macros. They tell mlockall which of its functions you want. All other bits must be zero.

MCL_CURRENT

Lock all pages which currently exist in the calling process’ virtual address space.

MCL_FUTURE

Set a mode such that any pages added to the process’ virtual address space in the future will be locked from birth. This mode does not affect future address spaces owned by the same process so exec, which replaces a process’ address space, wipes out MCL_FUTURE. See Executing a File.

When the function returns successfully, and you specified MCL_CURRENT, all of the process’ pages are backed by (connected to) real frames (they are resident) and are marked to stay that way. This means the function may cause page-ins and have to wait for them.

When the process is in MCL_FUTURE mode because it successfully executed this function and specified MCL_CURRENT, any system call by the process that requires space be added to its virtual address space fails with errno = ENOMEM if locking the additional space would cause the process to exceed its locked page limit. In the case that the address space addition that can’t be accommodated is stack expansion, the stack expansion fails and the kernel sends a SIGSEGV signal to the process.

When the function fails, it does not affect the lock status of any pages or the future locking mode.

The return value is zero if the function succeeds. Otherwise, it is -1 and errno is set accordingly. errno values specific to this function are:

ENOMEM
  • At least some of the specified address range does not exist in the calling process’ virtual address space.
  • The locking would cause the process to exceed its locked page limit.
EPERM

The calling process is not superuser.

EINVAL

Undefined bits in flags are not zero.

ENOSYS

The kernel does not provide mlockall capability.

You can lock just specific pages with mlock. You unlock pages with munlockall and munlock.

Function: int munlockall (void)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

munlockall unlocks every page in the calling process’ virtual address space and turns off MCL_FUTURE future locking mode.

The return value is zero if the function succeeds. Otherwise, it is -1 and errno is set accordingly. The only way this function can fail is for generic reasons that all functions and system calls can fail, so there are no specific errno values.


Next: , Previous: , Up: Top   [Contents][Index]

4 Character Handling

Programs that work with characters and strings often need to classify a character—is it alphabetic, is it a digit, is it whitespace, and so on—and perform case conversion operations on characters. The functions in the header file ctype.h are provided for this purpose.

Since the choice of locale and character set can alter the classifications of particular character codes, all of these functions are affected by the current locale. (More precisely, they are affected by the locale currently selected for character classification—the LC_CTYPE category; see Locale Categories.)

The ISO C standard specifies two different sets of functions. The one set works on char type characters, the other one on wchar_t wide characters (see Extended Char Intro).


4.1 Classification of Characters

This section explains the library functions for classifying characters. For example, isalpha is the function to test for an alphabetic character. It takes one argument, the character to test, and returns a nonzero integer if the character is alphabetic, and zero otherwise. You would use it like this:

if (isalpha (c))
  printf ("The character `%c' is alphabetic.\n", c);

Each of the functions in this section tests for membership in a particular class of characters; each has a name starting with ‘is’. Each of them takes one argument, which is a character to test, and returns an int which is treated as a boolean value. The character argument is passed as an int, and it may be the constant value EOF instead of a real character.

The attributes of any given character can vary between locales. See Locales, for more information on locales.

These functions are declared in the header file ctype.h.

Function: int islower (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

Function: int isupper (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

Function: int isalpha (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is an alphabetic character (a letter). If islower or isupper is true of a character, then isalpha is also true.

In some locales, there may be additional characters for which isalpha is true—letters which are neither upper case nor lower case. But in the standard "C" locale, there are no such additional characters.

Function: int isdigit (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a decimal digit (‘0’ through ‘9’).

Function: int isalnum (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is an alphanumeric character (a letter or number); in other words, if either isalpha or isdigit is true of a character, then isalnum is also true.

Function: int isxdigit (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a hexadecimal digit. Hexadecimal digits include the normal decimal digits ‘0’ through ‘9’ and the letters ‘A’ through ‘F’ and ‘a’ through ‘f’.

Function: int ispunct (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a punctuation character. This means any printing character that is not alphanumeric or a space character.

Function: int isspace (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a whitespace character. In the standard "C" locale, isspace returns true for only the standard whitespace characters:

' '

space

'\f'

formfeed

'\n'

newline

'\r'

carriage return

'\t'

horizontal tab

'\v'

vertical tab

Function: int isblank (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a blank character; that is, a space or a tab. This function was originally a GNU extension, but was added in ISO C99.

Function: int isgraph (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic.

Function: int isprint (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a printing character. Printing characters include all the graphic characters, plus the space (‘ ’) character.

Function: int iscntrl (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a control character (that is, a character that is not a printing character).

Function: int isascii (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if c is a 7-bit unsigned char value that fits into the US/UK ASCII character set. This function is a BSD extension and is also an SVID extension.


4.2 Case Conversion

This section explains the library functions for performing conversions such as case mappings on characters. For example, toupper converts any character to upper case if possible. If the character can’t be converted, toupper returns it unchanged.

These functions take one argument of type int, which is the character to convert, and return the converted character as an int. If the conversion is not applicable to the argument given, the argument is returned unchanged.

Compatibility Note: In pre-ISO C dialects, instead of returning the argument unchanged, these functions may fail when the argument is not suitable for the conversion. Thus for portability, you may need to write islower(c) ? toupper(c) : c rather than just toupper(c).

These functions are declared in the header file ctype.h.

Function: int tolower (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

If c is an upper-case letter, tolower returns the corresponding lower-case letter. If c is not an upper-case letter, c is returned unchanged.

Function: int toupper (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

If c is a lower-case letter, toupper returns the corresponding upper-case letter. Otherwise c is returned unchanged.

Function: int toascii (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function converts c to a 7-bit unsigned char value that fits into the US/UK ASCII character set, by clearing the high-order bits. This function is a BSD extension and is also an SVID extension.

Function: int _tolower (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is identical to tolower, and is provided for compatibility with the SVID. See SVID.

Function: int _toupper (int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is identical to toupper, and is provided for compatibility with the SVID.


4.3 Character class determination for wide characters

Amendment 1 to ISO C90 defines functions to classify wide characters. Although the original ISO C90 standard already defined the type wchar_t, no functions operating on them were defined.

The general design of the classification functions for wide characters is more general. It allows extensions to the set of available classifications, beyond those which are always available. The POSIX standard specifies how extensions can be made, and this is already implemented in the GNU C Library implementation of the localedef program.

The character class functions are normally implemented with bitsets, with a bitset per character. For a given character, the appropriate bitset is read from a table and a test is performed as to whether a certain bit is set. Which bit is tested for is determined by the class.

For the wide character classification functions this is made visible. There is a type classification type defined, a function to retrieve this value for a given class, and a function to test whether a given character is in this class, using the classification value. On top of this the normal character classification functions as used for char objects can be defined.

Data type: wctype_t

The wctype_t can hold a value which represents a character class. The only defined way to generate such a value is by using the wctype function.

This type is defined in wctype.h.

Function: wctype_t wctype (const char *property)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

wctype returns a value representing a class of wide characters which is identified by the string property. Besides some standard properties each locale can define its own ones. In case no property with the given name is known for the current locale selected for the LC_CTYPE category, the function returns zero.

The properties known in every locale are:

"alnum""alpha""cntrl""digit"
"graph""lower""print""punct"
"space""upper""xdigit"

This function is declared in wctype.h.

To test the membership of a character to one of the non-standard classes the ISO C standard defines a completely new function.

Function: int iswctype (wint_t wc, wctype_t desc)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function returns a nonzero value if wc is in the character class specified by desc. desc must previously be returned by a successful call to wctype.

This function is declared in wctype.h.

To make it easier to use the commonly-used classification functions, they are defined in the C library. There is no need to use wctype if the property string is one of the known character classes. In some situations it is desirable to construct the property strings, and then it is important that wctype can also handle the standard classes.

Function: int iswalnum (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function returns a nonzero value if wc is an alphanumeric character (a letter or number); in other words, if either iswalpha or iswdigit is true of a character, then iswalnum is also true.

This function can be implemented using

iswctype (wc, wctype ("alnum"))

It is declared in wctype.h.

Function: int iswalpha (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is an alphabetic character (a letter). If iswlower or iswupper is true of a character, then iswalpha is also true.

In some locales, there may be additional characters for which iswalpha is true—letters which are neither upper case nor lower case. But in the standard "C" locale, there are no such additional characters.

This function can be implemented using

iswctype (wc, wctype ("alpha"))

It is declared in wctype.h.

Function: int iswcntrl (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is a control character (that is, a character that is not a printing character).

This function can be implemented using

iswctype (wc, wctype ("cntrl"))

It is declared in wctype.h.

Function: int iswdigit (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is a digit (e.g., ‘0’ through ‘9’). Please note that this function does not only return a nonzero value for decimal digits, but for all kinds of digits. A consequence is that code like the following will not work unconditionally for wide characters:

n = 0;
while (iswdigit (*wc))
  {
    n *= 10;
    n += *wc++ - L'0';
  }

This function can be implemented using

iswctype (wc, wctype ("digit"))

It is declared in wctype.h.

Function: int iswgraph (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic.

This function can be implemented using

iswctype (wc, wctype ("graph"))

It is declared in wctype.h.

Function: int iswlower (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

This function can be implemented using

iswctype (wc, wctype ("lower"))

It is declared in wctype.h.

Function: int iswprint (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is a printing character. Printing characters include all the graphic characters, plus the space (‘ ’) character.

This function can be implemented using

iswctype (wc, wctype ("print"))

It is declared in wctype.h.

Function: int iswpunct (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is a punctuation character. This means any printing character that is not alphanumeric or a space character.

This function can be implemented using

iswctype (wc, wctype ("punct"))

It is declared in wctype.h.

Function: int iswspace (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is a whitespace character. In the standard "C" locale, iswspace returns true for only the standard whitespace characters:

L' '

space

L'\f'

formfeed

L'\n'

newline

L'\r'

carriage return

L'\t'

horizontal tab

L'\v'

vertical tab

This function can be implemented using

iswctype (wc, wctype ("space"))

It is declared in wctype.h.

Function: int iswupper (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

This function can be implemented using

iswctype (wc, wctype ("upper"))

It is declared in wctype.h.

Function: int iswxdigit (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is a hexadecimal digit. Hexadecimal digits include the normal decimal digits ‘0’ through ‘9’ and the letters ‘A’ through ‘F’ and ‘a’ through ‘f’.

This function can be implemented using

iswctype (wc, wctype ("xdigit"))

It is declared in wctype.h.

The GNU C Library also provides a function which is not defined in the ISO C standard but which is available as a version for single byte characters as well.

Function: int iswblank (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns true if wc is a blank character; that is, a space or a tab. This function was originally a GNU extension, but was added in ISO C99. It is declared in wchar.h.


4.4 Notes on using the wide character classes

The first note is probably not astonishing but still occasionally a cause of problems. The isw<var>XXX</var> functions can be implemented using macros and in fact, the GNU C Library does this. They are still available as real functions but when the wctype.h header is included the macros will be used. This is the same as the char type versions of these functions.

The second note covers something new. It can be best illustrated by a (real-world) example. The first piece of code is an excerpt from the original code. It is truncated a bit but the intention should be clear.

int
is_in_class (int c, const char *class)
{
  if (strcmp (class, "alnum") == 0)
    return isalnum (c);
  if (strcmp (class, "alpha") == 0)
    return isalpha (c);
  if (strcmp (class, "cntrl") == 0)
    return iscntrl (c);
  …
  return 0;
}

Now, with the wctype and iswctype you can avoid the if cascades, but rewriting the code as follows is wrong:

int
is_in_class (int c, const char *class)
{
  wctype_t desc = wctype (class);
  return desc ? iswctype ((wint_t) c, desc) : 0;
}

The problem is that it is not guaranteed that the wide character representation of a single-byte character can be found using casting. In fact, usually this fails miserably. The correct solution to this problem is to write the code as follows:

int
is_in_class (int c, const char *class)
{
  wctype_t desc = wctype (class);
  return desc ? iswctype (btowc (c), desc) : 0;
}

See Converting a Character, for more information on btowc. Note that this change probably does not improve the performance of the program a lot since the wctype function still has to make the string comparisons. It gets really interesting if the is_in_class function is called more than once for the same class name. In this case the variable desc could be computed once and reused for all the calls. Therefore the above form of the function is probably not the final one.


4.5 Mapping of wide characters.

The classification functions are also generalized by the ISO C standard. Instead of just allowing the two standard mappings, a locale can contain others. Again, the localedef program already supports generating such locale data files.

Data Type: wctrans_t

This data type is defined as a scalar type which can hold a value representing the locale-dependent character mapping. There is no way to construct such a value apart from using the return value of the wctrans function.

This type is defined in wctype.h.

Function: wctrans_t wctrans (const char *property)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wctrans function has to be used to find out whether a named mapping is defined in the current locale selected for the LC_CTYPE category. If the returned value is non-zero, you can use it afterwards in calls to towctrans. If the return value is zero no such mapping is known in the current locale.

Beside locale-specific mappings there are two mappings which are guaranteed to be available in every locale:

"tolower""toupper"

These functions are declared in wctype.h.

Function: wint_t towctrans (wint_t wc, wctrans_t desc)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

towctrans maps the input character wc according to the rules of the mapping for which desc is a descriptor, and returns the value it finds. desc must be obtained by a successful call to wctrans.

This function is declared in wctype.h.

For the generally available mappings, the ISO C standard defines convenient shortcuts so that it is not necessary to call wctrans for them.

Function: wint_t towlower (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

If wc is an upper-case letter, towlower returns the corresponding lower-case letter. If wc is not an upper-case letter, wc is returned unchanged.

towlower can be implemented using

towctrans (wc, wctrans ("tolower"))

This function is declared in wctype.h.

Function: wint_t towupper (wint_t wc)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

If wc is a lower-case letter, towupper returns the corresponding upper-case letter. Otherwise wc is returned unchanged.

towupper can be implemented using

towctrans (wc, wctrans ("toupper"))

This function is declared in wctype.h.

The same warnings given in the last section for the use of the wide character classification functions apply here. It is not possible to simply cast a char type value to a wint_t and use it as an argument to towctrans calls.


5 String and Array Utilities

Operations on strings (null-terminated byte sequences) are an important part of many programs. The GNU C Library provides an extensive set of string utility functions, including functions for copying, concatenating, comparing, and searching strings. Many of these functions can also operate on arbitrary regions of storage; for example, the memcpy function can be used to copy the contents of any kind of array.

It’s fairly common for beginning C programmers to “reinvent the wheel” by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.

For instance, you could easily compare one string to another in two lines of C code, but if you use the built-in strcmp function, you’re less likely to make a mistake. And, since these library functions are typically highly optimized, your program may run faster too.


5.1 Representation of Strings

This section is a quick summary of string concepts for beginning C programmers. It describes how strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.

A string is a null-terminated array of bytes of type char, including the terminating null byte. String-valued variables are usually declared to be pointers of type char *. Such variables do not include space for the text of a string; that has to be stored somewhere else—in an array variable, a string constant, or dynamically allocated memory (see Memory Allocation). It’s up to you to store the address of the chosen memory space into the pointer variable. Alternatively you can store a null pointer in the pointer variable. The null pointer does not point anywhere, so attempting to reference the string it points to gets an error.

A multibyte character is a sequence of one or more bytes that represents a single character using the locale’s encoding scheme; a null byte always represents the null character. A multibyte string is a string that consists entirely of multibyte characters. In contrast, a wide string is a null-terminated sequence of wchar_t objects. A wide-string variable is usually declared to be a pointer of type wchar_t *, by analogy with string variables and char *. See Extended Char Intro.

By convention, the null byte, '\0', marks the end of a string and the null wide character, L'\0', marks the end of a wide string. For example, in testing to see whether the char * variable p points to a null byte marking the end of a string, you can write !*<var>p</var> or *<var>p</var> == '\0'.

A null byte is quite different conceptually from a null pointer, although both are represented by the integer constant  .

A string literal appears in C program source as a multibyte string between double-quote characters (‘"’). If the initial double-quote character is immediately preceded by a capital ‘L’ (ell) character (as in L"foo"), it is a wide string literal. String literals can also contribute to string concatenation: "a" "b" is the same as "ab". For wide strings one can use either L"a" L"b" or L"a" "b". Modification of string literals is not allowed by the GNU C compiler, because literals are placed in read-only storage.

Arrays that are declared const cannot be modified either. It’s generally good style to declare non-modifiable string pointers to be of type const char *, since this often allows the C compiler to detect accidental modifications as well as providing some amount of documentation about what your program intends to do with the string.

The amount of memory allocated for a byte array may extend past the null byte that marks the end of the string that the array contains. In this document, the term allocated size is always used to refer to the total amount of memory allocated for an array, while the term length refers to the number of bytes up to (but not including) the terminating null byte. Wide strings are similar, except their sizes and lengths count wide characters, not bytes.

A notorious source of program bugs is trying to put more bytes into a string than fit in its allocated size. When writing code that extends strings or moves bytes into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null byte that marks the end of the string.

Originally strings were sequences of bytes where each byte represented a single character. This is still true today if the strings are encoded using a single-byte character encoding. Things are different if the strings are encoded using a multibyte encoding (for more information on encodings see Extended Char Intro). There is no difference in the programming interface for these two kind of strings; the programmer has to be aware of this and interpret the byte sequences accordingly.

But since there is no separate interface taking care of these differences the byte-based string functions are sometimes hard to use. Since the count parameters of these functions specify bytes a call to memcpy could cut a multibyte character in the middle and put an incomplete (and therefore unusable) byte sequence in the target buffer.

To avoid these problems later versions of the ISO C standard introduce a second set of functions which are operating on wide characters (see Extended Char Intro). These functions don’t have the problems the single-byte versions have since every wide character is a legal, interpretable value. This does not mean that cutting wide strings at arbitrary points is without problems. It normally is for alphabet-based languages (except for non-normalized text) but languages based on syllables still have the problem that more than one wide character is necessary to complete a logical unit. This is a higher level problem which the C library functions are not designed to solve. But it is at least good that no invalid byte sequences can be created. Also, the higher level functions can also much more easily operate on wide characters than on multibyte characters so that a common strategy is to use wide characters internally whenever text is more than simply copied.

The remaining of this chapter will discuss the functions for handling wide strings in parallel with the discussion of strings since there is almost always an exact equivalent available.


5.2 String and Array Conventions

This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to strings and wide strings.

Functions that operate on arbitrary blocks of memory have names beginning with ‘mem’ and ‘wmem’ (such as memcpy and wmemcpy) and invariably take an argument which specifies the size (in bytes and wide characters respectively) of the block of memory to operate on. The array arguments and return values for these functions have type void * or wchar_t. As a matter of style, the elements of the arrays used with the ‘mem’ functions are referred to as “bytes”. You can pass any kind of pointer to these functions, and the sizeof operator is useful in computing the value for the size argument. Parameters to the ‘wmem’ functions must be of type wchar_t *. These functions are not really usable with anything but arrays of this type.

In contrast, functions that operate specifically on strings and wide strings have names beginning with ‘str’ and ‘wcs’ respectively (such as strcpy and wcscpy) and look for a terminating null byte or null wide character instead of requiring an explicit size argument to be passed. (Some of these functions accept a specified maximum length, but they also check for premature termination.) The array arguments and return values for these functions have type char * and wchar_t * respectively, and the array elements are referred to as “bytes” and “wide characters”.

In many cases, there are both ‘mem’ and ‘str’/‘wcs’ versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the ‘mem’ functions. On the other hand, when you are manipulating strings it is usually more convenient to use the ‘str’/‘wcs’ functions, unless you already know the length of the string in advance. The ‘wmem’ functions should be used for wide character arrays with known size.

Some of the memory and string functions take single characters as arguments. Since a value of type char is automatically promoted into a value of type int when used as a parameter, the functions are declared with int as the type of the parameter in question. In case of the wide character functions the situation is similar: the parameter type for a single wide character is wint_t and not wchar_t. This would for many implementations not be necessary since wchar_t is large enough to not be automatically promoted, but since the ISO C standard does not require such a choice of types the wint_t type is used.


5.3 String Length

You can get the length of a string using the strlen function. This function is declared in the header file string.h.

Function: size_t strlen (const char *s)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The strlen function returns the length of the string s in bytes. (In other words, it returns the offset of the terminating null byte within the array.)

For example,

strlen ("hello, world")
    ⇒ 12

When applied to an array, the strlen function returns the length of the string stored there, not its allocated size. You can get the allocated size of the array that holds a string using the sizeof operator:

char string[32] = "hello, world";
sizeof (string)
    ⇒ 32
strlen (string)
    ⇒ 12

But beware, this will not work unless string is the array itself, not a pointer to it. For example:

char string[32] = "hello, world";
char *ptr = string;
sizeof (string)
    ⇒ 32
sizeof (ptr)
    ⇒ 4  /* <span class="roman">(on a machine with 4 byte pointers)</span> */

This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays.

It must also be noted that for multibyte encoded strings the return value does not have to correspond to the number of characters in the string. To get this value the string can be converted to wide characters and wcslen can be used or something like the following code can be used:

/* <span class="roman">The input is in <code>string</code>.</span>
   <span class="roman">The length is expected in <code>n</code>.</span>  */
{
  mbstate_t t;
  char *scopy = string;
  /* In initial state.  */
  memset (&amp;t, '\0', sizeof (t));
  /* Determine number of characters.  */
  n = mbsrtowcs (NULL, &amp;scopy, strlen (scopy), &amp;t);
}

This is cumbersome to do so if the number of characters (as opposed to bytes) is needed often it is better to work with wide characters.

The wide character equivalent is declared in wchar.h.

Function: size_t wcslen (const wchar_t *ws)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wcslen function is the wide character equivalent to strlen. The return value is the number of wide characters in the wide string pointed to by ws (this is also the offset of the terminating null wide character of ws).

Since there are no multi wide character sequences making up one wide character the return value is not only the offset in the array, it is also the number of wide characters.

This function was introduced in Amendment 1 to ISO C90.

Function: size_t strnlen (const char *s, size_t maxlen)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

If the array s of size maxlen contains a null byte, the strnlen function returns the length of the string s in bytes. Otherwise it returns maxlen. Therefore this function is equivalent to (strlen (<var>s</var>) &lt; <var>maxlen</var> ? strlen (<var>s</var>) : <var>maxlen</var>) but it is more efficient and works even if s is not null-terminated so long as maxlen does not exceed the size of s’s array.

char string[32] = "hello, world";
strnlen (string, 32)
    ⇒ 12
strnlen (string, 5)
    ⇒ 5

This function is a GNU extension and is declared in string.h.

Function: size_t wcsnlen (const wchar_t *ws, size_t maxlen)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

wcsnlen is the wide character equivalent to strnlen. The maxlen parameter specifies the maximum number of wide characters.

This function is a GNU extension and is declared in wchar.h.


5.4 Copying Strings and Arrays

You can use the functions described in this section to copy the contents of strings, wide strings, and arrays. The ‘str’ and ‘mem’ functions are declared in string.h while the ‘w’ functions are declared in wchar.h.

A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. Most of these functions return the address of the destination array; a few return the address of the destination’s terminating null, or of just past the destination.

Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null byte marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.

All functions that have problems copying between overlapping arrays are explicitly identified in this manual. In addition to functions in this section, there are a few others like sprintf (see Formatted Output Functions) and scanf (see Formatted Input Functions).

Function: void * memcpy (void *restrict to, const void *restrict from, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The memcpy function copies size bytes from the object beginning at from into the object beginning at to. The behavior of this function is undefined if the two arrays to and from overlap; use memmove instead if overlapping is possible.

The value returned by memcpy is the value of to.

Here is an example of how you might use memcpy to copy the contents of an array:

struct foo *oldarray, *newarray;
int arraysize;
…
memcpy (new, old, arraysize * sizeof (struct foo));
Function: wchar_t * wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wmemcpy function copies size wide characters from the object beginning at wfrom into the object beginning at wto. The behavior of this function is undefined if the two arrays wto and wfrom overlap; use wmemmove instead if overlapping is possible.

The following is a possible implementation of wmemcpy but there are more optimizations possible.

wchar_t *
wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
         size_t size)
{
  return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
}

The value returned by wmemcpy is the value of wto.

This function was introduced in Amendment 1 to ISO C90.

Function: void * mempcpy (void *restrict to, const void *restrict from, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The mempcpy function is nearly identical to the memcpy function. It copies size bytes from the object beginning at from into the object pointed to by to. But instead of returning the value of to it returns a pointer to the byte following the last written byte in the object beginning at to. I.e., the value is ((void *) ((char *) <var>to</var> + <var>size</var>)).

This function is useful in situations where a number of objects shall be copied to consecutive memory positions.

void *
combine (void *o1, size_t s1, void *o2, size_t s2)
{
  void *result = malloc (s1 + s2);
  if (result != NULL)
    mempcpy (mempcpy (result, o1, s1), o2, s2);
  return result;
}

This function is a GNU extension.

Function: wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wmempcpy function is nearly identical to the wmemcpy function. It copies size wide characters from the object beginning at wfrom into the object pointed to by wto. But instead of returning the value of wto it returns a pointer to the wide character following the last written wide character in the object beginning at wto. I.e., the value is <var>wto</var> + <var>size</var>.

This function is useful in situations where a number of objects shall be copied to consecutive memory positions.

The following is a possible implementation of wmemcpy but there are more optimizations possible.

wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
          size_t size)
{
  return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}

This function is a GNU extension.

Function: void * memmove (void *to, const void *from, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

memmove copies the size bytes at from into the size bytes at to, even if those two blocks of space overlap. In the case of overlap, memmove is careful to copy the original values of the bytes in the block at from, including those bytes which also belong to the block at to.

The value returned by memmove is the value of to.

Function: wchar_t * wmemmove (wchar_t *wto, const wchar_t *wfrom, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

wmemmove copies the size wide characters at wfrom into the size wide characters at wto, even if those two blocks of space overlap. In the case of overlap, wmemmove is careful to copy the original values of the wide characters in the block at wfrom, including those wide characters which also belong to the block at wto.

The following is a possible implementation of wmemcpy but there are more optimizations possible.

wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
          size_t size)
{
  return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}

The value returned by wmemmove is the value of wto.

This function is a GNU extension.

Function: void * memccpy (void *restrict to, const void *restrict from, int c, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function copies no more than size bytes from from to to, stopping if a byte matching c is found. The return value is a pointer into to one byte past where c was copied, or a null pointer if no byte matching c appeared in the first size bytes of from.

Function: void * memset (void *block, int c, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function copies the value of c (converted to an unsigned char) into each of the first size bytes of the object beginning at block. It returns the value of block.

Function: wchar_t * wmemset (wchar_t *block, wchar_t wc, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function copies the value of wc into each of the first size wide characters of the object beginning at block. It returns the value of block.

Function: char * strcpy (char *restrict to, const char *restrict from)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This copies bytes from the string from (up to and including the terminating null byte) into the string to. Like memcpy, this function has undefined results if the strings overlap. The return value is the value of to.

Function: wchar_t * wcscpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This copies wide characters from the wide string wfrom (up to and including the terminating null wide character) into the string wto. Like wmemcpy, this function has undefined results if the strings overlap. The return value is the value of wto.

Function: char * strdup (const char *s)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

This function copies the string s into a newly allocated string. The string is allocated using malloc; see Unconstrained Allocation. If malloc cannot allocate space for the new string, strdup returns a null pointer. Otherwise it returns a pointer to the new string.

Function: wchar_t * wcsdup (const wchar_t *ws)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

This function copies the wide string ws into a newly allocated string. The string is allocated using malloc; see Unconstrained Allocation. If malloc cannot allocate space for the new string, wcsdup returns a null pointer. Otherwise it returns a pointer to the new wide string.

This function is a GNU extension.

Function: char * stpcpy (char *restrict to, const char *restrict from)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is like strcpy, except that it returns a pointer to the end of the string to (that is, the address of the terminating null byte to + strlen (from)) rather than the beginning.

For example, this program uses stpcpy to concatenate ‘foo’ and ‘bar’ to produce ‘foobar’, which it then prints.

#include &lt;string.h&gt;
#include &lt;stdio.h&gt;
 
int
main (void)
{
  char buffer[10];
  char *to = buffer;
  to = stpcpy (to, "foo");
  to = stpcpy (to, "bar");
  puts (buffer);
  return 0;
}

This function is part of POSIX.1-2008 and later editions, but was available in the GNU C Library and other systems as an extension long before it was standardized.

Its behavior is undefined if the strings overlap. The function is declared in string.h.

Function: wchar_t * wcpcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is like wcscpy, except that it returns a pointer to the end of the string wto (that is, the address of the terminating null wide character wto + wcslen (wfrom)) rather than the beginning.

This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

The behavior of wcpcpy is undefined if the strings overlap.

wcpcpy is a GNU extension and is declared in wchar.h.

Macro: char * strdupa (const char *s)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This macro is similar to strdup but allocates the new string using alloca instead of malloc (see Variable Size Automatic). This means of course the returned string has the same limitations as any block of memory allocated using alloca.

For obvious reasons strdupa is implemented only as a macro; you cannot get the address of this function. Despite this limitation it is a useful function. The following code shows a situation where using malloc would be a lot more expensive.

#include &lt;paths.h&gt;
#include &lt;string.h&gt;
#include &lt;stdio.h&gt;
 
const char path[] = _PATH_STDPATH;
 
int
main (void)
{
  char *wr_path = strdupa (path);
  char *cp = strtok (wr_path, ":");
 
  while (cp != NULL)
    {
      puts (cp);
      cp = strtok (NULL, ":");
    }
  return 0;
}

Please note that calling strtok using path directly is invalid. It is also not allowed to call strdupa in the argument list of strtok since strdupa uses alloca (see Variable Size Automatic) can interfere with the parameter passing.

This function is only available if GNU CC is used.

Function: void bcopy (const void *from, void *to, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is a partially obsolete alternative for memmove, derived from BSD. Note that it is not quite equivalent to memmove, because the arguments are not in the same order and there is no return value.

Function: void bzero (void *block, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is a partially obsolete alternative for memset, derived from BSD. Note that it is not as general as memset, because the only value it can store is zero.


5.5 Concatenating Strings

The functions described in this section concatenate the contents of a string or wide string to another. They follow the string-copying functions in their conventions. See Copying Strings and Arrays. ‘strcat’ is declared in the header file string.h while ‘wcscat’ is declared in wchar.h.

Function: char * strcat (char *restrict to, const char *restrict from)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The strcat function is similar to strcpy, except that the bytes from from are concatenated or appended to the end of to, instead of overwriting it. That is, the first byte from from overwrites the null byte marking the end of to.

An equivalent definition for strcat would be:

char *
strcat (char *restrict to, const char *restrict from)
{
  strcpy (to + strlen (to), from);
  return to;
}

This function has undefined results if the strings overlap.

As noted below, this function has significant performance issues.

Function: wchar_t * wcscat (wchar_t *restrict wto, const wchar_t *restrict wfrom)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wcscat function is similar to wcscpy, except that the wide characters from wfrom are concatenated or appended to the end of wto, instead of overwriting it. That is, the first wide character from wfrom overwrites the null wide character marking the end of wto.

An equivalent definition for wcscat would be:

wchar_t *
wcscat (wchar_t *wto, const wchar_t *wfrom)
{
  wcscpy (wto + wcslen (wto), wfrom);
  return wto;
}

This function has undefined results if the strings overlap.

As noted below, this function has significant performance issues.

Programmers using the strcat or wcscat function (or the strncat or wcsncat functions defined in a later section, for that matter) can easily be recognized as lazy and reckless. In almost all situations the lengths of the participating strings are known (it better should be since how can one otherwise ensure the allocated size of the buffer is sufficient?) Or at least, one could know them if one keeps track of the results of the various function calls. But then it is very inefficient to use strcat/wcscat. A lot of time is wasted finding the end of the destination string so that the actual copying can start. This is a common example:

/* <span class="roman">This function concatenates arbitrarily many strings.  The last</span>
   <span class="roman">parameter must be <code>NULL</code>.</span>  */
char *
concat (const char *str, …)
{
  va_list ap, ap2;
  size_t total = 1;
  const char *s;
  char *result;
 
  va_start (ap, str);
  va_copy (ap2, ap);
 
  /* <span class="roman">Determine how much space we need.</span>  */
  for (s = str; s != NULL; s = va_arg (ap, const char *))
    total += strlen (s);
 
  va_end (ap);
 
  result = (char *) malloc (total);
  if (result != NULL)
    {
      result[0] = '\0';
 
      /* <span class="roman">Copy the strings.</span>  */
      for (s = str; s != NULL; s = va_arg (ap2, const char *))
        strcat (result, s);
    }
 
  va_end (ap2);
 
  return result;
}

This looks quite simple, especially the second loop where the strings are actually copied. But these innocent lines hide a major performance penalty. Just imagine that ten strings of 100 bytes each have to be concatenated. For the second string we search the already stored 100 bytes for the end of the string so that we can append the next string. For all strings in total the comparisons necessary to find the end of the intermediate results sums up to 5500! If we combine the copying with the search for the allocation we can write this function more efficiently:

char *
concat (const char *str, …)
{
  va_list ap;
  size_t allocated = 100;
  char *result = (char *) malloc (allocated);
 
  if (result != NULL)
    {
      char *newp;
      char *wp;
      const char *s;
 
      va_start (ap, str);
 
      wp = result;
      for (s = str; s != NULL; s = va_arg (ap, const char *))
        {
          size_t len = strlen (s);
 
          /* <span class="roman">Resize the allocated memory if necessary.</span>  */
          if (wp + len + 1 &gt; result + allocated)
            {
              allocated = (allocated + len) * 2;
              newp = (char *) realloc (result, allocated);
              if (newp == NULL)
                {
                  free (result);
                  return NULL;
                }
              wp = newp + (wp - result);
              result = newp;
            }
 
          wp = mempcpy (wp, s, len);
        }
 
      /* <span class="roman">Terminate the result string.</span>  */
      *wp++ = '\0';
 
      /* <span class="roman">Resize memory to the optimal size.</span>  */
      newp = realloc (result, wp - result);
      if (newp != NULL)
        result = newp;
 
      va_end (ap);
    }
 
  return result;
}

With a bit more knowledge about the input strings one could fine-tune the memory allocation. The difference we are pointing to here is that we don’t use strcat anymore. We always keep track of the length of the current intermediate result so we can save ourselves the search for the end of the string and use mempcpy. Please note that we also don’t use stpcpy which might seem more natural since we are handling strings. But this is not necessary since we already know the length of the string and therefore can use the faster memory copying function. The example would work for wide characters the same way.

Whenever a programmer feels the need to use strcat she or he should think twice and look through the program to see whether the code cannot be rewritten to take advantage of already calculated results. Again: it is almost always unnecessary to use strcat.


5.6 Truncating Strings while Copying

The functions described in this section copy or concatenate the possibly-truncated contents of a string or array to another, and similarly for wide strings. They follow the string-copying functions in their header conventions. See Copying Strings and Arrays. The ‘str’ functions are declared in the header file string.h and the ‘wc’ functions are declared in the file wchar.h.

Function: char * strncpy (char *restrict to, const char *restrict from, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is similar to strcpy but always copies exactly size bytes into to.

If from does not contain a null byte in its first size bytes, strncpy copies just the first size bytes. In this case no null terminator is written into to.

Otherwise from must be a string with length less than size. In this case strncpy copies all of from, followed by enough null bytes to add up to size bytes in all.

The behavior of strncpy is undefined if the strings overlap.

This function was designed for now-rarely-used arrays consisting of non-null bytes followed by zero or more null bytes. It needs to set all size bytes of the destination, even when size is much greater than the length of from. As noted below, this function is generally a poor choice for processing text.

Function: wchar_t * wcsncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is similar to wcscpy but always copies exactly size wide characters into wto.

If wfrom does not contain a null wide character in its first size wide characters, then wcsncpy copies just the first size wide characters. In this case no null terminator is written into wto.

Otherwise wfrom must be a wide string with length less than size. In this case wcsncpy copies all of wfrom, followed by enough null wide characters to add up to size wide characters in all.

The behavior of wcsncpy is undefined if the strings overlap.

This function is the wide-character counterpart of strncpy and suffers from most of the problems that strncpy does. For example, as noted below, this function is generally a poor choice for processing text.

Function: char * strndup (const char *s, size_t size)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

This function is similar to strdup but always copies at most size bytes into the newly allocated string.

If the length of s is more than size, then strndup copies just the first size bytes and adds a closing null byte. Otherwise all bytes are copied and the string is terminated.

This function differs from strncpy in that it always terminates the destination string.

As noted below, this function is generally a poor choice for processing text.

strndup is a GNU extension.

Macro: char * strndupa (const char *s, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is similar to strndup but like strdupa it allocates the new string using alloca see Variable Size Automatic. The same advantages and limitations of strdupa are valid for strndupa, too.

This function is implemented only as a macro, just like strdupa. Just as strdupa this macro also must not be used inside the parameter list in a function call.

As noted below, this function is generally a poor choice for processing text.

strndupa is only available if GNU CC is used.

Function: char * stpncpy (char *restrict to, const char *restrict from, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is similar to stpcpy but copies always exactly size bytes into to.

If the length of from is more than size, then stpncpy copies just the first size bytes and returns a pointer to the byte directly following the one which was copied last. Note that in this case there is no null terminator written into to.

If the length of from is less than size, then stpncpy copies all of from, followed by enough null bytes to add up to size bytes in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the strncpy is used. stpncpy returns a pointer to the first written null byte.

This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

Its behavior is undefined if the strings overlap. The function is declared in string.h.

As noted below, this function is generally a poor choice for processing text.

Function: wchar_t * wcpncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is similar to wcpcpy but copies always exactly wsize wide characters into wto.

If the length of wfrom is more than size, then wcpncpy copies just the first size wide characters and returns a pointer to the wide character directly following the last non-null wide character which was copied last. Note that in this case there is no null terminator written into wto.

If the length of wfrom is less than size, then wcpncpy copies all of wfrom, followed by enough null wide characters to add up to size wide characters in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the wcsncpy is used. wcpncpy returns a pointer to the first written null wide character.

This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

Its behavior is undefined if the strings overlap.

As noted below, this function is generally a poor choice for processing text.

wcpncpy is a GNU extension.

Function: char * strncat (char *restrict to, const char *restrict from, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is like strcat except that not more than size bytes from from are appended to the end of to, and from need not be null-terminated. A single null byte is also always appended to to, so the total allocated size of to must be at least <var>size</var> + 1 bytes longer than its initial length.

The strncat function could be implemented like this:

char *
strncat (char *to, const char *from, size_t size)
{
  size_t len = strlen (to);
  memcpy (to + len, from, strnlen (from, size));
  to[len + strnlen (from, size)] = '\0';
  return to;
}

The behavior of strncat is undefined if the strings overlap.

As a companion to strncpy, strncat was designed for now-rarely-used arrays consisting of non-null bytes followed by zero or more null bytes. As noted below, this function is generally a poor choice for processing text. Also, this function has significant performance issues. See Concatenating Strings.

Function: wchar_t * wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is like wcscat except that not more than size wide characters from from are appended to the end of to, and from need not be null-terminated. A single null wide character is also always appended to to, so the total allocated size of to must be at least

wcsnlen (<var>wfrom</var>,
<var>size</var>) + 1
wide characters longer than its initial length.

The wcsncat function could be implemented like this:

wchar_t *
wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
         size_t size)
{
  size_t len = wcslen (wto);
  memcpy (wto + len, wfrom, wcsnlen (wfrom, size) * sizeof (wchar_t));
  wto[len + wcsnlen (wfrom, size)] = L'\0';
  return wto;
}

The behavior of wcsncat is undefined if the strings overlap.

As noted below, this function is generally a poor choice for processing text. Also, this function has significant performance issues. See Concatenating Strings.

Because these functions can abruptly truncate strings or wide strings, they are generally poor choices for processing text. When coping or concatening multibyte strings, they can truncate within a multibyte character so that the result is not a valid multibyte string. When combining or concatenating multibyte or wide strings, they may truncate the output after a combining character, resulting in a corrupted grapheme. They can cause bugs even when processing single-byte strings: for example, when calculating an ASCII-only user name, a truncated name can identify the wrong user.

Although some buffer overruns can be prevented by manually replacing calls to copying functions with calls to truncation functions, there are often easier and safer automatic techniques that cause buffer overruns to reliably terminate a program, such as GCC’s -fcheck-pointer-bounds and -fsanitize=address options. See Options for Debugging Your Program or GCC in Using GCC. Because truncation functions can mask application bugs that would otherwise be caught by the automatic techniques, these functions should be used only when the application’s underlying logic requires truncation.

Note: GNU programs should not truncate strings or wide strings to fit arbitrary size limits. See Writing Robust Programs in The GNU Coding Standards. Instead of string-truncation functions, it is usually better to use dynamic memory allocation (see Unconstrained Allocation) and functions such as strdup or asprintf to construct strings.


5.7 String/Array Comparison

You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. See Searching and Sorting, for an example of this.

Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are. The sign of the value indicates the relative ordering of the first part of the strings that are not equivalent: a negative value indicates that the first string is “less” than the second, while a positive value indicates that the first string is “greater”.

The most common use of these functions is to check only for equality. This is canonically done with an expression like ‘! strcmp (s1, s2).

All of these functions are declared in the header file string.h.

Function: int memcmp (const void *a1, const void *a2, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function memcmp compares the size bytes of memory beginning at a1 against the size bytes of memory beginning at a2. The value returned has the same sign as the difference between the first differing pair of bytes (interpreted as

unsigned
char
objects, then promoted to int).

If the contents of the two blocks are equal, memcmp returns  .

Function: int wmemcmp (const wchar_t *a1, const wchar_t *a2, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function wmemcmp compares the size wide characters beginning at a1 against the size wide characters beginning at a2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is a1 is smaller or larger than the corresponding wide character in a2.

If the contents of the two blocks are equal, wmemcmp returns  .

On arbitrary arrays, the memcmp function is mostly useful for testing equality. It usually isn’t meaningful to do byte-wise ordering comparisons on arrays of things other than bytes. For example, a byte-wise comparison on the bytes that make up floating-point numbers isn’t likely to tell you anything about the relationship between the values of the floating-point numbers.

wmemcmp is really only useful to compare arrays of type wchar_t since the function looks at sizeof (wchar_t) bytes at a time and this number of bytes is system dependent.

You should also be careful about using memcmp to compare objects that can contain “holes”, such as the padding inserted into structure objects to enforce alignment requirements, extra space at the end of unions, and extra bytes at the ends of strings whose length is less than their allocated size. The contents of these “holes” are indeterminate and may cause strange behavior when performing byte-wise comparisons. For more predictable results, perform an explicit component-wise comparison.

For example, given a structure type definition like:

struct foo
  {
    unsigned char tag;
    union
      {
        double f;
        long i;
        char *p;
      } value;
  };

you are better off writing a specialized comparison function to compare struct foo objects instead of comparing them with memcmp.

Function: int strcmp (const char *s1, const char *s2)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The strcmp function compares the string s1 against s2, returning a value that has the same sign as the difference between the first differing pair of bytes (interpreted as unsigned char objects, then promoted to int).

If the two strings are equal, strcmp returns  .

A consequence of the ordering used by strcmp is that if s1 is an initial substring of s2, then s1 is considered to be “less than” s2.

strcmp does not take sorting conventions of the language the strings are written in into account. To get that one has to use strcoll.

Function: int wcscmp (const wchar_t *ws1, const wchar_t *ws2)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wcscmp function compares the wide string ws1 against ws2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is ws1 is smaller or larger than the corresponding wide character in ws2.

If the two strings are equal, wcscmp returns  .

A consequence of the ordering used by wcscmp is that if ws1 is an initial substring of ws2, then ws1 is considered to be “less than” ws2.

wcscmp does not take sorting conventions of the language the strings are written in into account. To get that one has to use wcscoll.

Function: int strcasecmp (const char *s1, const char *s2)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is like strcmp, except that differences in case are ignored, and its arguments must be multibyte strings. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard "C" locale the characters Ä and ä do not match but in a locale which regards these characters as parts of the alphabet they do match.

strcasecmp is derived from BSD.

Function: int wcscasecmp (const wchar_t *ws1, const wchar_t *ws2)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is like wcscmp, except that differences in case are ignored. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard "C" locale the characters Ä and ä do not match but in a locale which regards these characters as parts of the alphabet they do match.

wcscasecmp is a GNU extension.

Function: int strncmp (const char *s1, const char *s2, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is the similar to strcmp, except that no more than size bytes are compared. In other words, if the two strings are the same in their first size bytes, the return value is zero.

Function: int wcsncmp (const wchar_t *ws1, const wchar_t *ws2, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is similar to wcscmp, except that no more than size wide characters are compared. In other words, if the two strings are the same in their first size wide characters, the return value is zero.

Function: int strncasecmp (const char *s1, const char *s2, size_t n)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is like strncmp, except that differences in case are ignored, and the compared parts of the arguments should consist of valid multibyte characters. Like strcasecmp, it is locale dependent how uppercase and lowercase characters are related.

strncasecmp is a GNU extension.

Function: int wcsncasecmp (const wchar_t *ws1, const wchar_t *s2, size_t n)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function is like wcsncmp, except that differences in case are ignored. Like wcscasecmp, it is locale dependent how uppercase and lowercase characters are related.

wcsncasecmp is a GNU extension.

Here are some examples showing the use of strcmp and strncmp (equivalent examples can be constructed for the wide character functions). These examples assume the use of the ASCII character set. (If some other character set—say, EBCDIC—is used instead, then the glyphs are associated with different numeric codes, and the return values and ordering may differ.)

strcmp ("hello", "hello")
    ⇒ 0    /* <span class="roman">These two strings are the same.</span> */
strcmp ("hello", "Hello")
    ⇒ 32   /* <span class="roman">Comparisons are case-sensitive.</span> */
strcmp ("hello", "world")
    ⇒ -15  /* <span class="roman">The byte <code>'h'</code> comes before <code>'w'</code>.</span> */
strcmp ("hello", "hello, world")
    ⇒ -44  /* <span class="roman">Comparing a null byte against a comma.</span> */
strncmp ("hello", "hello, world", 5)
    ⇒ 0    /* <span class="roman">The initial 5 bytes are the same.</span> */
strncmp ("hello, world", "hello, stupid world!!!", 5)
    ⇒ 0    /* <span class="roman">The initial 5 bytes are the same.</span> */
Function: int strverscmp (const char *s1, const char *s2)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The strverscmp function compares the string s1 against s2, considering them as holding indices/version numbers. The return value follows the same conventions as found in the strcmp function. In fact, if s1 and s2 contain no digits, strverscmp behaves like strcmp (in the sense that the sign of the result is the same).

The comparison algorithm which the strverscmp function implements differs slightly from other version-comparison algorithms. The implementation is based on a finite-state machine, whose behavior is approximated below.

  • The input strings are each split into sequences of non-digits and digits. These sequences can be empty at the beginning and end of the string. Digits are determined by the isdigit function and are thus subject to the current locale.
  • Comparison starts with a (possibly empty) non-digit sequence. The first non-equal sequences of non-digits or digits determines the outcome of the comparison.
  • Corresponding non-digit sequences in both strings are compared lexicographically if their lengths are equal. If the lengths differ, the shorter non-digit sequence is extended with the input string character immediately following it (which may be the null terminator), the other sequence is truncated to be of the same (extended) length, and these two sequences are compared lexicographically. In the last case, the sequence comparison determines the result of the function because the extension character (or some character before it) is necessarily different from the character at the same offset in the other input string.
  • For two sequences of digits, the number of leading zeros is counted (which can be zero). If the count differs, the string with more leading zeros in the digit sequence is considered smaller than the other string.
  • If the two sequences of digits have no leading zeros, they are compared as integers, that is, the string with the longer digit sequence is deemed larger, and if both sequences are of equal length, they are compared lexicographically.
  • If both digit sequences start with a zero and have an equal number of leading zeros, they are compared lexicographically if their lengths are the same. If the lengths differ, the shorter sequence is extended with the following character in its input string, and the other sequence is truncated to the same length, and both sequences are compared lexicographically (similar to the non-digit sequence case above).

The treatment of leading zeros and the tie-breaking extension characters (which in effect propagate across non-digit/digit sequence boundaries) differs from other version-comparison algorithms.

strverscmp ("no digit", "no digit")
    ⇒ 0    /* <span class="roman">same behavior as strcmp.</span> */
strverscmp ("item#99", "item#100")
    ⇒ &lt;0   /* <span class="roman">same prefix, but 99 &lt; 100.</span> */
strverscmp ("alpha1", "alpha001")
    ⇒ &gt;0   /* <span class="roman">different number of leading zeros (0 and 2).</span> */
strverscmp ("part1_f012", "part1_f01")
    ⇒ &gt;0   /* <span class="roman">lexicographical comparison with leading zeros.</span> */
strverscmp ("foo.009", "foo.0")
    ⇒ &lt;0   /* <span class="roman">different number of leading zeros (2 and 1).</span> */

strverscmp is a GNU extension.

Function: int bcmp (const void *a1, const void *a2, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is an obsolete alias for memcmp, derived from BSD.


5.8 Collation Functions

In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence ‘ll’ is treated as a single letter that is collated immediately after ‘l’.

You can use the functions strcoll and strxfrm (declared in the headers file string.h) and wcscoll and wcsxfrm (declared in the headers file wchar) to compare strings using a collation ordering appropriate for the current locale. The locale used by these functions in particular can be specified by setting the locale for the LC_COLLATE category; see Locales.

In the standard C locale, the collation sequence for strcoll is the same as that for strcmp. Similarly, wcscoll and wcscmp are the same in this situation.

Effectively, the way these functions work is by applying a mapping to transform the characters in a multibyte string to a byte sequence that represents the string’s position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale’s collating sequence.

The functions strcoll and wcscoll perform this translation implicitly, in order to do one comparison. By contrast, strxfrm and wcsxfrm perform the mapping explicitly. If you are making multiple comparisons using the same string or set of strings, it is likely to be more efficient to use strxfrm or wcsxfrm to transform all the strings just once, and subsequently compare the transformed strings with strcmp or wcscmp.

Function: int strcoll (const char *s1, const char *s2)

Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The strcoll function is similar to strcmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale). The arguments are multibyte strings.

Function: int wcscoll (const wchar_t *ws1, const wchar_t *ws2)

Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The wcscoll function is similar to wcscmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale).

Here is an example of sorting an array of strings, using strcoll to compare them. The actual sort algorithm is not written here; it comes from qsort (see Array Sort Function). The job of the code shown here is to say how to compare the strings while sorting them. (Later on in this section, we will show a way to do this more efficiently using strxfrm.)

/* <span class="roman">This is the comparison function used with <code>qsort</code>.</span> */
 
int
compare_elements (const void *v1, const void *v2)
{
  char * const *p1 = v1;
  char * const *p2 = v2;
 
  return strcoll (*p1, *p2);
}
 
/* <span class="roman">This is the entry point—the function to sort</span>
   <span class="roman">strings using the locale’s collating sequence.</span> */
 
void
sort_strings (char **array, int nstrings)
{
  /* <span class="roman">Sort <code>temp_array</code> by comparing the strings.</span> */
  qsort (array, nstrings,
         sizeof (char *), compare_elements);
}
Function: size_t strxfrm (char *restrict to, const char *restrict from, size_t size)

Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The function strxfrm transforms the multibyte string from using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array to. Up to size bytes (including a terminating null byte) are stored.

The behavior is undefined if the strings to and from overlap; see Copying Strings and Arrays.

The return value is the length of the entire transformed string. This value is not affected by the value of size, but if it is greater or equal than size, it means that the transformed string did not entirely fit in the array to. In this case, only as much of the string as actually fits was stored. To get the whole transformed string, call strxfrm again with a bigger output array.

The transformed string may be longer than the original string, and it may also be shorter.

If size is zero, no bytes are stored in to. In this case, strxfrm simply returns the number of bytes that would be the length of the transformed string. This is useful for determining what size the allocated array should be. It does not matter what to is if size is zero; to may even be a null pointer.

Function: size_t wcsxfrm (wchar_t *restrict wto, const wchar_t *wfrom, size_t size)

Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The function wcsxfrm transforms wide string wfrom using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array wto. Up to size wide characters (including a terminating null wide character) are stored.

The behavior is undefined if the strings wto and wfrom overlap; see Copying Strings and Arrays.

The return value is the length of the entire transformed wide string. This value is not affected by the value of size, but if it is greater or equal than size, it means that the transformed wide string did not entirely fit in the array wto. In this case, only as much of the wide string as actually fits was stored. To get the whole transformed wide string, call wcsxfrm again with a bigger output array.

The transformed wide string may be longer than the original wide string, and it may also be shorter.

If size is zero, no wide characters are stored in to. In this case, wcsxfrm simply returns the number of wide characters that would be the length of the transformed wide string. This is useful for determining what size the allocated array should be (remember to multiply with sizeof (wchar_t)). It does not matter what wto is if size is zero; wto may even be a null pointer.

Here is an example of how you can use strxfrm when you plan to do many comparisons. It does the same thing as the previous example, but much faster, because it has to transform each string only once, no matter how many times it is compared with other strings. Even the time needed to allocate and free storage is much less than the time we save, when there are many strings.

struct sorter { char *input; char *transformed; };
 
/* <span class="roman">This is the comparison function used with <code>qsort</code></span>
   <span class="roman">to sort an array of <code>struct sorter</code>.</span> */
 
int
compare_elements (const void *v1, const void *v2)
{
  const struct sorter *p1 = v1;
  const struct sorter *p2 = v2;
 
  return strcmp (p1-&gt;transformed, p2-&gt;transformed);
}
 
/* <span class="roman">This is the entry point—the function to sort</span>
   <span class="roman">strings using the locale’s collating sequence.</span> */
 
void
sort_strings_fast (char **array, int nstrings)
{
  struct sorter temp_array[nstrings];
  int i;
 
  /* <span class="roman">Set up <code>temp_array</code>.  Each element contains</span>
     <span class="roman">one input string and its transformed string.</span> */
  for (i = 0; i &lt; nstrings; i++)
    {
      size_t length = strlen (array[i]) * 2;
      char *transformed;
      size_t transformed_length;
 
      temp_array[i].input = array[i];
 
      /* <span class="roman">First try a buffer perhaps big enough.</span>  */
      transformed = (char *) xmalloc (length);
 
      /* <span class="roman">Transform <code>array[i]</code>.</span>  */
      transformed_length = strxfrm (transformed, array[i], length);
 
      /* <span class="roman">If the buffer was not large enough, resize it</span>
         <span class="roman">and try again.</span>  */
      if (transformed_length &gt;= length)
        {
          /* <span class="roman">Allocate the needed space. +1 for terminating</span>
             <span class="roman"><code>'\0'</code> byte.</span>  */
          transformed = (char *) xrealloc (transformed,
                                           transformed_length + 1);
 
          /* <span class="roman">The return value is not interesting because we know</span>
             <span class="roman">how long the transformed string is.</span>  */
          (void) strxfrm (transformed, array[i],
                          transformed_length + 1);
        }
 
      temp_array[i].transformed = transformed;
    }
 
  /* <span class="roman">Sort <code>temp_array</code> by comparing transformed strings.</span> */
  qsort (temp_array, nstrings,
         sizeof (struct sorter), compare_elements);
 
  /* <span class="roman">Put the elements back in the permanent array</span>
     <span class="roman">in their sorted order.</span> */
  for (i = 0; i &lt; nstrings; i++)
    array[i] = temp_array[i].input;
 
  /* <span class="roman">Free the strings we allocated.</span> */
  for (i = 0; i &lt; nstrings; i++)
    free (temp_array[i].transformed);
}

The interesting part of this code for the wide character version would look like this:

void
sort_strings_fast (wchar_t **array, int nstrings)
{
  …
      /* <span class="roman">Transform <code>array[i]</code>.</span>  */
      transformed_length = wcsxfrm (transformed, array[i], length);
 
      /* <span class="roman">If the buffer was not large enough, resize it</span>
         <span class="roman">and try again.</span>  */
      if (transformed_length &gt;= length)
        {
          /* <span class="roman">Allocate the needed space. +1 for terminating</span>
             <span class="roman"><code>L'\0'</code> wide character.</span>  */
          transformed = (wchar_t *) xrealloc (transformed,
                                              (transformed_length + 1)
                                              * sizeof (wchar_t));
 
          /* <span class="roman">The return value is not interesting because we know</span>
             <span class="roman">how long the transformed string is.</span>  */
          (void) wcsxfrm (transformed, array[i],
                          transformed_length + 1);
        }
  …

Note the additional multiplication with sizeof (wchar_t) in the realloc call.

Compatibility Note: The string collation functions are a new feature of ISO C90. Older C dialects have no equivalent feature. The wide character versions were introduced in Amendment 1 to ISO C90.


5.9 Search Functions

This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file string.h.

Function: void * memchr (const void *block, int c, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function finds the first occurrence of the byte c (converted to an unsigned char) in the initial size bytes of the object beginning at block. The return value is a pointer to the located byte, or a null pointer if no match was found.

Function: wchar_t * wmemchr (const wchar_t *block, wchar_t wc, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function finds the first occurrence of the wide character wc in the initial size wide characters of the object beginning at block. The return value is a pointer to the located wide character, or a null pointer if no match was found.

Function: void * rawmemchr (const void *block, int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Often the memchr function is used with the knowledge that the byte c is available in the memory block specified by the parameters. But this means that the size parameter is not really needed and that the tests performed with it at runtime (to check whether the end of the block is reached) are not needed.

The rawmemchr function exists for just this situation which is surprisingly frequent. The interface is similar to memchr except that the size parameter is missing. The function will look beyond the end of the block pointed to by block in case the programmer made an error in assuming that the byte c is present in the block. In this case the result is unspecified. Otherwise the return value is a pointer to the located byte.

This function is of special interest when looking for the end of a string. Since all strings are terminated by a null byte a call like

   rawmemchr (str, '\0')

will never go beyond the end of the string.

This function is a GNU extension.

Function: void * memrchr (const void *block, int c, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function memrchr is like memchr, except that it searches backwards from the end of the block defined by block and size (instead of forwards from the front).

This function is a GNU extension.

Function: char * strchr (const char *string, int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The strchr function finds the first occurrence of the byte c (converted to a char) in the string beginning at string. The return value is a pointer to the located byte, or a null pointer if no match was found.

For example,

strchr ("hello, world", 'l')
    ⇒ "llo, world"
strchr ("hello, world", '?')
    ⇒ NULL

The terminating null byte is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying zero as the value of the c argument.

When strchr returns a null pointer, it does not let you know the position of the terminating null byte it has found. If you need that information, it is better (but less portable) to use strchrnul than to search for it a second time.

Function: wchar_t * wcschr (const wchar_t *wstring, int wc)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wcschr function finds the first occurrence of the wide character wc in the wide string beginning at wstring. The return value is a pointer to the located wide character, or a null pointer if no match was found.

The terminating null wide character is considered to be part of the wide string, so you can use this function get a pointer to the end of a wide string by specifying a null wide character as the value of the wc argument. It would be better (but less portable) to use wcschrnul in this case, though.

Function: char * strchrnul (const char *string, int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

strchrnul is the same as strchr except that if it does not find the byte, it returns a pointer to string’s terminating null byte rather than a null pointer.

This function is a GNU extension.

Function: wchar_t * wcschrnul (const wchar_t *wstring, wchar_t wc)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

wcschrnul is the same as wcschr except that if it does not find the wide character, it returns a pointer to the wide string’s terminating null wide character rather than a null pointer.

This function is a GNU extension.

One useful, but unusual, use of the strchr function is when one wants to have a pointer pointing to the null byte terminating a string. This is often written in this way:

  s += strlen (s);

This is almost optimal but the addition operation duplicated a bit of the work already done in the strlen function. A better solution is this:

  s = strchr (s, '\0');

There is no restriction on the second parameter of strchr so it could very well also be zero. Those readers thinking very hard about this might now point out that the strchr function is more expensive than the strlen function since we have two abort criteria. This is right. But in the GNU C Library the implementation of strchr is optimized in a special way so that strchr actually is faster.

Function: char * strrchr (const char *string, int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function strrchr is like strchr, except that it searches backwards from the end of the string string (instead of forwards from the front).

For example,

strrchr ("hello, world", 'l')
    ⇒ "ld"
Function: wchar_t * wcsrchr (const wchar_t *wstring, wchar_t c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function wcsrchr is like wcschr, except that it searches backwards from the end of the string wstring (instead of forwards from the front).

Function: char * strstr (const char *haystack, const char *needle)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is like strchr, except that it searches haystack for a substring needle rather than just a single byte. It returns a pointer into the string haystack that is the first byte of the substring, or a null pointer if no match was found. If needle is an empty string, the function returns haystack.

For example,

strstr ("hello, world", "l")
    ⇒ "llo, world"
strstr ("hello, world", "wo")
    ⇒ "world"
Function: wchar_t * wcsstr (const wchar_t *haystack, const wchar_t *needle)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is like wcschr, except that it searches haystack for a substring needle rather than just a single wide character. It returns a pointer into the string haystack that is the first wide character of the substring, or a null pointer if no match was found. If needle is an empty string, the function returns haystack.

Function: wchar_t * wcswcs (const wchar_t *haystack, const wchar_t *needle)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

wcswcs is a deprecated alias for wcsstr. This is the name originally used in the X/Open Portability Guide before the Amendment 1 to ISO C90 was published.

Function: char * strcasestr (const char *haystack, const char *needle)

Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is like strstr, except that it ignores case in searching for the substring. Like strcasecmp, it is locale dependent how uppercase and lowercase characters are related, and arguments are multibyte strings.

For example,

strcasestr ("hello, world", "L")
    ⇒ "llo, world"
strcasestr ("hello, World", "wo")
    ⇒ "World"
Function: void * memmem (const void *haystack, size_t haystack-len,
const void *needle, size_t needle-len)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is like strstr, but needle and haystack are byte arrays rather than strings. needle-len is the length of needle and haystack-len is the length of haystack.

This function is a GNU extension.

Function: size_t strspn (const char *string, const char *skipset)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The strspn (“string span”) function returns the length of the initial substring of string that consists entirely of bytes that are members of the set specified by the string skipset. The order of the bytes in skipset is not important.

For example,

strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
    ⇒ 5

In a multibyte string, characters consisting of more than one byte are not treated as single entities. Each byte is treated separately. The function is not locale-dependent.

Function: size_t wcsspn (const wchar_t *wstring, const wchar_t *skipset)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wcsspn (“wide character string span”) function returns the length of the initial substring of wstring that consists entirely of wide characters that are members of the set specified by the string skipset. The order of the wide characters in skipset is not important.

Function: size_t strcspn (const char *string, const char *stopset)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The strcspn (“string complement span”) function returns the length of the initial substring of string that consists entirely of bytes that are not members of the set specified by the string stopset. (In other words, it returns the offset of the first byte in string that is a member of the set stopset.)

For example,

strcspn ("hello, world", " \t\n,.;!?")
    ⇒ 5

In a multibyte string, characters consisting of more than one byte are not treated as a single entities. Each byte is treated separately. The function is not locale-dependent.

Function: size_t wcscspn (const wchar_t *wstring, const wchar_t *stopset)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wcscspn (“wide character string complement span”) function returns the length of the initial substring of wstring that consists entirely of wide characters that are not members of the set specified by the string stopset. (In other words, it returns the offset of the first wide character in string that is a member of the set stopset.)

Function: char * strpbrk (const char *string, const char *stopset)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The strpbrk (“string pointer break”) function is related to strcspn, except that it returns a pointer to the first byte in string that is a member of the set stopset instead of the length of the initial substring. It returns a null pointer if no such byte from stopset is found.

For example,

strpbrk ("hello, world", " \t\n,.;!?")
    ⇒ ", world"

In a multibyte string, characters consisting of more than one byte are not treated as single entities. Each byte is treated separately. The function is not locale-dependent.

Function: wchar_t * wcspbrk (const wchar_t *wstring, const wchar_t *stopset)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The wcspbrk (“wide character string pointer break”) function is related to wcscspn, except that it returns a pointer to the first wide character in wstring that is a member of the set stopset instead of the length of the initial substring. It returns a null pointer if no such wide character from stopset is found.

5.9.1 Compatibility String Search Functions

Function: char * index (const char *string, int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

index is another name for strchr; they are exactly the same. New code should always use strchr since this name is defined in ISO C while index is a BSD invention which never was available on System V derived systems.

Function: char * rindex (const char *string, int c)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

rindex is another name for strrchr; they are exactly the same. New code should always use strrchr since this name is defined in ISO C while rindex is a BSD invention which never was available on System V derived systems.


5.10 Finding Tokens in a String

It’s fairly common for programs to have a need to do some simple kinds of lexical analysis and parsing, such as splitting a command string up into tokens. You can do this with the strtok function, declared in the header file string.h.

Function: char * strtok (char *restrict newstring, const char *restrict delimiters)

Preliminary: | MT-Unsafe race:strtok | AS-Unsafe | AC-Safe | See POSIX Safety Concepts.

A string can be split into tokens by making a series of calls to the function strtok.

The string to be split up is passed as the newstring argument on the first call only. The strtok function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same string are indicated by passing a null pointer as the newstring argument. Calling strtok with another non-null newstring argument reinitializes the state information. It is guaranteed that no other library function ever calls strtok behind your back (which would mess up this internal state information).

The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial bytes that are members of this set are discarded. The first byte that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next byte that is a member of the delimiter set. This byte in the original string newstring is overwritten by a null byte, and the pointer to the beginning of the token in newstring is returned.

On the next call to strtok, the searching begins at the next byte beyond the one that marked the end of the previous token. Note that the set of delimiters delimiters do not have to be the same on every call in a series of calls to strtok.

If the end of the string newstring is reached, or if the remainder of string consists only of delimiter bytes, strtok returns a null pointer.

In a multibyte string, characters consisting of more than one byte are not treated as single entities. Each byte is treated separately. The function is not locale-dependent.

Function: wchar_t * wcstok (wchar_t *newstring, const wchar_t *delimiters, wchar_t **save_ptr)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

A string can be split into tokens by making a series of calls to the function wcstok.

The string to be split up is passed as the newstring argument on the first call only. The wcstok function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same wide string are indicated by passing a null pointer as the newstring argument, which causes the pointer previously stored in save_ptr to be used instead.

The delimiters argument is a wide string that specifies a set of delimiters that may surround the token being extracted. All the initial wide characters that are members of this set are discarded. The first wide character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next wide character that is a member of the delimiter set. This wide character in the original wide string newstring is overwritten by a null wide character, the pointer past the overwritten wide character is saved in save_ptr, and the pointer to the beginning of the token in newstring is returned.

On the next call to wcstok, the searching begins at the next wide character beyond the one that marked the end of the previous token. Note that the set of delimiters delimiters do not have to be the same on every call in a series of calls to wcstok.

If the end of the wide string newstring is reached, or if the remainder of string consists only of delimiter wide characters, wcstok returns a null pointer.

Warning: Since strtok and wcstok alter the string they is parsing, you should always copy the string to a temporary buffer before parsing it with strtok/wcstok (see Copying Strings and Arrays). If you allow strtok or wcstok to modify a string that came from another part of your program, you are asking for trouble; that string might be used for other purposes after strtok or wcstok has modified it, and it would not have the expected value.

The string that you are operating on might even be a constant. Then when strtok or wcstok tries to modify it, your program will get a fatal signal for writing in read-only memory. See Program Error Signals. Even if the operation of strtok or wcstok would not require a modification of the string (e.g., if there is exactly one token) the string can (and in the GNU C Library case will) be modified.

This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.

The function strtok is not reentrant, whereas wcstok is. See Nonreentrancy, for a discussion of where and why reentrancy is important.

Here is a simple example showing the use of strtok.

#include &lt;string.h&gt;
#include &lt;stddef.h&gt;
 
…
 
const char string[] = "words separated by spaces -- and, punctuation!";
const char delimiters[] = " .,;:!-";
char *token, *cp;
 
…
 
cp = strdupa (string);                /* Make writable copy.  */
token = strtok (cp, delimiters);      /* token =&gt; "words" */
token = strtok (NULL, delimiters);    /* token =&gt; "separated" */
token = strtok (NULL, delimiters);    /* token =&gt; "by" */
token = strtok (NULL, delimiters);    /* token =&gt; "spaces" */
token = strtok (NULL, delimiters);    /* token =&gt; "and" */
token = strtok (NULL, delimiters);    /* token =&gt; "punctuation" */
token = strtok (NULL, delimiters);    /* token =&gt; NULL */

The GNU C Library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy. They are not available available for wide strings.

Function: char * strtok_r (char *newstring, const char *delimiters, char **save_ptr)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Just like strtok, this function splits the string into several tokens which can be accessed by successive calls to strtok_r. The difference is that, as in wcstok, the information about the next token is stored in the space pointed to by the third argument, save_ptr, which is a pointer to a string pointer. Calling strtok_r with a null pointer for newstring and leaving save_ptr between the calls unchanged does the job without hindering reentrancy.

This function is defined in POSIX.1 and can be found on many systems which support multi-threading.

Function: char * strsep (char **string_ptr, const char *delimiter)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function has a similar functionality as strtok_r with the newstring argument replaced by the save_ptr argument. The initialization of the moving pointer has to be done by the user. Successive calls to strsep move the pointer along the tokens separated by delimiter, returning the address of the next token and updating string_ptr to point to the beginning of the next token.

One difference between strsep and strtok_r is that if the input string contains more than one byte from delimiter in a row strsep returns an empty string for each pair of bytes from delimiter. This means that a program normally should test for strsep returning an empty string before processing it.

This function was introduced in 4.3BSD and therefore is widely available.

Here is how the above example looks like when strsep is used.

#include &lt;string.h&gt;
#include &lt;stddef.h&gt;
 
…
 
const char string[] = "words separated by spaces -- and, punctuation!";
const char delimiters[] = " .,;:!-";
char *running;
char *token;
 
…
 
running = strdupa (string);
token = strsep (&amp;running, delimiters);    /* token =&gt; "words" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "separated" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "by" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "spaces" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "and" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "punctuation" */
token = strsep (&amp;running, delimiters);    /* token =&gt; "" */
token = strsep (&amp;running, delimiters);    /* token =&gt; NULL */
Function: char * basename (const char *filename)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The GNU version of the basename function returns the last component of the path in filename. This function is the preferred usage, since it does not modify the argument, filename, and respects trailing slashes. The prototype for basename can be found in string.h. Note, this function is overridden by the XPG version, if libgen.h is included.

Example of using GNU basename:

#include &lt;string.h&gt;
 
int
main (int argc, char *argv[])
{
  char *prog = basename (argv[0]);
 
  if (argc &lt; 2)
    {
      fprintf (stderr, "Usage %s &lt;arg&gt;\n", prog);
      exit (1);
    }
 
  …
}

Portability Note: This function may produce different results on different systems.

Function: char * basename (char *path)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This is the standard XPG defined basename. It is similar in spirit to the GNU version, but may modify the path by removing trailing ’/’ bytes. If the path is made up entirely of ’/’ bytes, then "/" will be returned. Also, if path is NULL or an empty string, then "." is returned. The prototype for the XPG version can be found in libgen.h.

Example of using XPG basename:

#include &lt;libgen.h&gt;
 
int
main (int argc, char *argv[])
{
  char *prog;
  char *path = strdupa (argv[0]);
 
  prog = basename (path);
 
  if (argc &lt; 2)
    {
      fprintf (stderr, "Usage %s &lt;arg&gt;\n", prog);
      exit (1);
    }
 
  …
 
}
Function: char * dirname (char *path)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The dirname function is the compliment to the XPG version of basename. It returns the parent directory of the file specified by path. If path is NULL, an empty string, or contains no ’/’ bytes, then "." is returned. The prototype for this function can be found in libgen.h.


5.11 Erasing Sensitive Data

Sensitive data, such as cryptographic keys, should be erased from memory after use, to reduce the risk that a bug will expose it to the outside world. However, compiler optimizations may determine that an erasure operation is “unnecessary,” and remove it from the generated code, because no correct program could access the variable or heap object containing the sensitive data after it’s deallocated. Since erasure is a precaution against bugs, this optimization is inappropriate.

The function explicit_bzero erases a block of memory, and guarantees that the compiler will not remove the erasure as “unnecessary.”

#include &lt;string.h&gt;
 
extern void encrypt (const char *key, const char *in,
                     char *out, size_t n);
extern void genkey (const char *phrase, char *key);
 
void encrypt_with_phrase (const char *phrase, const char *in,
                          char *out, size_t n)
{
  char key[16];
  genkey (phrase, key);
  encrypt (key, in, out, n);
  explicit_bzero (key, 16);
}

In this example, if memset, bzero, or a hand-written loop had been used, the compiler might remove them as “unnecessary.”

Warning: explicit_bzero does not guarantee that sensitive data is completely erased from the computer’s memory. There may be copies in temporary storage areas, such as registers and “scratch” stack space; since these are invisible to the source code, a library function cannot erase them.

Also, explicit_bzero only operates on RAM. If a sensitive data object never needs to have its address taken other than to call explicit_bzero, it might be stored entirely in CPU registers until the call to explicit_bzero. Then it will be copied into RAM, the copy will be erased, and the original will remain intact. Data in RAM is more likely to be exposed by a bug than data in registers, so this creates a brief window where the data is at greater risk of exposure than it would have been if the program didn’t try to erase it at all.

Declaring sensitive variables as volatile will make both the above problems worse; a volatile variable will be stored in memory for its entire lifetime, and the compiler will make more copies of it than it would otherwise have. Attempting to erase a normal variable “by hand” through a volatile-qualified pointer doesn’t work at all—because the variable itself is not volatile, some compilers will ignore the qualification on the pointer and remove the erasure anyway.

Having said all that, in most situations, using explicit_bzero is better than not using it. At present, the only way to do a more thorough job is to write the entire sensitive operation in assembly language. We anticipate that future compilers will recognize calls to explicit_bzero and take appropriate steps to erase all the copies of the affected data, whereever they may be.

Function: void explicit_bzero (void *block, size_t len)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

explicit_bzero writes zero into len bytes of memory beginning at block, just as bzero would. The zeroes are always written, even if the compiler could determine that this is “unnecessary” because no correct program could read them back.

Note: The only optimization that explicit_bzero disables is removal of “unnecessary” writes to memory. The compiler can perform all the other optimizations that it could for a call to memset. For instance, it may replace the function call with inline memory writes, and it may assume that block cannot be a null pointer.

Portability Note: This function first appeared in OpenBSD 5.5 and has not been standardized. Other systems may provide the same functionality under a different name, such as explicit_memset, memset_s, or SecureZeroMemory.

The GNU C Library declares this function in string.h, but on other systems it may be in strings.h instead.


5.12 strfry

The function below addresses the perennial programming quandary: “How do I take good data in string form and painlessly turn it into garbage?” This is actually a fairly simple task for C programmers who do not use the GNU C Library string functions, but for programs based on the GNU C Library, the strfry function is the preferred method for destroying string data.

The prototype for this function is in string.h.

Function: char * strfry (char *string)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

strfry creates a pseudorandom anagram of a string, replacing the input with the anagram in place. For each position in the string, strfry swaps it with a position in the string selected at random (from a uniform distribution). The two positions may be the same.

The return value of strfry is always string.

Portability Note: This function is unique to the GNU C Library.


5.13 Trivial Encryption

The memfrob function converts an array of data to something unrecognizable and back again. It is not encryption in its usual sense since it is easy for someone to convert the encrypted data back to clear text. The transformation is analogous to Usenet’s “Rot13” encryption method for obscuring offensive jokes from sensitive eyes and such. Unlike Rot13, memfrob works on arbitrary binary data, not just text.

For true encryption, See Cryptographic Functions.

This function is declared in string.h.

Function: void * memfrob (void *mem, size_t length)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

memfrob transforms (frobnicates) each byte of the data structure at mem, which is length bytes long, by bitwise exclusive oring it with binary 00101010. It does the transformation in place and its return value is always mem.

Note that memfrob a second time on the same data structure returns it to its original state.

This is a good function for hiding information from someone who doesn’t want to see it or doesn’t want to see it very much. To really prevent people from retrieving the information, use stronger encryption such as that described in See Cryptographic Functions.

Portability Note: This function is unique to the GNU C Library.


5.14 Encode Binary Data

To store or transfer binary data in environments which only support text one has to encode the binary data by mapping the input bytes to bytes in the range allowed for storing or transferring. SVID systems (and nowadays XPG compliant systems) provide minimal support for this task.

Function: char * l64a (long int n)

Preliminary: | MT-Unsafe race:l64a | AS-Unsafe | AC-Safe | See POSIX Safety Concepts.

This function encodes a 32-bit input value using bytes from the basic character set. It returns a pointer to a 7 byte buffer which contains an encoded version of n. To encode a series of bytes the user must copy the returned string to a destination buffer. It returns the empty string if n is zero, which is somewhat bizarre but mandated by the standard.
Warning: Since a static buffer is used this function should not be used in multi-threaded programs. There is no thread-safe alternative to this function in the C library.
Compatibility Note: The XPG standard states that the return value of l64a is undefined if n is negative. In the GNU implementation, l64a treats its argument as unsigned, so it will return a sensible encoding for any nonzero n; however, portable programs should not rely on this.

To encode a large buffer l64a must be called in a loop, once for each 32-bit word of the buffer. For example, one could do something like this:

char *
encode (const void *buf, size_t len)
{
  /* <span class="roman">We know in advance how long the buffer has to be.</span> */
  unsigned char *in = (unsigned char *) buf;
  char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
  char *cp = out, *p;
 
  /* <span class="roman">Encode the length.</span> */
  /* <span class="roman">Using ‘htonl’ is necessary so that the data can be</span>
     <span class="roman">decoded even on machines with different byte order.</span>
     <span class="roman">‘l64a’ can return a string shorter than 6 bytes, so </span>
     <span class="roman">we pad it with encoding of 0 (</span>'.'<span class="roman">) at the end by </span>
     <span class="roman">hand.</span> */
 
  p = stpcpy (cp, l64a (htonl (len)));
  cp = mempcpy (p, "......", 6 - (p - cp));
 
  while (len &gt; 3)
    {
      unsigned long int n = *in++;
      n = (n &lt;&lt; 8) | *in++;
      n = (n &lt;&lt; 8) | *in++;
      n = (n &lt;&lt; 8) | *in++;
      len -= 4;
      p = stpcpy (cp, l64a (htonl (n)));
      cp = mempcpy (p, "......", 6 - (p - cp));
    }
  if (len &gt; 0)
    {
      unsigned long int n = *in++;
      if (--len &gt; 0)
        {
          n = (n &lt;&lt; 8) | *in++;
          if (--len &gt; 0)
            n = (n &lt;&lt; 8) | *in;
        }
      cp = stpcpy (cp, l64a (htonl (n)));
    }
  *cp = '\0';
  return out;
}

It is strange that the library does not provide the complete functionality needed but so be it.

To decode data produced with l64a the following function should be used.

Function: long int a64l (const char *string)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The parameter string should contain a string which was produced by a call to l64a. The function processes at least 6 bytes of this string, and decodes the bytes it finds according to the table below. It stops decoding when it finds a byte not in the table, rather like atoi; if you have a buffer which has been broken into lines, you must be careful to skip over the end-of-line bytes.

The decoded number is returned as a long int value.

The l64a and a64l functions use a base 64 encoding, in which each byte of an encoded string represents six bits of an input word. These symbols are used for the base 64 digits:

01234567
0./ 12345
86789ABCD
16EFGHIJKL
24MNOPQRST
32UVWXYZab
40cdefghij
48klmnopqr
56stuvwxyz

This encoding scheme is not standard. There are some other encoding methods which are much more widely used (UU encoding, MIME encoding). Generally, it is better to use one of these encodings.


5.15 Argz and Envz Vectors

argz vectors are vectors of strings in a contiguous block of memory, each element separated from its neighbors by null bytes ('\0').

Envz vectors are an extension of argz vectors where each element is a name-value pair, separated by a '=' byte (as in a Unix environment).


5.15.1 Argz Functions

Each argz vector is represented by a pointer to the first element, of type char *, and a size, of type size_t, both of which can be initialized to   to represent an empty argz vector. All argz functions accept either a pointer and a size argument, or pointers to them, if they will be modified.

The argz functions use malloc/realloc to allocate/grow argz vectors, and so any argz vector created using these functions may be freed by using free; conversely, any argz function that may grow a string expects that string to have been allocated using malloc (those argz functions that only examine their arguments or modify them in place will work on any sort of memory). See Unconstrained Allocation.

All argz functions that do memory allocation have a return type of error_t, and return   for success, and ENOMEM if an allocation error occurs.

These functions are declared in the standard include file argz.h.

Function: error_t argz_create (char *const argv[], char **argz, size_t *argz_len)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The argz_create function converts the Unix-style argument vector argv (a vector of pointers to normal C strings, terminated by (char *)0; see Program Arguments) into an argz vector with the same elements, which is returned in argz and argz_len.

Function: error_t argz_create_sep (const char *string, int sep, char **argz, size_t *argz_len)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The argz_create_sep function converts the string string into an argz vector (returned in argz and argz_len) by splitting it into elements at every occurrence of the byte sep.

Function: size_t argz_count (const char *argz, size_t argz_len)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

Returns the number of elements in the argz vector argz and argz_len.

Function: void argz_extract (const char *argz, size_t argz_len, char **argv)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The argz_extract function converts the argz vector argz and argz_len into a Unix-style argument vector stored in argv, by putting pointers to every element in argz into successive positions in argv, followed by a terminator of  . Argv must be pre-allocated with enough space to hold all the elements in argz plus the terminating (char *)0 ((argz_count (<var>argz</var>, <var>argz_len</var>) + 1) * sizeof (char *) bytes should be enough). Note that the string pointers stored into argv point into argz—they are not copies—and so argz must be copied if it will be changed while argv is still active. This function is useful for passing the elements in argz to an exec function (see Executing a File).

Function: void argz_stringify (char *argz, size_t len, int sep)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The argz_stringify converts argz into a normal string with the elements separated by the byte sep, by replacing each '\0' inside argz (except the last one, which terminates the string) with sep. This is handy for printing argz in a readable manner.

Function: error_t argz_add (char **argz, size_t *argz_len, const char *str)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The argz_add function adds the string str to the end of the argz vector *<var>argz</var>, and updates *<var>argz</var> and *<var>argz_len</var> accordingly.

Function: error_t argz_add_sep (char **argz, size_t *argz_len, const char *str, int delim)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The argz_add_sep function is similar to argz_add, but str is split into separate elements in the result at occurrences of the byte delim. This is useful, for instance, for adding the components of a Unix search path to an argz vector, by using a value of ':' for delim.

Function: error_t argz_append (char **argz, size_t *argz_len, const char *buf, size_t buf_len)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The argz_append function appends buf_len bytes starting at buf to the argz vector *<var>argz</var>, reallocating *<var>argz</var> to accommodate it, and adding buf_len to *<var>argz_len</var>.

Function: void argz_delete (char **argz, size_t *argz_len, char *entry)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

If entry points to the beginning of one of the elements in the argz vector *<var>argz</var>, the argz_delete function will remove this entry and reallocate *<var>argz</var>, modifying *<var>argz</var> and *<var>argz_len</var> accordingly. Note that as destructive argz functions usually reallocate their argz argument, pointers into argz vectors such as entry will then become invalid.

Function: error_t argz_insert (char **argz, size_t *argz_len, char *before, const char *entry)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The argz_insert function inserts the string entry into the argz vector *<var>argz</var> at a point just before the existing element pointed to by before, reallocating *<var>argz</var> and updating *<var>argz</var> and *<var>argz_len</var>. If before is  , entry is added to the end instead (as if by argz_add). Since the first element is in fact the same as *<var>argz</var>, passing in *<var>argz</var> as the value of before will result in entry being inserted at the beginning.

Function: char * argz_next (const char *argz, size_t argz_len, const char *entry)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The argz_next function provides a convenient way of iterating over the elements in the argz vector argz. It returns a pointer to the next element in argz after the element entry, or   if there are no elements following entry. If entry is  , the first element of argz is returned.

This behavior suggests two styles of iteration:

    char *entry = 0;
    while ((entry = argz_next (<var>argz</var>, <var>argz_len</var>, entry)))
      <var>action</var>;

(the double parentheses are necessary to make some C compilers shut up about what they consider a questionable while-test) and:

    char *entry;
    for (entry = <var>argz</var>;
         entry;
         entry = argz_next (<var>argz</var>, <var>argz_len</var>, entry))
      <var>action</var>;

Note that the latter depends on argz having a value of   if it is empty (rather than a pointer to an empty block of memory); this invariant is maintained for argz vectors created by the functions here.

Function: error_t argz_replace (char **argzsize_t *argz_len, const char *str, const char *with, unsigned *replace_count)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

Replace any occurrences of the string str in argz with with, reallocating argz as necessary. If replace_count is non-zero, *<var>replace_count</var> will be incremented by the number of replacements performed.


5.15.2 Envz Functions

Envz vectors are just argz vectors with additional constraints on the form of each element; as such, argz functions can also be used on them, where it makes sense.

Each element in an envz vector is a name-value pair, separated by a '=' byte; if multiple '=' bytes are present in an element, those after the first are considered part of the value, and treated like all other non-'\0' bytes.

If no '=' bytes are present in an element, that element is considered the name of a “null” entry, as distinct from an entry with an empty value: envz_get will return   if given the name of null entry, whereas an entry with an empty value would result in a value of ""; envz_entry will still find such entries, however. Null entries can be removed with the envz_strip function.

As with argz functions, envz functions that may allocate memory (and thus fail) have a return type of error_t, and return either   or ENOMEM.

These functions are declared in the standard include file envz.h.

Function: char * envz_entry (const char *envz, size_t envz_len, const char *name)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The envz_entry function finds the entry in envz with the name name, and returns a pointer to the whole entry—that is, the argz element which begins with name followed by a '=' byte. If there is no entry with that name,   is returned.

Function: char * envz_get (const char *envz, size_t envz_len, const char *name)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The envz_get function finds the entry in envz with the name name (like envz_entry), and returns a pointer to the value portion of that entry (following the '='). If there is no entry with that name (or only a null entry),   is returned.

Function: error_t envz_add (char **envz, size_t *envz_len, const char *name, const char *value)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The envz_add function adds an entry to *<var>envz</var> (updating *<var>envz</var> and *<var>envz_len</var>) with the name name, and value value. If an entry with the same name already exists in envz, it is removed first. If value is  , then the new entry will be the special null type of entry (mentioned above).

Function: error_t envz_merge (char **envz, size_t *envz_len, const char *envz2, size_t envz2_len, int override)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The envz_merge function adds each entry in envz2 to envz, as if with envz_add, updating *<var>envz</var> and *<var>envz_len</var>. If override is true, then values in envz2 will supersede those with the same name in envz, otherwise not.

Null entries are treated just like other entries in this respect, so a null entry in envz can prevent an entry of the same name in envz2 from being added to envz, if override is false.

Function: void envz_strip (char **envz, size_t *envz_len)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The envz_strip function removes any null entries from envz, updating *<var>envz</var> and *<var>envz_len</var>.

Function: void envz_remove (char **envz, size_t *envz_len, const char *name)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The envz_remove function removes an entry named name from envz, updating *<var>envz</var> and *<var>envz_len</var>.


Next: , Previous: , Up: Top   [Contents][Index]

6 Character Set Handling

Character sets used in the early days of computing had only six, seven, or eight bits for each character: there was never a case where more than eight bits (one byte) were used to represent a single character. The limitations of this approach became more apparent as more people grappled with non-Roman character sets, where not all the characters that make up a language’s character set can be represented by 2^8 choices. This chapter shows the functionality that was added to the C library to support multiple character sets.


6.1 Introduction to Extended Characters

A variety of solutions are available to overcome the differences between character sets with a 1:1 relation between bytes and characters and character sets with ratios of 2:1 or 4:1. The remainder of this section gives a few examples to help understand the design decisions made while developing the functionality of the C library.

A distinction we have to make right away is between internal and external representation. Internal representation means the representation used by a program while keeping the text in memory. External representations are used when text is stored or transmitted through some communication channel. Examples of external representations include files waiting in a directory to be read and parsed.

Traditionally there has been no difference between the two representations. It was equally comfortable and useful to use the same single-byte representation internally and externally. This comfort level decreases with more and larger character sets.

One of the problems to overcome with the internal representation is handling text that is externally encoded using different character sets. Assume a program that reads two texts and compares them using some metric. The comparison can be usefully done only if the texts are internally kept in a common format.

For such a common format (= character set) eight bits are certainly no longer enough. So the smallest entity will have to grow: wide characters will now be used. Instead of one byte per character, two or four will be used instead. (Three are not good to address in memory and more than four bytes seem not to be necessary).

As shown in some other part of this manual, a completely new family has been created of functions that can handle wide character texts in memory. The most commonly used character sets for such internal wide character representations are Unicode and ISO 10646 (also known as UCS for Universal Character Set). Unicode was originally planned as a 16-bit character set; whereas, ISO 10646 was designed to be a 31-bit large code space. The two standards are practically identical. They have the same character repertoire and code table, but Unicode specifies added semantics. At the moment, only characters in the first 0x10000 code positions (the so-called Basic Multilingual Plane, BMP) have been assigned, but the assignment of more specialized characters outside this 16-bit space is already in progress. A number of encodings have been defined for Unicode and ISO 10646 characters: UCS-2 is a 16-bit word that can only represent characters from the BMP, UCS-4 is a 32-bit word than can represent any Unicode and ISO 10646 character, UTF-8 is an ASCII compatible encoding where ASCII characters are represented by ASCII bytes and non-ASCII characters by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension of UCS-2 in which pairs of certain UCS-2 words can be used to encode non-BMP characters up to 0x10ffff.

To represent wide characters the char type is not suitable. For this reason the ISO C standard introduces a new type that is designed to keep one character of a wide character string. To maintain the similarity there is also a type corresponding to int for those functions that take a single wide character.

Data type: wchar_t

This data type is used as the base type for wide character strings. In other words, arrays of objects of this type are the equivalent of char[] for multibyte character strings. The type is defined in stddef.h.

The ISO C90 standard, where wchar_t was introduced, does not say anything specific about the representation. It only requires that this type is capable of storing all elements of the basic character set. Therefore it would be legitimate to define wchar_t as char, which might make sense for embedded systems.

But in the GNU C Library wchar_t is always 32 bits wide and, therefore, capable of representing all UCS-4 values and, therefore, covering all of ISO 10646. Some Unix systems define wchar_t as a 16-bit type and thereby follow Unicode very strictly. This definition is perfectly fine with the standard, but it also means that to represent all characters from Unicode and ISO 10646 one has to use UTF-16 surrogate characters, which is in fact a multi-wide-character encoding. But resorting to multi-wide-character encoding contradicts the purpose of the wchar_t type.

Data type: wint_t

wint_t is a data type used for parameters and variables that contain a single wide character. As the name suggests this type is the equivalent of int when using the normal char strings. The types wchar_t and wint_t often have the same representation if their size is 32 bits wide but if wchar_t is defined as char the type wint_t must be defined as int due to the parameter promotion.

This type is defined in wchar.h and was introduced in Amendment 1 to ISO C90.

As there are for the char data type macros are available for specifying the minimum and maximum value representable in an object of type wchar_t.

Macro: wint_t WCHAR_MIN

The macro WCHAR_MIN evaluates to the minimum value representable by an object of type wint_t.

This macro was introduced in Amendment 1 to ISO C90.

Macro: wint_t WCHAR_MAX

The macro WCHAR_MAX evaluates to the maximum value representable by an object of type wint_t.

This macro was introduced in Amendment 1 to ISO C90.

Another special wide character value is the equivalent to EOF.

Macro: wint_t WEOF

The macro WEOF evaluates to a constant expression of type wint_t whose value is different from any member of the extended character set.

WEOF need not be the same value as EOF and unlike EOF it also need not be negative. In other words, sloppy code like

{
  int c;
  …
  while ((c = getc (fp)) &lt; 0)
    …
}

has to be rewritten to use WEOF explicitly when wide characters are used:

{
  wint_t c;
  …
  while ((c = wgetc (fp)) != WEOF)
    …
}

This macro was introduced in Amendment 1 to ISO C90 and is defined in wchar.h.

These internal representations present problems when it comes to storage and transmittal. Because each single wide character consists of more than one byte, they are affected by byte-ordering. Thus, machines with different endianesses would see different values when accessing the same data. This byte ordering concern also applies for communication protocols that are all byte-based and therefore require that the sender has to decide about splitting the wide character in bytes. A last (but not least important) point is that wide characters often require more storage space than a customized byte-oriented character set.

For all the above reasons, an external encoding that is different from the internal encoding is often used if the latter is UCS-2 or UCS-4. The external encoding is byte-based and can be chosen appropriately for the environment and for the texts to be handled. A variety of different character sets can be used for this external encoding (information that will not be exhaustively presented here–instead, a description of the major groups will suffice). All of the ASCII-based character sets fulfill one requirement: they are "filesystem safe." This means that the character '/' is used in the encoding only to represent itself. Things are a bit different for character sets like EBCDIC (Extended Binary Coded Decimal Interchange Code, a character set family used by IBM), but if the operating system does not understand EBCDIC directly the parameters-to-system calls have to be converted first anyhow.

  • The simplest character sets are single-byte character sets. There can be only up to 256 characters (for 8 bit character sets), which is not sufficient to cover all languages but might be sufficient to handle a specific text. Handling of a 8 bit character sets is simple. This is not true for other kinds presented later, and therefore, the application one uses might require the use of 8 bit character sets.
  • The ISO 2022 standard defines a mechanism for extended character sets where one character can be represented by more than one byte. This is achieved by associating a state with the text. Characters that can be used to change the state can be embedded in the text. Each byte in the text might have a different interpretation in each state. The state might even influence whether a given byte stands for a character on its own or whether it has to be combined with some more bytes.

    In most uses of ISO 2022 the defined character sets do not allow state changes that cover more than the next character. This has the big advantage that whenever one can identify the beginning of the byte sequence of a character one can interpret a text correctly. Examples of character sets using this policy are the various EUC character sets (used by Sun’s operating systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN) or Shift_JIS (SJIS, a Japanese encoding).

    But there are also character sets using a state that is valid for more than one character and has to be changed by another byte sequence. Examples for this are ISO-2022-JP, ISO-2022-KR, and ISO-2022-CN.

  • Early attempts to fix 8 bit character sets for other languages using the Roman alphabet lead to character sets like ISO 6937. Here bytes representing characters like the acute accent do not produce output themselves: one has to combine them with other characters to get the desired result. For example, the byte sequence 0xc2 0x61 (non-spacing acute accent, followed by lower-case ‘a’) to get the “small a with acute” character. To get the acute accent character on its own, one has to write 0xc2 0x20 (the non-spacing acute followed by a space).

    Character sets like ISO 6937 are used in some embedded systems such as teletex.

  • Instead of converting the Unicode or ISO 10646 text used internally, it is often also sufficient to simply use an encoding different than UCS-2/UCS-4. The Unicode and ISO 10646 standards even specify such an encoding: UTF-8. This encoding is able to represent all of ISO 10646 31 bits in a byte string of length one to six.

    There were a few other attempts to encode ISO 10646 such as UTF-7, but UTF-8 is today the only encoding that should be used. In fact, with any luck UTF-8 will soon be the only external encoding that has to be supported. It proves to be universally usable and its only disadvantage is that it favors Roman languages by making the byte string representation of other scripts (Cyrillic, Greek, Asian scripts) longer than necessary if using a specific character set for these scripts. Methods like the Unicode compression scheme can alleviate these problems.

The question remaining is: how to select the character set or encoding to use. The answer: you cannot decide about it yourself, it is decided by the developers of the system or the majority of the users. Since the goal is interoperability one has to use whatever the other people one works with use. If there are no constraints, the selection is based on the requirements the expected circle of users will have. In other words, if a project is expected to be used in only, say, Russia it is fine to use KOI8-R or a similar character set. But if at the same time people from, say, Greece are participating one should use a character set that allows all people to collaborate.

The most widely useful solution seems to be: go with the most general character set, namely ISO 10646. Use UTF-8 as the external encoding and problems about users not being able to use their own language adequately are a thing of the past.

One final comment about the choice of the wide character representation is necessary at this point. We have said above that the natural choice is using Unicode or ISO 10646. This is not required, but at least encouraged, by the ISO C standard. The standard defines at least a macro __STDC_ISO_10646__ that is only defined on systems where the wchar_t type encodes ISO 10646 characters. If this symbol is not defined one should avoid making assumptions about the wide character representation. If the programmer uses only the functions provided by the C library to handle wide character strings there should be no compatibility problems with other systems.


6.2 Overview about Character Handling Functions

A Unix C library contains three different sets of functions in two families to handle character set conversion. One of the function families (the most commonly used) is specified in the ISO C90 standard and, therefore, is portable even beyond the Unix world. Unfortunately this family is the least useful one. These functions should be avoided whenever possible, especially when developing libraries (as opposed to applications).

The second family of functions got introduced in the early Unix standards (XPG2) and is still part of the latest and greatest Unix standard: Unix 98. It is also the most powerful and useful set of functions. But we will start with the functions defined in Amendment 1 to ISO C90.


6.3 Restartable Multibyte Conversion Functions

The ISO C standard defines functions to convert strings from a multibyte representation to wide character strings. There are a number of peculiarities:

  • The character set assumed for the multibyte encoding is not specified as an argument to the functions. Instead the character set specified by the LC_CTYPE category of the current locale is used; see Locale Categories.
  • The functions handling more than one character at a time require NUL terminated strings as the argument (i.e., converting blocks of text does not work unless one can add a NUL byte at an appropriate place). The GNU C Library contains some extensions to the standard that allow specifying a size, but basically they also expect terminated strings.

Despite these limitations the ISO C functions can be used in many contexts. In graphical user interfaces, for instance, it is not uncommon to have functions that require text to be displayed in a wide character string if the text is not simple ASCII. The text itself might come from a file with translations and the user should decide about the current locale, which determines the translation and therefore also the external encoding used. In such a situation (and many others) the functions described here are perfect. If more freedom while performing the conversion is necessary take a look at the iconv functions (see Generic Charset Conversion).


6.3.1 Selecting the conversion and its properties

We already said above that the currently selected locale for the LC_CTYPE category decides the conversion that is performed by the functions we are about to describe. Each locale uses its own character set (given as an argument to localedef) and this is the one assumed as the external multibyte encoding. The wide character set is always UCS-4 in the GNU C Library.

A characteristic of each multibyte character set is the maximum number of bytes that can be necessary to represent one character. This information is quite important when writing code that uses the conversion functions (as shown in the examples below). The ISO C standard defines two macros that provide this information.

Macro: int MB_LEN_MAX

MB_LEN_MAX specifies the maximum number of bytes in the multibyte sequence for a single character in any of the supported locales. It is a compile-time constant and is defined in limits.h.

Macro: int MB_CUR_MAX

MB_CUR_MAX expands into a positive integer expression that is the maximum number of bytes in a multibyte character in the current locale. The value is never greater than MB_LEN_MAX. Unlike MB_LEN_MAX this macro need not be a compile-time constant, and in the GNU C Library it is not.

MB_CUR_MAX is defined in stdlib.h.

Two different macros are necessary since strictly ISO C90 compilers do not allow variable length array definitions, but still it is desirable to avoid dynamic allocation. This incomplete piece of code shows the problem:

{
  char buf[MB_LEN_MAX];
  ssize_t len = 0;
 
  while (! feof (fp))
    {
      fread (&amp;buf[len], 1, MB_CUR_MAX - len, fp);
      /* <span class="roman">… process</span> buf */
      len -= used;
    }
}

The code in the inner loop is expected to have always enough bytes in the array buf to convert one multibyte character. The array buf has to be sized statically since many compilers do not allow a variable size. The fread call makes sure that MB_CUR_MAX bytes are always available in buf. Note that it isn’t a problem if MB_CUR_MAX is not a compile-time constant.


6.3.2 Representing the state of the conversion

In the introduction of this chapter it was said that certain character sets use a stateful encoding. That is, the encoded values depend in some way on the previous bytes in the text.

Since the conversion functions allow converting a text in more than one step we must have a way to pass this information from one call of the functions to another.

Data type: mbstate_t

A variable of type mbstate_t can contain all the information about the shift state needed from one call to a conversion function to another.

mbstate_t is defined in wchar.h. It was introduced in Amendment 1 to ISO C90.

To use objects of type mbstate_t the programmer has to define such objects (normally as local variables on the stack) and pass a pointer to the object to the conversion functions. This way the conversion function can update the object if the current multibyte character set is stateful.

There is no specific function or initializer to put the state object in any specific state. The rules are that the object should always represent the initial state before the first use, and this is achieved by clearing the whole variable with code such as follows:

{
  mbstate_t state;
  memset (&amp;state, '\0', sizeof (state));
  /* <span class="roman">from now on <var>state</var> can be used.</span>  */
  …
}

When using the conversion functions to generate output it is often necessary to test whether the current state corresponds to the initial state. This is necessary, for example, to decide whether to emit escape sequences to set the state to the initial state at certain sequence points. Communication protocols often require this.

Function: int mbsinit (const mbstate_t *ps)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The mbsinit function determines whether the state object pointed to by ps is in the initial state. If ps is a null pointer or the object is in the initial state the return value is nonzero. Otherwise it is zero.

mbsinit was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Code using mbsinit often looks similar to this:

{
  mbstate_t state;
  memset (&amp;state, '\0', sizeof (state));
  /* <span class="roman">Use <var>state</var>.</span>  */
  …
  if (! mbsinit (&amp;state))
    {
      /* <span class="roman">Emit code to return to initial state.</span>  */
      const wchar_t empty[] = L"";
      const wchar_t *srcp = empty;
      wcsrtombs (outbuf, &amp;srcp, outbuflen, &amp;state);
    }
  …
}

The code to emit the escape sequence to get back to the initial state is interesting. The wcsrtombs function can be used to determine the necessary output code (see Converting Strings). Please note that with the GNU C Library it is not necessary to perform this extra action for the conversion from multibyte text to wide character text since the wide character encoding is not stateful. But there is nothing mentioned in any standard that prohibits making wchar_t use a stateful encoding.


6.3.3 Converting Single Characters

The most fundamental of the conversion functions are those dealing with single characters. Please note that this does not always mean single bytes. But since there is very often a subset of the multibyte character set that consists of single byte sequences, there are functions to help with converting bytes. Frequently, ASCII is a subset of the multibyte character set. In such a scenario, each ASCII character stands for itself, and all other characters have at least a first byte that is beyond the range 0 to 127.

Function: wint_t btowc (int c)

Preliminary: | MT-Safe | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The btowc function (“byte to wide character”) converts a valid single byte character c in the initial shift state into the wide character equivalent using the conversion rules from the currently selected locale of the LC_CTYPE category.

If (unsigned char) <var>c</var> is no valid single byte multibyte character or if c is EOF, the function returns WEOF.

Please note the restriction of c being tested for validity only in the initial shift state. No mbstate_t object is used from which the state information is taken, and the function also does not use any static state.

The btowc function was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Despite the limitation that the single byte value is always interpreted in the initial state, this function is actually useful most of the time. Most characters are either entirely single-byte character sets or they are extensions to ASCII. But then it is possible to write code like this (not that this specific example is very useful):

wchar_t *
itow (unsigned long int val)
{
  static wchar_t buf[30];
  wchar_t *wcp = &amp;buf[29];
  *wcp = L'\0';
  while (val != 0)
    {
      *--wcp = btowc ('0' + val % 10);
      val /= 10;
    }
  if (wcp == &amp;buf[29])
    *--wcp = L'0';
  return wcp;
}

Why is it necessary to use such a complicated implementation and not simply cast '0' + val % 10 to a wide character? The answer is that there is no guarantee that one can perform this kind of arithmetic on the character of the character set used for wchar_t representation. In other situations the bytes are not constant at compile time and so the compiler cannot do the work. In situations like this, using btowc is required.

There is also a function for the conversion in the other direction.

Function: int wctob (wint_t c)

Preliminary: | MT-Safe | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The wctob function (“wide character to byte”) takes as the parameter a valid wide character. If the multibyte representation for this character in the initial state is exactly one byte long, the return value of this function is this character. Otherwise the return value is EOF.

wctob was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

There are more general functions to convert single characters from multibyte representation to wide characters and vice versa. These functions pose no limit on the length of the multibyte representation and they also do not require it to be in the initial state.

Function: size_t mbrtowc (wchar_t *restrict pwc, const char *restrict s, size_t n, mbstate_t *restrict ps)

Preliminary: | MT-Unsafe race:mbrtowc/!ps | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The mbrtowc function (“multibyte restartable to wide character”) converts the next multibyte character in the string pointed to by s into a wide character and stores it in the wide character string pointed to by pwc. The conversion is performed according to the locale currently selected for the LC_CTYPE category. If the conversion for the character set used in the locale requires a state, the multibyte string is interpreted in the state represented by the object pointed to by ps. If ps is a null pointer, a static, internal state variable used only by the mbrtowc function is used.

If the next multibyte character corresponds to the NUL wide character, the return value of the function is 0 and the state object is afterwards in the initial state. If the next n or fewer bytes form a correct multibyte character, the return value is the number of bytes starting from s that form the multibyte character. The conversion state is updated according to the bytes consumed in the conversion. In both cases the wide character (either the L'\0' or the one found in the conversion) is stored in the string pointed to by pwc if pwc is not null.

If the first n bytes of the multibyte string possibly form a valid multibyte character but there are more than n bytes needed to complete it, the return value of the function is (size_t) -2 and no value is stored. Please note that this can happen even if n has a value greater than or equal to MB_CUR_MAX since the input might contain redundant shift sequences.

If the first n bytes of the multibyte string cannot possibly form a valid multibyte character, no value is stored, the global variable errno is set to the value EILSEQ, and the function returns (size_t) -1. The conversion state is afterwards undefined.

mbrtowc was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Use of mbrtowc is straightforward. A function that copies a multibyte string into a wide character string while at the same time converting all lowercase characters into uppercase could look like this (this is not the final version, just an example; it has no error checking, and sometimes leaks memory):

wchar_t *
mbstouwcs (const char *s)
{
  size_t len = strlen (s);
  wchar_t *result = malloc ((len + 1) * sizeof (wchar_t));
  wchar_t *wcp = result;
  wchar_t tmp[1];
  mbstate_t state;
  size_t nbytes;
 
  memset (&amp;state, '\0', sizeof (state));
  while ((nbytes = mbrtowc (tmp, s, len, &amp;state)) &gt; 0)
    {
      if (nbytes &gt;= (size_t) -2)
        /* Invalid input string.  */
        return NULL;
      *wcp++ = towupper (tmp[0]);
      len -= nbytes;
      s += nbytes;
    }
  return result;
}

The use of mbrtowc should be clear. A single wide character is stored in <var>tmp</var>[0], and the number of consumed bytes is stored in the variable nbytes. If the conversion is successful, the uppercase variant of the wide character is stored in the result array and the pointer to the input string and the number of available bytes is adjusted.

The only non-obvious thing about mbrtowc might be the way memory is allocated for the result. The above code uses the fact that there can never be more wide characters in the converted result than there are bytes in the multibyte input string. This method yields a pessimistic guess about the size of the result, and if many wide character strings have to be constructed this way or if the strings are long, the extra memory required to be allocated because the input string contains multibyte characters might be significant. The allocated memory block can be resized to the correct size before returning it, but a better solution might be to allocate just the right amount of space for the result right away. Unfortunately there is no function to compute the length of the wide character string directly from the multibyte string. There is, however, a function that does part of the work.

Function: size_t mbrlen (const char *restrict s, size_t n, mbstate_t *ps)

Preliminary: | MT-Unsafe race:mbrlen/!ps | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The mbrlen function (“multibyte restartable length”) computes the number of at most n bytes starting at s, which form the next valid and complete multibyte character.

If the next multibyte character corresponds to the NUL wide character, the return value is 0. If the next n bytes form a valid multibyte character, the number of bytes belonging to this multibyte character byte sequence is returned.

If the first n bytes possibly form a valid multibyte character but the character is incomplete, the return value is (size_t) -2. Otherwise the multibyte character sequence is invalid and the return value is (size_t) -1.

The multibyte sequence is interpreted in the state represented by the object pointed to by ps. If ps is a null pointer, a state object local to mbrlen is used.

mbrlen was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The attentive reader now will note that mbrlen can be implemented as

mbrtowc (NULL, s, n, ps != NULL ? ps : &amp;internal)

This is true and in fact is mentioned in the official specification. How can this function be used to determine the length of the wide character string created from a multibyte character string? It is not directly usable, but we can define a function mbslen using it:

size_t
mbslen (const char *s)
{
  mbstate_t state;
  size_t result = 0;
  size_t nbytes;
  memset (&amp;state, '\0', sizeof (state));
  while ((nbytes = mbrlen (s, MB_LEN_MAX, &amp;state)) &gt; 0)
    {
      if (nbytes &gt;= (size_t) -2)
        /* <span class="roman">Something is wrong.</span>  */
        return (size_t) -1;
      s += nbytes;
      ++result;
    }
  return result;
}

This function simply calls mbrlen for each multibyte character in the string and counts the number of function calls. Please note that we here use MB_LEN_MAX as the size argument in the mbrlen call. This is acceptable since a) this value is larger than the length of the longest multibyte character sequence and b) we know that the string s ends with a NUL byte, which cannot be part of any other multibyte character sequence but the one representing the NUL wide character. Therefore, the mbrlen function will never read invalid memory.

Now that this function is available (just to make this clear, this function is not part of the GNU C Library) we can compute the number of wide characters required to store the converted multibyte character string s using

wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t);

Please note that the mbslen function is quite inefficient. The implementation of mbstouwcs with mbslen would have to perform the conversion of the multibyte character input string twice, and this conversion might be quite expensive. So it is necessary to think about the consequences of using the easier but imprecise method before doing the work twice.

Function: size_t wcrtomb (char *restrict s, wchar_t wc, mbstate_t *restrict ps)

Preliminary: | MT-Unsafe race:wcrtomb/!ps | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The wcrtomb function (“wide character restartable to multibyte”) converts a single wide character into a multibyte string corresponding to that wide character.

If s is a null pointer, the function resets the state stored in the object pointed to by ps (or the internal mbstate_t object) to the initial state. This can also be achieved by a call like this:

wcrtombs (temp_buf, L'\0', ps)

since, if s is a null pointer, wcrtomb performs as if it writes into an internal buffer, which is guaranteed to be large enough.

If wc is the NUL wide character, wcrtomb emits, if necessary, a shift sequence to get the state ps into the initial state followed by a single NUL byte, which is stored in the string s.

Otherwise a byte sequence (possibly including shift sequences) is written into the string s. This only happens if wc is a valid wide character (i.e., it has a multibyte representation in the character set selected by locale of the LC_CTYPE category). If wc is no valid wide character, nothing is stored in the strings s, errno is set to EILSEQ, the conversion state in ps is undefined and the return value is (size_t) -1.

If no error occurred the function returns the number of bytes stored in the string s. This includes all bytes representing shift sequences.

One word about the interface of the function: there is no parameter specifying the length of the array s. Instead the function assumes that there are at least MB_CUR_MAX bytes available since this is the maximum length of any byte sequence representing a single character. So the caller has to make sure that there is enough space available, otherwise buffer overruns can occur.

wcrtomb was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Using wcrtomb is as easy as using mbrtowc. The following example appends a wide character string to a multibyte character string. Again, the code is not really useful (or correct), it is simply here to demonstrate the use and some problems.

char *
mbscatwcs (char *s, size_t len, const wchar_t *ws)
{
  mbstate_t state;
  /* <span class="roman">Find the end of the existing string.</span>  */
  char *wp = strchr (s, '\0');
  len -= wp - s;
  memset (&amp;state, '\0', sizeof (state));
  do
    {
      size_t nbytes;
      if (len &lt; MB_CUR_LEN)
        {
          /* <span class="roman">We cannot guarantee that the next</span>
             <span class="roman">character fits into the buffer, so</span>
             <span class="roman">return an error.</span>  */
          errno = E2BIG;
          return NULL;
        }
      nbytes = wcrtomb (wp, *ws, &amp;state);
      if (nbytes == (size_t) -1)
        /* <span class="roman">Error in the conversion.</span>  */
        return NULL;
      len -= nbytes;
      wp += nbytes;
    }
  while (*ws++ != L'\0');
  return s;
}

First the function has to find the end of the string currently in the array s. The strchr call does this very efficiently since a requirement for multibyte character representations is that the NUL byte is never used except to represent itself (and in this context, the end of the string).

After initializing the state object the loop is entered where the first task is to make sure there is enough room in the array s. We abort if there are not at least MB_CUR_LEN bytes available. This is not always optimal but we have no other choice. We might have less than MB_CUR_LEN bytes available but the next multibyte character might also be only one byte long. At the time the wcrtomb call returns it is too late to decide whether the buffer was large enough. If this solution is unsuitable, there is a very slow but more accurate solution.

  …
  if (len &lt; MB_CUR_LEN)
    {
      mbstate_t temp_state;
      memcpy (&amp;temp_state, &amp;state, sizeof (state));
      if (wcrtomb (NULL, *ws, &amp;temp_state) &gt; len)
        {
          /* <span class="roman">We cannot guarantee that the next</span>
             <span class="roman">character fits into the buffer, so</span>
             <span class="roman">return an error.</span>  */
          errno = E2BIG;
          return NULL;
        }
    }
  …

Here we perform the conversion that might overflow the buffer so that we are afterwards in the position to make an exact decision about the buffer size. Please note the NULL argument for the destination buffer in the new wcrtomb call; since we are not interested in the converted text at this point, this is a nice way to express this. The most unusual thing about this piece of code certainly is the duplication of the conversion state object, but if a change of the state is necessary to emit the next multibyte character, we want to have the same shift state change performed in the real conversion. Therefore, we have to preserve the initial shift state information.

There are certainly many more and even better solutions to this problem. This example is only provided for educational purposes.


6.3.4 Converting Multibyte and Wide Character Strings

The functions described in the previous section only convert a single character at a time. Most operations to be performed in real-world programs include strings and therefore the ISO C standard also defines conversions on entire strings. However, the defined set of functions is quite limited; therefore, the GNU C Library contains a few extensions that can help in some important situations.

Function: size_t mbsrtowcs (wchar_t *restrict dst, const char **restrict src, size_t len, mbstate_t *restrict ps)

Preliminary: | MT-Unsafe race:mbsrtowcs/!ps | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The mbsrtowcs function (“multibyte string restartable to wide character string”) converts the NUL-terminated multibyte character string at *<var>src</var> into an equivalent wide character string, including the NUL wide character at the end. The conversion is started using the state information from the object pointed to by ps or from an internal object of mbsrtowcs if ps is a null pointer. Before returning, the state object is updated to match the state after the last converted character. The state is the initial state if the terminating NUL byte is reached and converted.

If dst is not a null pointer, the result is stored in the array pointed to by dst; otherwise, the conversion result is not available since it is stored in an internal buffer.

If len wide characters are stored in the array dst before reaching the end of the input string, the conversion stops and len is returned. If dst is a null pointer, len is never checked.

Another reason for a premature return from the function call is if the input string contains an invalid multibyte sequence. In this case the global variable errno is set to EILSEQ and the function returns (size_t) -1.

In all other cases the function returns the number of wide characters converted during this call. If dst is not null, mbsrtowcs stores in the pointer pointed to by src either a null pointer (if the NUL byte in the input string was reached) or the address of the byte following the last converted multibyte character.

mbsrtowcs was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The definition of the mbsrtowcs function has one important limitation. The requirement that dst has to be a NUL-terminated string provides problems if one wants to convert buffers with text. A buffer is not normally a collection of NUL-terminated strings but instead a continuous collection of lines, separated by newline characters. Now assume that a function to convert one line from a buffer is needed. Since the line is not NUL-terminated, the source pointer cannot directly point into the unmodified text buffer. This means, either one inserts the NUL byte at the appropriate place for the time of the mbsrtowcs function call (which is not doable for a read-only buffer or in a multi-threaded application) or one copies the line in an extra buffer where it can be terminated by a NUL byte. Note that it is not in general possible to limit the number of characters to convert by setting the parameter len to any specific value. Since it is not known how many bytes each multibyte character sequence is in length, one can only guess.

There is still a problem with the method of NUL-terminating a line right after the newline character, which could lead to very strange results. As said in the description of the mbsrtowcs function above, the conversion state is guaranteed to be in the initial shift state after processing the NUL byte at the end of the input string. But this NUL byte is not really part of the text (i.e., the conversion state after the newline in the original text could be something different than the initial shift state and therefore the first character of the next line is encoded using this state). But the state in question is never accessible to the user since the conversion stops after the NUL byte (which resets the state). Most stateful character sets in use today require that the shift state after a newline be the initial state–but this is not a strict guarantee. Therefore, simply NUL-terminating a piece of a running text is not always an adequate solution and, therefore, should never be used in generally used code.

The generic conversion interface (see Generic Charset Conversion) does not have this limitation (it simply works on buffers, not strings), and the GNU C Library contains a set of functions that take additional parameters specifying the maximal number of bytes that are consumed from the input string. This way the problem of mbsrtowcs’s example above could be solved by determining the line length and passing this length to the function.

Function: size_t wcsrtombs (char *restrict dst, const wchar_t **restrict src, size_t len, mbstate_t *restrict ps)

Preliminary: | MT-Unsafe race:wcsrtombs/!ps | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The wcsrtombs function (“wide character string restartable to multibyte string”) converts the NUL-terminated wide character string at *<var>src</var> into an equivalent multibyte character string and stores the result in the array pointed to by dst. The NUL wide character is also converted. The conversion starts in the state described in the object pointed to by ps or by a state object local to wcsrtombs in case ps is a null pointer. If dst is a null pointer, the conversion is performed as usual but the result is not available. If all characters of the input string were successfully converted and if dst is not a null pointer, the pointer pointed to by src gets assigned a null pointer.

If one of the wide characters in the input string has no valid multibyte character equivalent, the conversion stops early, sets the global variable errno to EILSEQ, and returns (size_t) -1.

Another reason for a premature stop is if dst is not a null pointer and the next converted character would require more than len bytes in total to the array dst. In this case (and if dst is not a null pointer) the pointer pointed to by src is assigned a value pointing to the wide character right after the last one successfully converted.

Except in the case of an encoding error the return value of the wcsrtombs function is the number of bytes in all the multibyte character sequences stored in dst. Before returning, the state in the object pointed to by ps (or the internal object in case ps is a null pointer) is updated to reflect the state after the last conversion. The state is the initial shift state in case the terminating NUL wide character was converted.

The wcsrtombs function was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The restriction mentioned above for the mbsrtowcs function applies here also. There is no possibility of directly controlling the number of input characters. One has to place the NUL wide character at the correct place or control the consumed input indirectly via the available output array size (the len parameter).

Function: size_t mbsnrtowcs (wchar_t *restrict dst, const char **restrict src, size_t nmc, size_t len, mbstate_t *restrict ps)

Preliminary: | MT-Unsafe race:mbsnrtowcs/!ps | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The mbsnrtowcs function is very similar to the mbsrtowcs function. All the parameters are the same except for nmc, which is new. The return value is the same as for mbsrtowcs.

This new parameter specifies how many bytes at most can be used from the multibyte character string. In other words, the multibyte character string *<var>src</var> need not be NUL-terminated. But if a NUL byte is found within the nmc first bytes of the string, the conversion stops there.

This function is a GNU extension. It is meant to work around the problems mentioned above. Now it is possible to convert a buffer with multibyte character text piece by piece without having to care about inserting NUL bytes and the effect of NUL bytes on the conversion state.

A function to convert a multibyte string into a wide character string and display it could be written like this (this is not a really useful example):

void
showmbs (const char *src, FILE *fp)
{
  mbstate_t state;
  int cnt = 0;
  memset (&amp;state, '\0', sizeof (state));
  while (1)
    {
      wchar_t linebuf[100];
      const char *endp = strchr (src, '\n');
      size_t n;
 
      /* <span class="roman">Exit if there is no more line.</span>  */
      if (endp == NULL)
        break;
 
      n = mbsnrtowcs (linebuf, &amp;src, endp - src, 99, &amp;state);
      linebuf[n] = L'\0';
      fprintf (fp, "line %d: \"%S\"\n", linebuf);
    }
}

There is no problem with the state after a call to mbsnrtowcs. Since we don’t insert characters in the strings that were not in there right from the beginning and we use state only for the conversion of the given buffer, there is no problem with altering the state.

Function: size_t wcsnrtombs (char *restrict dst, const wchar_t **restrict src, size_t nwc, size_t len, mbstate_t *restrict ps)

Preliminary: | MT-Unsafe race:wcsnrtombs/!ps | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The wcsnrtombs function implements the conversion from wide character strings to multibyte character strings. It is similar to wcsrtombs but, just like mbsnrtowcs, it takes an extra parameter, which specifies the length of the input string.

No more than nwc wide characters from the input string *<var>src</var> are converted. If the input string contains a NUL wide character in the first nwc characters, the conversion stops at this place.

The wcsnrtombs function is a GNU extension and just like mbsnrtowcs helps in situations where no NUL-terminated input strings are available.


6.3.5 A Complete Multibyte Conversion Example

The example programs given in the last sections are only brief and do not contain all the error checking, etc. Presented here is a complete and documented example. It features the mbrtowc function but it should be easy to derive versions using the other functions.

int
file_mbsrtowcs (int input, int output)
{
  /* <span class="roman">Note the use of <code>MB_LEN_MAX</code>.</span>
     <span class="roman"><code>MB_CUR_MAX</code> cannot portably be used here.</span>  */
  char buffer[BUFSIZ + MB_LEN_MAX];
  mbstate_t state;
  int filled = 0;
  int eof = 0;
 
  /* <span class="roman">Initialize the state.</span>  */
  memset (&amp;state, '\0', sizeof (state));
 
  while (!eof)
    {
      ssize_t nread;
      ssize_t nwrite;
      char *inp = buffer;
      wchar_t outbuf[BUFSIZ];
      wchar_t *outp = outbuf;
 
      /* <span class="roman">Fill up the buffer from the input file.</span>  */
      nread = read (input, buffer + filled, BUFSIZ);
      if (nread &lt; 0)
        {
          perror ("read");
          return 0;
        }
      /* <span class="roman">If we reach end of file, make a note to read no more.</span> */
      if (nread == 0)
        eof = 1;
 
      /* <span class="roman"><code>filled</code> is now the number of bytes in <code>buffer</code>.</span> */
      filled += nread;
 
      /* <span class="roman">Convert those bytes to wide characters–as many as we can.</span> */
      while (1)
        {
          size_t thislen = mbrtowc (outp, inp, filled, &amp;state);
          /* <span class="roman">Stop converting at invalid character;</span>
             <span class="roman">this can mean we have read just the first part</span>
             <span class="roman">of a valid character.</span>  */
          if (thislen == (size_t) -1)
            break;
          /* <span class="roman">We want to handle embedded NUL bytes</span>
             <span class="roman">but the return value is 0.  Correct this.</span>  */
          if (thislen == 0)
            thislen = 1;
          /* <span class="roman">Advance past this character.</span> */
          inp += thislen;
          filled -= thislen;
          ++outp;
        }
 
      /* <span class="roman">Write the wide characters we just made.</span>  */
      nwrite = write (output, outbuf,
                      (outp - outbuf) * sizeof (wchar_t));
      if (nwrite &lt; 0)
        {
          perror ("write");
          return 0;
        }
 
      /* <span class="roman">See if we have a <em>real</em> invalid character.</span> */
      if ((eof &amp;&amp; filled &gt; 0) || filled &gt;= MB_CUR_MAX)
        {
          error (0, 0, "invalid multibyte character");
          return 0;
        }
 
      /* <span class="roman">If any characters must be carried forward,</span>
         <span class="roman">put them at the beginning of <code>buffer</code>.</span> */
      if (filled &gt; 0)
        memmove (buffer, inp, filled);
    }
 
  return 1;
}

6.4 Non-reentrant Conversion Function

The functions described in the previous chapter are defined in Amendment 1 to ISO C90, but the original ISO C90 standard also contained functions for character set conversion. The reason that these original functions are not described first is that they are almost entirely useless.

The problem is that all the conversion functions described in the original ISO C90 use a local state. Using a local state implies that multiple conversions at the same time (not only when using threads) cannot be done, and that you cannot first convert single characters and then strings since you cannot tell the conversion functions which state to use.

These original functions are therefore usable only in a very limited set of situations. One must complete converting the entire string before starting a new one, and each string/text must be converted with the same function (there is no problem with the library itself; it is guaranteed that no library function changes the state of any of these functions). For the above reasons it is highly requested that the functions described in the previous section be used in place of non-reentrant conversion functions.


6.4.1 Non-reentrant Conversion of Single Characters

Function: int mbtowc (wchar_t *restrict result, const char *restrict string, size_t size)

Preliminary: | MT-Unsafe race | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The mbtowc (“multibyte to wide character”) function when called with non-null string converts the first multibyte character beginning at string to its corresponding wide character code. It stores the result in *<var>result</var>.

mbtowc never examines more than size bytes. (The idea is to supply for size the number of bytes of data you have in hand.)

mbtowc with non-null string distinguishes three possibilities: the first size bytes at string start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or string points to an empty string (a null character).

For a valid multibyte character, mbtowc converts it to a wide character and stores that in *<var>result</var>, and returns the number of bytes in that character (always at least 1 and never more than size).

For an invalid byte sequence, mbtowc returns -1. For an empty string, it returns 0, also storing '\0' in *<var>result</var>.

If the multibyte character code uses shift characters, then mbtowc maintains and updates a shift state as it scans. If you call mbtowc with a null pointer for string, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. See Shift State.

Function: int wctomb (char *string, wchar_t wchar)

Preliminary: | MT-Unsafe race | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The wctomb (“wide character to multibyte”) function converts the wide character code wchar to its corresponding multibyte character sequence, and stores the result in bytes starting at string. At most MB_CUR_MAX characters are stored.

wctomb with non-null string distinguishes three possibilities for wchar: a valid wide character code (one that can be translated to a multibyte character), an invalid code, and L'\0'.

Given a valid code, wctomb converts it to a multibyte character, storing the bytes starting at string. Then it returns the number of bytes in that character (always at least 1 and never more than MB_CUR_MAX).

If wchar is an invalid wide character code, wctomb returns -1. If wchar is L'\0', it returns  , also storing '\0' in *<var>string</var>.

If the multibyte character code uses shift characters, then wctomb maintains and updates a shift state as it scans. If you call wctomb with a null pointer for string, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. See Shift State.

Calling this function with a wchar argument of zero when string is not null has the side-effect of reinitializing the stored shift state as well as storing the multibyte character '\0' and returning 0.

Similar to mbrlen there is also a non-reentrant function that computes the length of a multibyte character. It can be defined in terms of mbtowc.

Function: int mblen (const char *string, size_t size)

Preliminary: | MT-Unsafe race | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The mblen function with a non-null string argument returns the number of bytes that make up the multibyte character beginning at string, never examining more than size bytes. (The idea is to supply for size the number of bytes of data you have in hand.)

The return value of mblen distinguishes three possibilities: the first size bytes at string start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or string points to an empty string (a null character).

For a valid multibyte character, mblen returns the number of bytes in that character (always at least 1 and never more than size). For an invalid byte sequence, mblen returns -1. For an empty string, it returns 0.

If the multibyte character code uses shift characters, then mblen maintains and updates a shift state as it scans. If you call mblen with a null pointer for string, that initializes the shift state to its standard initial value. It also returns a nonzero value if the multibyte character code in use actually has a shift state. See Shift State.

The function mblen is declared in stdlib.h.


6.4.2 Non-reentrant Conversion of Strings

For convenience the ISO C90 standard also defines functions to convert entire strings instead of single characters. These functions suffer from the same problems as their reentrant counterparts from Amendment 1 to ISO C90; see Converting Strings.

Function: size_t mbstowcs (wchar_t *wstring, const char *string, size_t size)

Preliminary: | MT-Safe | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The mbstowcs (“multibyte string to wide character string”) function converts the null-terminated string of multibyte characters string to an array of wide character codes, storing not more than size wide characters into the array beginning at wstring. The terminating null character counts towards the size, so if size is less than the actual number of wide characters resulting from string, no terminating null character is stored.

The conversion of characters from string begins in the initial shift state.

If an invalid multibyte character sequence is found, the mbstowcs function returns a value of -1. Otherwise, it returns the number of wide characters stored in the array wstring. This number does not include the terminating null character, which is present if the number is less than size.

Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result.

wchar_t *
mbstowcs_alloc (const char *string)
{
  size_t size = strlen (string) + 1;
  wchar_t *buf = xmalloc (size * sizeof (wchar_t));
 
  size = mbstowcs (buf, string, size);
  if (size == (size_t) -1)
    return NULL;
  buf = xrealloc (buf, (size + 1) * sizeof (wchar_t));
  return buf;
}
Function: size_t wcstombs (char *string, const wchar_t *wstring, size_t size)

Preliminary: | MT-Safe | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The wcstombs (“wide character string to multibyte string”) function converts the null-terminated wide character array wstring into a string containing multibyte characters, storing not more than size bytes starting at string, followed by a terminating null character if there is room. The conversion of characters begins in the initial shift state.

The terminating null character counts towards the size, so if size is less than or equal to the number of bytes needed in wstring, no terminating null character is stored.

If a code that does not correspond to a valid multibyte character is found, the wcstombs function returns a value of -1. Otherwise, the return value is the number of bytes stored in the array string. This number does not include the terminating null character, which is present if the number is less than size.


6.4.3 States in Non-reentrant Functions

In some multibyte character codes, the meaning of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string. Typically there are just a few sequences that can change the meaning of other sequences; these few are called shift sequences and we say that they set the shift state for other sequences that follow.

To illustrate shift state and shift sequences, suppose we decide that the sequence 0200 (just one byte) enters Japanese mode, in which pairs of bytes in the range from 0240 to 0377 are single characters, while 0201 enters Latin-1 mode, in which single bytes in the range from 0240 to 0377 are characters, and interpreted according to the ISO Latin-1 character set. This is a multibyte code that has two alternative shift states (“Japanese mode” and “Latin-1 mode”), and two shift sequences that specify particular shift states.

When the multibyte character code in use has shift states, then mblen, mbtowc, and wctomb must maintain and update the current shift state as they scan the string. To make this work properly, you must follow these rules:

  • Before starting to scan a string, call the function with a null pointer for the multibyte character address—for example,
    mblen (NULL,
    0)
    . This initializes the shift state to its standard initial value.
  • Scan the string one character at a time, in order. Do not “back up” and rescan characters already scanned, and do not intersperse the processing of different strings.

Here is an example of using mblen following these rules:

void
scan_string (char *s)
{
  int length = strlen (s);
 
  /* <span class="roman">Initialize shift state.</span>  */
  mblen (NULL, 0);
 
  while (1)
    {
      int thischar = mblen (s, length);
      /* <span class="roman">Deal with end of string and invalid characters.</span>  */
      if (thischar == 0)
        break;
      if (thischar == -1)
        {
          error ("invalid multibyte character");
          break;
        }
      /* <span class="roman">Advance past this character.</span>  */
      s += thischar;
      length -= thischar;
    }
}

The functions mblen, mbtowc and wctomb are not reentrant when using a multibyte code that uses a shift state. However, no other library functions call these functions, so you don’t have to worry that the shift state will be changed mysteriously.


6.5 Generic Charset Conversion

The conversion functions mentioned so far in this chapter all had in common that they operate on character sets that are not directly specified by the functions. The multibyte encoding used is specified by the currently selected locale for the LC_CTYPE category. The wide character set is fixed by the implementation (in the case of the GNU C Library it is always UCS-4 encoded ISO 10646).

This has of course several problems when it comes to general character conversion:

  • For every conversion where neither the source nor the destination character set is the character set of the locale for the LC_CTYPE category, one has to change the LC_CTYPE locale using setlocale.

    Changing the LC_CTYPE locale introduces major problems for the rest of the programs since several more functions (e.g., the character classification functions, see Classification of Characters) use the LC_CTYPE category.

  • Parallel conversions to and from different character sets are not possible since the LC_CTYPE selection is global and shared by all threads.
  • If neither the source nor the destination character set is the character set used for wchar_t representation, there is at least a two-step process necessary to convert a text using the functions above. One would have to select the source character set as the multibyte encoding, convert the text into a wchar_t text, select the destination character set as the multibyte encoding, and convert the wide character text to the multibyte (= destination) character set.

    Even if this is possible (which is not guaranteed) it is a very tiring work. Plus it suffers from the other two raised points even more due to the steady changing of the locale.

The XPG2 standard defines a completely new set of functions, which has none of these limitations. They are not at all coupled to the selected locales, and they have no constraints on the character sets selected for source and destination. Only the set of available conversions limits them. The standard does not specify that any conversion at all must be available. Such availability is a measure of the quality of the implementation.

In the following text first the interface to iconv and then the conversion function, will be described. Comparisons with other implementations will show what obstacles stand in the way of portable applications. Finally, the implementation is described in so far as might interest the advanced user who wants to extend conversion capabilities.


6.5.1 Generic Character Set Conversion Interface

This set of functions follows the traditional cycle of using a resource: open–use–close. The interface consists of three functions, each of which implements one step.

Before the interfaces are described it is necessary to introduce a data type. Just like other open–use–close interfaces the functions introduced here work using handles and the iconv.h header defines a special type for the handles used.

Data Type: iconv_t

This data type is an abstract type defined in iconv.h. The user must not assume anything about the definition of this type; it must be completely opaque.

Objects of this type can be assigned handles for the conversions using the iconv functions. The objects themselves need not be freed, but the conversions for which the handles stand for have to.

The first step is the function to create a handle.

Function: iconv_t iconv_open (const char *tocode, const char *fromcode)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem fd | See POSIX Safety Concepts.

The iconv_open function has to be used before starting a conversion. The two parameters this function takes determine the source and destination character set for the conversion, and if the implementation has the possibility to perform such a conversion, the function returns a handle.

If the wanted conversion is not available, the iconv_open function returns (iconv_t) -1. In this case the global variable errno can have the following values:

EMFILE

The process already has OPEN_MAX file descriptors open.

ENFILE

The system limit of open files is reached.

ENOMEM

Not enough memory to carry out the operation.

EINVAL

The conversion from fromcode to tocode is not supported.

It is not possible to use the same descriptor in different threads to perform independent conversions. The data structures associated with the descriptor include information about the conversion state. This must not be messed up by using it in different conversions.

An iconv descriptor is like a file descriptor as for every use a new descriptor must be created. The descriptor does not stand for all of the conversions from fromset to toset.

The GNU C Library implementation of iconv_open has one significant extension to other implementations. To ease the extension of the set of available conversions, the implementation allows storing the necessary files with data and code in an arbitrary number of directories. How this extension must be written will be explained below (see glibc iconv Implementation). Here it is only important to say that all directories mentioned in the GCONV_PATH environment variable are considered only if they contain a file gconv-modules. These directories need not necessarily be created by the system administrator. In fact, this extension is introduced to help users writing and using their own, new conversions. Of course, this does not work for security reasons in SUID binaries; in this case only the system directory is considered and this normally is prefix/lib/gconv. The GCONV_PATH environment variable is examined exactly once at the first call of the iconv_open function. Later modifications of the variable have no effect.

The iconv_open function was introduced early in the X/Open Portability Guide, version 2. It is supported by all commercial Unices as it is required for the Unix branding. However, the quality and completeness of the implementation varies widely. The iconv_open function is declared in iconv.h.

The iconv implementation can associate large data structure with the handle returned by iconv_open. Therefore, it is crucial to free all the resources once all conversions are carried out and the conversion is not needed anymore.

Function: int iconv_close (iconv_t cd)

Preliminary: | MT-Safe | AS-Unsafe corrupt heap lock dlopen | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.

The iconv_close function frees all resources associated with the handle cd, which must have been returned by a successful call to the iconv_open function.

If the function call was successful the return value is 0. Otherwise it is -1 and errno is set appropriately. Defined errors are:

EBADF

The conversion descriptor is invalid.

The iconv_close function was introduced together with the rest of the iconv functions in XPG2 and is declared in iconv.h.

The standard defines only one actual conversion function. This has, therefore, the most general interface: it allows conversion from one buffer to another. Conversion from a file to a buffer, vice versa, or even file to file can be implemented on top of it.

Function: size_t iconv (iconv_t cd, char **inbuf, size_t *inbytesleft, char **outbuf, size_t *outbytesleft)

Preliminary: | MT-Safe race:cd | AS-Safe | AC-Unsafe corrupt | See POSIX Safety Concepts.

The iconv function converts the text in the input buffer according to the rules associated with the descriptor cd and stores the result in the output buffer. It is possible to call the function for the same text several times in a row since for stateful character sets the necessary state information is kept in the data structures associated with the descriptor.

The input buffer is specified by *<var>inbuf</var> and it contains *<var>inbytesleft</var> bytes. The extra indirection is necessary for communicating the used input back to the caller (see below). It is important to note that the buffer pointer is of type char and the length is measured in bytes even if the input text is encoded in wide characters.

The output buffer is specified in a similar way. *<var>outbuf</var> points to the beginning of the buffer with at least *<var>outbytesleft</var> bytes room for the result. The buffer pointer again is of type char and the length is measured in bytes. If outbuf or *<var>outbuf</var> is a null pointer, the conversion is performed but no output is available.

If inbuf is a null pointer, the iconv function performs the necessary action to put the state of the conversion into the initial state. This is obviously a no-op for non-stateful encodings, but if the encoding has a state, such a function call might put some byte sequences in the output buffer, which perform the necessary state changes. The next call with inbuf not being a null pointer then simply goes on from the initial state. It is important that the programmer never makes any assumption as to whether the conversion has to deal with states. Even if the input and output character sets are not stateful, the implementation might still have to keep states. This is due to the implementation chosen for the GNU C Library as it is described below. Therefore an iconv call to reset the state should always be performed if some protocol requires this for the output text.

The conversion stops for one of three reasons. The first is that all characters from the input buffer are converted. This actually can mean two things: either all bytes from the input buffer are consumed or there are some bytes at the end of the buffer that possibly can form a complete character but the input is incomplete. The second reason for a stop is that the output buffer is full. And the third reason is that the input contains invalid characters.

In all of these cases the buffer pointers after the last successful conversion, for the input and output buffers, are stored in inbuf and outbuf, and the available room in each buffer is stored in inbytesleft and outbytesleft.

Since the character sets selected in the iconv_open call can be almost arbitrary, there can be situations where the input buffer contains valid characters, which have no identical representation in the output character set. The behavior in this situation is undefined. The current behavior of the GNU C Library in this situation is to return with an error immediately. This certainly is not the most desirable solution; therefore, future versions will provide better ones, but they are not yet finished.

If all input from the input buffer is successfully converted and stored in the output buffer, the function returns the number of non-reversible conversions performed. In all other cases the return value is (size_t) -1 and errno is set appropriately. In such cases the value pointed to by inbytesleft is nonzero.

EILSEQ

The conversion stopped because of an invalid byte sequence in the input. After the call, *<var>inbuf</var> points at the first byte of the invalid byte sequence.

E2BIG

The conversion stopped because it ran out of space in the output buffer.

EINVAL

The conversion stopped because of an incomplete byte sequence at the end of the input buffer.

EBADF

The cd argument is invalid.

The iconv function was introduced in the XPG2 standard and is declared in the iconv.h header.

The definition of the iconv function is quite good overall. It provides quite flexible functionality. The only problems lie in the boundary cases, which are incomplete byte sequences at the end of the input buffer and invalid input. A third problem, which is not really a design problem, is the way conversions are selected. The standard does not say anything about the legitimate names, a minimal set of available conversions. We will see how this negatively impacts other implementations, as demonstrated below.


6.5.2 A complete iconv example

The example below features a solution for a common problem. Given that one knows the internal encoding used by the system for wchar_t strings, one often is in the position to read text from a file and store it in wide character buffers. One can do this using mbsrtowcs, but then we run into the problems discussed above.

int
file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail)
{
  char inbuf[BUFSIZ];
  size_t insize = 0;
  char *wrptr = (char *) outbuf;
  int result = 0;
  iconv_t cd;
 
  cd = iconv_open ("WCHAR_T", charset);
  if (cd == (iconv_t) -1)
    {
      /* <span class="roman">Something went wrong.</span>  */
      if (errno == EINVAL)
        error (0, 0, "conversion from '%s' to wchar_t not available",
               charset);
      else
        perror ("iconv_open");
 
      /* <span class="roman">Terminate the output string.</span>  */
      *outbuf = L'\0';
 
      return -1;
    }
 
  while (avail &gt; 0)
    {
      size_t nread;
      size_t nconv;
      char *inptr = inbuf;
 
      /* <span class="roman">Read more input.</span>  */
      nread = read (fd, inbuf + insize, sizeof (inbuf) - insize);
      if (nread == 0)
        {
          /* <span class="roman">When we come here the file is completely read.</span>
             <span class="roman">This still could mean there are some unused</span>
             <span class="roman">characters in the <code>inbuf</code>.  Put them back.</span>  */
          if (lseek (fd, -insize, SEEK_CUR) == -1)
            result = -1;
 
          /* <span class="roman">Now write out the byte sequence to get into the</span>
             <span class="roman">initial state if this is necessary.</span>  */
          iconv (cd, NULL, NULL, &amp;wrptr, &amp;avail);
 
          break;
        }
      insize += nread;
 
      /* <span class="roman">Do the conversion.</span>  */
      nconv = iconv (cd, &amp;inptr, &amp;insize, &amp;wrptr, &amp;avail);
      if (nconv == (size_t) -1)
        {
          /* <span class="roman">Not everything went right.  It might only be</span>
             <span class="roman">an unfinished byte sequence at the end of the</span>
             <span class="roman">buffer.  Or it is a real problem.</span>  */
          if (errno == EINVAL)
            /* <span class="roman">This is harmless.  Simply move the unused</span>
               <span class="roman">bytes to the beginning of the buffer so that</span>
               <span class="roman">they can be used in the next round.</span>  */
            memmove (inbuf, inptr, insize);
          else
            {
              /* <span class="roman">It is a real problem.  Maybe we ran out of</span>
                 <span class="roman">space in the output buffer or we have invalid</span>
                 <span class="roman">input.  In any case back the file pointer to</span>
                 <span class="roman">the position of the last processed byte.</span>  */
              lseek (fd, -insize, SEEK_CUR);
              result = -1;
              break;
            }
        }
    }
 
  /* <span class="roman">Terminate the output string.</span>  */
  if (avail &gt;= sizeof (wchar_t))
    *((wchar_t *) wrptr) = L'\0';
 
  if (iconv_close (cd) != 0)
    perror ("iconv_close");
 
  return (wchar_t *) wrptr - outbuf;
}

This example shows the most important aspects of using the iconv functions. It shows how successive calls to iconv can be used to convert large amounts of text. The user does not have to care about stateful encodings as the functions take care of everything.

An interesting point is the case where iconv returns an error and errno is set to EINVAL. This is not really an error in the transformation. It can happen whenever the input character set contains byte sequences of more than one byte for some character and texts are not processed in one piece. In this case there is a chance that a multibyte sequence is cut. The caller can then simply read the remainder of the takes and feed the offending bytes together with new character from the input to iconv and continue the work. The internal state kept in the descriptor is not unspecified after such an event as is the case with the conversion functions from the ISO C standard.

The example also shows the problem of using wide character strings with iconv. As explained in the description of the iconv function above, the function always takes a pointer to a char array and the available space is measured in bytes. In the example, the output buffer is a wide character buffer; therefore, we use a local variable wrptr of type char *, which is used in the iconv calls.

This looks rather innocent but can lead to problems on platforms that have tight restriction on alignment. Therefore the caller of iconv has to make sure that the pointers passed are suitable for access of characters from the appropriate character set. Since, in the above case, the input parameter to the function is a wchar_t pointer, this is the case (unless the user violates alignment when computing the parameter). But in other situations, especially when writing generic functions where one does not know what type of character set one uses and, therefore, treats text as a sequence of bytes, it might become tricky.


6.5.3 Some Details about other iconv Implementations

This is not really the place to discuss the iconv implementation of other systems but it is necessary to know a bit about them to write portable programs. The above mentioned problems with the specification of the iconv functions can lead to portability issues.

The first thing to notice is that, due to the large number of character sets in use, it is certainly not practical to encode the conversions directly in the C library. Therefore, the conversion information must come from files outside the C library. This is usually done in one or both of the following ways:

  • The C library contains a set of generic conversion functions that can read the needed conversion tables and other information from data files. These files get loaded when necessary.

    This solution is problematic as it requires a great deal of effort to apply to all character sets (potentially an infinite set). The differences in the structure of the different character sets is so large that many different variants of the table-processing functions must be developed. In addition, the generic nature of these functions make them slower than specifically implemented functions.

  • The C library only contains a framework that can dynamically load object files and execute the conversion functions contained therein.

    This solution provides much more flexibility. The C library itself contains only very little code and therefore reduces the general memory footprint. Also, with a documented interface between the C library and the loadable modules it is possible for third parties to extend the set of available conversion modules. A drawback of this solution is that dynamic loading must be available.

Some implementations in commercial Unices implement a mixture of these possibilities; the majority implement only the second solution. Using loadable modules moves the code out of the library itself and keeps the door open for extensions and improvements, but this design is also limiting on some platforms since not many platforms support dynamic loading in statically linked programs. On platforms without this capability it is therefore not possible to use this interface in statically linked programs. The GNU C Library has, on ELF platforms, no problems with dynamic loading in these situations; therefore, this point is moot. The danger is that one gets acquainted with this situation and forgets about the restrictions on other systems.

A second thing to know about other iconv implementations is that the number of available conversions is often very limited. Some implementations provide, in the standard release (not special international or developer releases), at most 100 to 200 conversion possibilities. This does not mean 200 different character sets are supported; for example, conversions from one character set to a set of 10 others might count as 10 conversions. Together with the other direction this makes 20 conversion possibilities used up by one character set. One can imagine the thin coverage these platforms provide. Some Unix vendors even provide only a handful of conversions, which renders them useless for almost all uses.

This directly leads to a third and probably the most problematic point. The way the iconv conversion functions are implemented on all known Unix systems and the availability of the conversion functions from character set A to B and the conversion from B to C does not imply that the conversion from A to C is available.

This might not seem unreasonable and problematic at first, but it is a quite big problem as one will notice shortly after hitting it. To show the problem we assume to write a program that has to convert from A to C. A call like

cd = iconv_open ("<em>C</em>", "<em>A</em>");

fails according to the assumption above. But what does the program do now? The conversion is necessary; therefore, simply giving up is not an option.

This is a nuisance. The iconv function should take care of this. But how should the program proceed from here on? If it tries to convert to character set B, first the two iconv_open calls

cd1 = iconv_open ("<em>B</em>", "<em>A</em>");

and

cd2 = iconv_open ("<em>C</em>", "<em>B</em>");

will succeed, but how to find B?

Unfortunately, the answer is: there is no general solution. On some systems guessing might help. On those systems most character sets can convert to and from UTF-8 encoded ISO 10646 or Unicode text. Besides this only some very system-specific methods can help. Since the conversion functions come from loadable modules and these modules must be stored somewhere in the filesystem, one could try to find them and determine from the available file which conversions are available and whether there is an indirect route from A to C.

This example shows one of the design errors of iconv mentioned above. It should at least be possible to determine the list of available conversions programmatically so that if iconv_open says there is no such conversion, one could make sure this also is true for indirect routes.


6.5.4 The iconv Implementation in the GNU C Library

After reading about the problems of iconv implementations in the last section it is certainly good to note that the implementation in the GNU C Library has none of the problems mentioned above. What follows is a step-by-step analysis of the points raised above. The evaluation is based on the current state of the development (as of January 1999). The development of the iconv functions is not complete, but basic functionality has solidified.

The GNU C Library’s iconv implementation uses shared loadable modules to implement the conversions. A very small number of conversions are built into the library itself but these are only rather trivial conversions.

All the benefits of loadable modules are available in the GNU C Library implementation. This is especially appealing since the interface is well documented (see below), and it, therefore, is easy to write new conversion modules. The drawback of using loadable objects is not a problem in the GNU C Library, at least on ELF systems. Since the library is able to load shared objects even in statically linked binaries, static linking need not be forbidden in case one wants to use iconv.

The second mentioned problem is the number of supported conversions. Currently, the GNU C Library supports more than 150 character sets. The way the implementation is designed the number of supported conversions is greater than 22350 (150 times 149). If any conversion from or to a character set is missing, it can be added easily.

Particularly impressive as it may be, this high number is due to the fact that the GNU C Library implementation of iconv does not have the third problem mentioned above (i.e., whenever there is a conversion from a character set A to B and from B to C it is always possible to convert from A to C directly). If the iconv_open returns an error and sets errno to EINVAL, there is no known way, directly or indirectly, to perform the wanted conversion.

Triangulation is achieved by providing for each character set a conversion from and to UCS-4 encoded ISO 10646. Using ISO 10646 as an intermediate representation it is possible to triangulate (i.e., convert with an intermediate representation).

There is no inherent requirement to provide a conversion to ISO 10646 for a new character set, and it is also possible to provide other conversions where neither source nor destination character set is ISO 10646. The existing set of conversions is simply meant to cover all conversions that might be of interest.

All currently available conversions use the triangulation method above, making conversion run unnecessarily slow. If, for example, somebody often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution would involve direct conversion between the two character sets, skipping the input to ISO 10646 first. The two character sets of interest are much more similar to each other than to ISO 10646.

In such a situation one easily can write a new conversion and provide it as a better alternative. The GNU C Library iconv implementation would automatically use the module implementing the conversion if it is specified to be more efficient.

6.5.4.1 Format of gconv-modules files

All information about the available conversions comes from a file named gconv-modules, which can be found in any of the directories along the GCONV_PATH. The gconv-modules files are line-oriented text files, where each of the lines has one of the following formats:

  • If the first non-whitespace character is a # the line contains only comments and is ignored.
  • Lines starting with alias define an alias name for a character set. Two more words are expected on the line. The first word defines the alias name, and the second defines the original name of the character set. The effect is that it is possible to use the alias name in the fromset or toset parameters of iconv_open and achieve the same result as when using the real character set name.

    This is quite important as a character set has often many different names. There is normally an official name but this need not correspond to the most popular name. Besides this many character sets have special names that are somehow constructed. For example, all character sets specified by the ISO have an alias of the form ISO-IR-<var>nnn</var> where nnn is the registration number. This allows programs that know about the registration number to construct character set names and use them in iconv_open calls. More on the available names and aliases follows below.

  • Lines starting with module introduce an available conversion module. These lines must contain three or four more words.

    The first word specifies the source character set, the second word the destination character set of conversion implemented in this module, and the third word is the name of the loadable module. The filename is constructed by appending the usual shared object suffix (normally .so) and this file is then supposed to be found in the same directory the gconv-modules file is in. The last word on the line, which is optional, is a numeric value representing the cost of the conversion. If this word is missing, a cost of 1 is assumed. The numeric value itself does not matter that much; what counts are the relative values of the sums of costs for all possible conversion paths. Below is a more precise description of the use of the cost value.

Returning to the example above where one has written a module to directly convert from ISO-2022-JP to EUC-JP and back. All that has to be done is to put the new module, let its name be ISO2022JP-EUCJP.so, in a directory and add a file gconv-modules with the following content in the same directory:

module  ISO-2022-JP//   EUC-JP//        ISO2022JP-EUCJP    1
module  EUC-JP//        ISO-2022-JP//   ISO2022JP-EUCJP    1

To see why this is sufficient, it is necessary to understand how the conversion used by iconv (and described in the descriptor) is selected. The approach to this problem is quite simple.

At the first call of the iconv_open function the program reads all available gconv-modules files and builds up two tables: one containing all the known aliases and another that contains the information about the conversions and which shared object implements them.

6.5.4.2 Finding the conversion path in iconv

The set of available conversions form a directed graph with weighted edges. The weights on the edges are the costs specified in the gconv-modules files. The iconv_open function uses an algorithm suitable for search for the best path in such a graph and so constructs a list of conversions that must be performed in succession to get the transformation from the source to the destination character set.

Explaining why the above gconv-modules files allows the iconv implementation to resolve the specific ISO-2022-JP to EUC-JP conversion module instead of the conversion coming with the library itself is straightforward. Since the latter conversion takes two steps (from ISO-2022-JP to ISO 10646 and then from ISO 10646