This file documents the GNU C library. This is Edition 0.09 DRAFT, last updated 28 Aug 1999, of `The GNU C Library Reference Manual', for Version 2.2 Beta. Copyright (C) 1993, '94, '95, '96, '97, '98, '99 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU Library General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the text of the translation of the section entitled "GNU Library General Public License" must be approved for accuracy by the Foundation. Main Menu ********* This is Edition 0.09 DRAFT, last updated 28 Aug 1999, of `The GNU C Library Reference Manual', for Version 2.2 Beta of the GNU C Library. 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 the GNU system. The purpose of this manual is to tell you how to use the facilities of the GNU 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. 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 (*note 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. 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 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. *Note 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. 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 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. *Note 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. *Note 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. POSIX (The Portable Operating System Interface) ----------------------------------------------- The GNU 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 (*note File System Interface::), device-specific terminal control functions (*note Low-Level Terminal Interface::), and process control functions (*note Processes::). Some facilities from `ISO/IEC 9945-2:1993, the POSIX Shell and Utilities standard' (POSIX.2) are also implemented in the GNU library. These include utilities for dealing with regular expressions and other pattern matching facilities (*note Pattern Matching::). 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 (*note Symbolic Links::), the `select' function (*note Waiting for I/O::), the BSD signal functions (*note BSD Signal Handling::), and sockets (*note Sockets::). 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 (*note 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. 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. Using the Library ================= This section describes some of the practical issues involved in using the GNU C library. 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 "HEADER" 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 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 (*note Feature Test Macros::). For more information about the use of header files and `#include' directives, *note Header Files: (cpp.info)Header Files.. 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. 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 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 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. 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 (*note Variadic Functions::) and non-local exits (*note Non-Local Exits::), actually require a considerable amount of cooperation on the part of the C compiler, and implementationally 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. *Note 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. *Note Character Handling::. * Names that begin with `LC_' followed by an uppercase letter may be used for additional macros specifying locale attributes. *Note Locales::. * Names of all existing mathematics functions (*note 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. *Note Standard Signals::. * Names that begin with `SIG_' followed by an uppercase letter are reserved for additional signal actions. *Note Basic Signal Handling::. * Names beginning with `str', `mem', or `wcs' followed by a lowercase letter are reserved for additional string and array functions. *Note 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. 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. *Note GNU CC Command Options: (gcc.info)Invoking GCC, 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: _BSD_SOURCE If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ISO C, POSIX.1, and POSIX.2 material. Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard. If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions. Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1, you need to use a special "BSD compatibility library" when linking programs compiled for BSD compatibility. This is because some functions must be defined in two different ways, one of them in the normal C library, and one of them in the compatibility library. If your program defines `_BSD_SOURCE', you must give the option `-lbsd-compat' to the compiler or linker when linking the program, to tell it to find functions in this special compatibility library before looking for them in the normal C library. - Macro: _SVID_SOURCE If you define this macro, functionality derived from SVID is included as well as the ISO C, POSIX.1, POSIX.2, and X/Open material. - 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 libc nevertheless has a complete implementation of the new standard and to enable the new features the macro `_ISOC99_SOURCE' should be defined. - 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. If you want to get the full effect of `_GNU_SOURCE' but make the BSD definitions take precedence over the POSIX definitions, use this sequence of definitions: #define _GNU_SOURCE #define _BSD_SOURCE #define _SVID_SOURCE Note that if you do this, you must link your program with the BSD compatibility library by passing the `-lbsd-compat' option to the compiler or linker. *Note:* If you forget to do this, you may get very strange errors at run time. - Macro: _REENTRANT - Macro: _THREAD_SAFE If you define one of these macros, reentrant versions of several functions get declared. Some of the functions are specified in POSIX.1c but many others are only available on a few other systems or are unique to GNU libc. The problem is the delay in the standardization of the thread safe C library interface. Unlike on some other systems, no special version of the C library must be used for linking. There is only one version but while compiling this it must have been specified to compile as thread safe. We recommend you use `_GNU_SOURCE' in new programs. If you don't specify the `-ansi' option to GCC and don't define any of these macros explicitly, the effect is the same as defining `_POSIX_C_SOURCE' to 2 and `_POSIX_SOURCE', `_SVID_SOURCE', and `_BSD_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' or `_SVID_SOURCE' as well has no effect. Note, however, that the features of `_BSD_SOURCE' are not a subset of any of the other feature test macros supported. This is because it defines BSD features that take precedence over the POSIX features that are requested by the other macros. For this reason, defining `_BSD_SOURCE' in addition to the other feature test macros does have an effect: it causes the BSD features to take priority over the conflicting POSIX features. Roadmap to the Manual ===================== Here is an overview of the contents of the remaining chapters of this manual. * *Note Error Reporting::, describes how errors detected by the library are reported. * *Note 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. * *Note Memory::, describes the GNU 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. * *Note Character Handling::, contains information about character classification functions (such as `isspace') and functions for performing case conversion. * *Note 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. * *Note 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. * *Note I/O on Streams::, describes I/O operations involving streams (or `FILE *' objects). These are the normal C library functions from `stdio.h'. * *Note 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. * *Note 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. * *Note 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. * *Note 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. * *Note 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. * *Note 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. * *Note Low-Level Arithmetic Functions: Arithmetic, describes functions for simple arithmetic, analysis of floating-point values, and reading numbers from strings. * *Note 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. * *Note Pattern Matching::, presents functions for matching regular expressions and shell file name patterns, and for expanding words as the shell does. * *Note Date and Time::, describes functions for measuring both calendar time and CPU time, as well as functions for setting alarms and timers. * *Note Character Set Handling::, contains information about manipulating characters and strings using character sets larger than will fit in the usual `char' data type. * *Note 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. * *Note 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. * *Note 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. * *Note Program Basics::, tells how your programs can access their command-line arguments and environment variables. * *Note Processes::, contains information about how to start new processes and run programs. * *Note 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. * *Note 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. * *Note User Database::, and *Note Group Database::, tell you how to access the system user and group databases. * *Note System Management::, describes functions for controlling and getting information about the hardware and software configuration your program is executing under. * *Note System Configuration::, tells you how you can get information about various operating system limits. Most of these parameters are provided for compatibility with POSIX. * *Note 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. * *Note Maintenance::, explains how to build and install the GNU C library on your system, how to report any bugs you might find, and 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 *Note 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. 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. 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 *Note 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 `*_errno ()'. In fact, that is what it is on the GNU system itself. The GNU library, on non-GNU systems, 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. *Note 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. On non-GNU 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 the GNU system, 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. 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 the GNU system, but they can occur using the GNU 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 *Note 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 (*note Executing a File::) occupy too much memory space. This condition never arises in the GNU system. - Macro: int ENOEXEC Invalid executable file format. This condition is detected by the `exec' functions; see *Note 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. *Note 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. In the GNU system, 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' (*note Hard Links::) but also when you rename a file with `rename' (*note 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; *note 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 *Note Linked Channels::. This error never occurs in the GNU system. - 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 in the GNU system; 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 (*note 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; *note 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'; *note 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; *note 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. *Note Socket Options::. - Macro: int EPROTONOSUPPORT The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). *Note 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. In the GNU system, 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. *Note Sockets::. - Macro: int EADDRINUSE The requested socket address is already in use. *Note 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. *Note 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. *Note 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'; *note Limits for Files::) or host name too long (in `gethostname' or `sethostname'; *note 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'. *Note 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 NFS file handle. This indicates an internal confusion in the NFS system which is due to file system rearrangements on the server host. Repairing this condition usually requires unmounting and remounting the NFS file system on the local host. - 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 the GNU system, 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 *Note File Locks::. This error is never generated by the GNU system, 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; *note 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 In the GNU system, 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. *Note Job Control::, for information on process groups and these signals. - Macro: int EDIED In the GNU system, 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 _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 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 `program_invocation_short_name' gives you convenient access to the name of the program that encountered the error. - Function: char * strerror (int ERRNUM) The `strerror' function maps the error code (*note 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) 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 the 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. This function `strerror_r' is a GNU extension and it is declared in `string.h'. - Function: void perror (const char *MESSAGE) This function prints an error message to the stream `stderr'; see *Note Standard Streams::. 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. On the GNU system, 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. *Compatibility Note:* The `strerror' function is a new feature of ISO C. Many older C systems do not support this function yet. 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. *Note Program Arguments::. - 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.) The library initialization code sets up both of these variables before calling `main'. *Portability Note:* These two variables are GNU extensions. 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. #include #include #include #include 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; } 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. *Note Memory-mapped I/O::. 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 *Note 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. *Note 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 (*note 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. *Note 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. *Note 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. *Note 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 *Note 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. 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 the GNU C library and GNU Compiler. 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. 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->name = x; ptr->next = current_foobar; current_foobar = ptr; } 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). 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) 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; *note 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' (*note Copying and Concatenation::): 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); *Note Representation of Strings::, for more information about this. 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) { register 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) { register 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. In the GNU system, the address is always a multiple of eight on most 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 `memalign', `posix_memalign' or `valloc' (*note 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' (*note Changing Block Size::). 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) The `free' function deallocates the block of memory pointed at by PTR. - Function: void cfree (void *PTR) 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->next; free (chain->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. 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) 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) { register 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. 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) 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. Efficiency Considerations for `malloc' ...................................... As opposed to other versions, the `malloc' in the GNU C Library does not round up block 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. Very large blocks (much larger than a page) are allocated with `mmap' (anonymous or via `/dev/zero') by this implementation. 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 can be adjusted with `mallopt'. The use of `mmap' can also be disabled completely. Allocating Aligned Memory Blocks ................................ The address of a block returned by `malloc' or `realloc' in the GNU system 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 `memalign', `posix_memalign', or `valloc'. These functions are declared in `stdlib.h'. With the GNU library, you can use `free' to free the blocks that `memalign', `posix_memalign', and `valloc' return. That does not work in BSD, however--BSD does not provide any way to free such blocks. - Function: void * memalign (size_t BOUNDARY, size_t SIZE) 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. - Function: int posix_memalign (void **MEMPTR, size_t ALIGNMENT, size_t SIZE) 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 `*MEMPTR' and the return value is zero. Otherwise the function returns an error value indicating the problem. This function was introduced in POSIX 1003.1d. - Function: void * valloc (size_t SIZE) Using `valloc' is like using `memalign' and passing the page size as the value of the second argument. It is implemented like this: void * valloc (size_t size) { return memalign (getpagesize (), size); } *Note Query Memory Parameters:: for more information about the memory subsystem. 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) 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_TRIM_THRESHOLD' This is the minimum size (in bytes) of the top-most, releasable chunk that will cause `sbrk' to be called with a negative argument in order to return memory to the system. `M_TOP_PAD' This parameter determines the amount of extra memory to obtain from the system when a call to `sbrk' is required. It also specifies the number of bytes to retain when shrinking the heap by calling `sbrk' with a negative argument. This provides the necessary hysteresis in heap size such that excessive amounts of system calls can be avoided. `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'. `M_MMAP_MAX' The maximum number of chunks to allocate with `mmap'. Setting this to zero disables all use of `mmap'. 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)) 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' (*note 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 `0' 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". >call mcheck(0) >continue >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) 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 `0', 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 behaviour it now writes something to the standard error desriptor. 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. 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 *FUNCTION (size_t SIZE, const void *CALLER) 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 *FUNCTION (void *PTR, size_t SIZE, const void *CALLER) 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 FUNCTION (void *PTR, const void *CALLER) 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 `memalign' uses whenever it is called. You should define this function to look like `memalign'; that is, like: void *FUNCTION (size_t SIZE, size_t ALIGNMENT, const void *CALLER) The value of CALLER is the return address found on the stack when the `memalign' function was 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. - Variable: __malloc_initialize_hook The value of this variable is a pointer to a function that is called once when the malloc implementation is initialized. This is a weak variable, so it can be overridden in the application with a definition like the following: void (*__MALLOC_INITIALIZE_HOOK) (void) = my_init_hook; 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. In both cases, the problem can be solved by setting up the hooks from within a user-defined function pointed to by `__malloc_initialize_hook'--then the hooks will be set up safely at the right time. 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 /* 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 *); /* Override initializing hook from the C library. */ void (*__malloc_initialize_hook) (void) = my_init_hook; static void my_init_hook (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 underlaying hooks */ old_malloc_hook = __malloc_hook; old_free_hook = __free_hook; /* `printf' might call `malloc', so protect it too. */ 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 underlaying hooks */ old_malloc_hook = __malloc_hook; old_free_hook = __free_hook; /* `printf' might call `free', so protect it too. */ printf ("freed pointer %p\n", ptr); /* Restore our own hooks */ __malloc_hook = my_malloc_hook; __free_hook = my_free_hook; } main () { ... } The `mcheck' function (*note Heap Consistency Checking::) works by installing such hooks. 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 *Note Efficiency and Malloc::.) `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. `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) This function returns information about the current dynamic memory usage in a structure of type `struct mallinfo'. Summary of `malloc'-Related Functions ..................................... Here is a summary of the functions that work with `malloc': `void *malloc (size_t SIZE)' Allocate a block of SIZE bytes. *Note Basic Allocation::. `void free (void *ADDR)' Free a block previously allocated by `malloc'. *Note Freeing after Malloc::. `void *realloc (void *ADDR, size_t SIZE)' Make a block previously allocated by `malloc' larger or smaller, possibly by copying it to a new location. *Note Changing Block Size::. `void *calloc (size_t COUNT, size_t ELTSIZE)' Allocate a block of COUNT * ELTSIZE bytes using `malloc', and set its contents to zero. *Note Allocating Cleared Space::. `void *valloc (size_t SIZE)' Allocate a block of SIZE bytes, starting on a page boundary. *Note Aligned Memory Blocks::. `void *memalign (size_t SIZE, size_t BOUNDARY)' Allocate a block of SIZE bytes, starting on an address that is a multiple of BOUNDARY. *Note Aligned Memory Blocks::. `int mallopt (int PARAM, int VALUE)' Adjust a tunable parameter. *Note Malloc Tunable Parameters::. `int mcheck (void (*ABORTFN) (void))' Tell `malloc' to perform occasional consistency checks on dynamically allocated memory, and to call ABORTFN when an inconsistency is found. *Note Heap Consistency Checking::. `void *(*__malloc_hook) (size_t SIZE, const void *CALLER)' A pointer to a function that `malloc' uses whenever it is called. `void *(*__realloc_hook) (void *PTR, size_t SIZE, const void *CALLER)' A pointer to a function that `realloc' uses whenever it is called. `void (*__free_hook) (void *PTR, const void *CALLER)' A pointer to a function that `free' uses whenever it is called. `void (*__memalign_hook) (size_t SIZE, size_t ALIGNMENT, const void *CALLER)' A pointer to a function that `memalign' uses whenever it is called. `struct mallinfo mallinfo (void)' Return information about the current dynamic memory usage. *Note Statistics of Malloc::. 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 assure 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. How to install the tracing functionality ........................................ - Function: void mtrace (void) 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 behaviour 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' (*note 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) The `muntrace' function can be called after `mtrace' was used to enable tracing the `malloc' calls. If no (succesful) 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'. Example program excerpts ........................ Even though the tracing functionality does not influence the runtime behaviour 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 int main (int argc, char *argv[]) { #ifdef DEBUGGING mtrace (); #endif ... } This is all what 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 no good idea to call `muntrace' before the program terminated. 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). 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 #include 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. Interpreting the traces ....................... If you take a look at the output it will look similar to this: = Start [0x8048209] - 0x8064cc8 [0x8048209] - 0x8064ce0 [0x8048209] - 0x8064cf8 [0x80481eb] + 0x8064c48 0x14 [0x80481eb] + 0x8064c60 0x14 [0x80481eb] + 0x8064c78 0x14 [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. 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. 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. 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 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' (*note 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. *Note 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) 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 (&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' (*note Program Termination::) or `longjmp' (*note Non-Local Exits::) and doesn't return. void my_obstack_alloc_failed (void) ... obstack_alloc_failed_handler = &my_obstack_alloc_failed; 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) 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 (&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) 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) 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 (&myobstack, addr, size); } Contrast this with the previous example of `savestring' using `malloc' (*note Basic Allocation::). 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) 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 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 (*note Preparing for Obstacks::). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk. 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) (); /* Use the macro. */ x = (char *) obstack_alloc (obptr, size); /* Call the function. */ x = (char *) (obstack_alloc) (obptr, size); /* Take the address of the function. */ funcp = obstack_alloc; This is the same situation that exists in ISO C for the standard library functions. *Note 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. 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) 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) 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) 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) 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) 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) 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) 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' (*note 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) 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. 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) 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 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) 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) 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) 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) 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 > 0) { int room = obstack_room (obstack); if (room == 0) { /* Not enough room. Add one character slowly, which may copy to a new chunk and make room. */ obstack_1grow (obstack, *ptr++); len--; } else { if (room > len) room = len; /* Add fast as much as we have room for. */ len -= room; while (room-- > 0) obstack_1grow_fast (obstack, *ptr++); } } } 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) 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) 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) This function returns the size in bytes of the currently growing object. This is equivalent to obstack_next_free (OBSTACK-PTR) - obstack_base (OBSTACK-PTR) 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 4 bytes. 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) 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 3, so that 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. 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' (*note 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) 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) < NEW-CHUNK-SIZE) obstack_chunk_size (obstack_ptr) = NEW-CHUNK-SIZE; 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 *OBSTACK-PTR)' Initialize use of an obstack. *Note Creating Obstacks::. `void *obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)' Allocate an object of SIZE uninitialized bytes. *Note Allocation in an Obstack::. `void *obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Allocate an object of SIZE bytes, with contents copied from ADDRESS. *Note Allocation in an Obstack::. `void *obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Allocate an object of SIZE+1 bytes, with SIZE of them copied from ADDRESS, followed by a null character at the end. *Note Allocation in an Obstack::. `void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)' Free OBJECT (and everything allocated in the specified obstack more recently than OBJECT). *Note Freeing Obstack Objects::. `void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)' Add SIZE uninitialized bytes to a growing object. *Note Growing Objects::. `void obstack_grow (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Add SIZE bytes, copied from ADDRESS, to a growing object. *Note Growing Objects::. `void obstack_grow0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Add SIZE bytes, copied from ADDRESS, to a growing object, and then add another byte containing a null character. *Note Growing Objects::. `void obstack_1grow (struct obstack *OBSTACK-PTR, char DATA-CHAR)' Add one byte containing DATA-CHAR to a growing object. *Note Growing Objects::. `void *obstack_finish (struct obstack *OBSTACK-PTR)' Finalize the object that is growing and return its permanent address. *Note Growing Objects::. `int obstack_object_size (struct obstack *OBSTACK-PTR)' Get the current size of the currently growing object. *Note Growing Objects::. `void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)' Add SIZE uninitialized bytes to a growing object without checking that there is enough room. *Note Extra Fast Growing::. `void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char DATA-CHAR)' Add one byte containing DATA-CHAR to a growing object without checking that there is enough room. *Note Extra Fast Growing::. `int obstack_room (struct obstack *OBSTACK-PTR)' Get the amount of room now available for growing the current object. *Note Extra Fast Growing::. `int obstack_alignment_mask (struct obstack *OBSTACK-PTR)' The mask used for aligning the beginning of an object. This is an lvalue. *Note Obstacks Data Alignment::. `int obstack_chunk_size (struct obstack *OBSTACK-PTR)' The size for allocating chunks. This is an lvalue. *Note Obstack Chunks::. `void *obstack_base (struct obstack *OBSTACK-PTR)' Tentative starting address of the currently growing object. *Note Status of an Obstack::. `void *obstack_next_free (struct obstack *OBSTACK-PTR)' Address just after the end of the currently growing object. *Note Status of an Obstack::. 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); 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)'. `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. 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 block, space used for any size block can be reused for any other size. `alloca' does not cause memory fragmentation. * Nonlocal exits done with `longjmp' (*note 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' (*note 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. 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 (*note 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. 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. *Note:* 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. 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 *Note 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) `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 (*note 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: int sbrk (ptrdiff_t DELTA) 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. 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. 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. *Note 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. 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). *Note 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. *Note 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. 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) `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) `mlock' 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) `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'. *Note 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 accomodated 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) `munlockall' unlocks every page in the calling process' virtual address space and turn 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. 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 *Note 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 (*note Extended Char Intro::). 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. *Note Locales::, for more information on locales. These functions are declared in the header file `ctype.h'. - Function: int islower (int C) 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) 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) 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) Returns true if C is a decimal digit (`0' through `9'). - Function: int isalnum (int C) 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) 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) 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) 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) Returns true if C is a blank character; that is, a space or a tab. This function is a GNU extension. - Function: int isgraph (int C) 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) Returns true if C is a printing character. Printing characters include all the graphic characters, plus the space (` ') character. - Function: int iscntrl (int C) Returns true if C is a control character (that is, a character that is not a printing character). - Function: int isascii (int C) 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. 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) 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) If C is a lower-case letter, `toupper' returns the corresponding upper-case letter. Otherwise C is returned unchanged. - Function: int toascii (int C) 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) This is identical to `tolower', and is provided for compatibility with the SVID. *Note SVID::. - Function: int _toupper (int C) This is identical to `toupper', and is provided for compatibility with the SVID. 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) The `wctype' returns a value representing a class of wide characters which is identified by the string PROPERTY. Beside 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) 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) 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) 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) 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) 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) 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) 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) 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) 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) 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) 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) 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) Returns true if WC is a blank character; that is, a space or a tab. This function is a GNU extension. It is declared in `wchar.h'. Notes on using the wide character classes ========================================= The first note is probably not astonishing but still occasionally a cause of problems. The `iswXXX' 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; } *Note 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. 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 apar from using the return value of the `wctrans' function. This type is defined in `wctype.h'. - Function: wctrans_t wctrans (const char *PROPERTY) 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) `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) 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) 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. String and Array Utilities ************************** Operations on strings (or arrays of characters) 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. Representation of Strings ========================= This section is a quick summary of string concepts for beginning C programmers. It describes how character 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 an array of `char' objects. But 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 (*note 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. "string" normally refers to multibyte character strings as opposed to wide character strings. Wide character strings are arrays of type `wchar_t' and as for multibyte character strings usually pointers of type `wchar_t *' are used. By convention, a "null character", `'\0'', marks the end of a multibyte character string and the "null wide character", `L'\0'', marks the end of a wide character string. For example, in testing to see whether the `char *' variable P points to a null character marking the end of a string, you can write `!*P' or `*P == '\0''. A null character is quite different conceptually from a null pointer, although both are represented by the integer `0'. "String literals" appear in C program source as strings of characters between double-quote characters (`"') where the initial double-quote character is immediately preceded by a capital `L' (ell) character (as in `L"foo"'). In ISO C, string literals can also be formed by "string concatenation": `"a" "b"' is the same as `"ab"'. For wide character strings one can either use `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. Character 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 the character array may extend past the null character that normally marks the end of the string. In this document, the term "allocated size" is always used to refer to the total amount of memory allocated for the string, while the term "length" refers to the number of characters up to (but not including) the terminating null character. A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters 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 character that marks the end of the string. Originally strings were sequences of bytes where each byte represents 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 *Note 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 `strncpy' 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" (*note 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 character 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 easier operate on wide character than on multibyte characters so that a general advise 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 character strings in parallel with the discussion of the multibyte character strings since there is almost always an exact equivalent available. String and Array Conventions ============================ This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters and wide characters. 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 character strings have names beginning with `str' and `wcs' respectively (such as `strcpy' and `wcscpy') and look for a null character to terminate the string 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 with a null character.) The array arguments and return values for these functions have type `char *' and `wchar_t *' respectively, and the array elements are referred to as "characters" 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 null-terminated 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 an 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 function the situation is similarly: the parameter type for a single wide character is `wint_t' and not `wchar_t'. This would for many implementations not be necessary since the `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. 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) The `strlen' function returns the length of the null-terminated string S in bytes. (In other words, it returns the offset of the terminating null character within the array.) For example, strlen ("hello, world") => 12 When applied to a character array, the `strlen' function returns the length of the string stored there, not its allocated size. You can get the allocated size of the character 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 character array itself, not a pointer to it. For example: char string[32] = "hello, world"; char *ptr = string; sizeof (string) => 32 sizeof (ptr) => 4 /* (on a machine with 4 byte pointers) */ 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: /* The input is in `string'. The length is expected in `n'. */ { mbstate_t t; char *scopy = string; /* In initial state. */ memset (&t, '\0', sizeof (t)); /* Determine number of characters. */ n = mbsrtowcs (NULL, &scopy, strlen (scopy), &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) The `wcslen' function is the wide character equivalent to `strlen'. The return value is the number of wide characters in the wide character 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 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) The `strnlen' function returns the length of the string S in bytes if this length is smaller than MAXLEN bytes. Otherwise it returns MAXLEN. Therefore this function is equivalent to `(strlen (S) < n ? strlen (S) : MAXLEN)' but it is more efficient and works even if the string S is not null-terminated. 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) `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'. Copying and Concatenation ========================= You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. The `str' and `mem' functions are declared in the header file `string.h' while the `wstr' and `wmem' functions are declared in the file `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. All of these functions return the address of the destination array. 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 character 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' (*note Formatted Output Functions::) and `scanf' (*note Formatted Input Functions::). - Function: void * memcpy (void *restrict TO, const void *restrict FROM, size_t SIZE) 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 *restruct WFROM, size_t SIZE) 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) 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 *) TO + SIZE))'. 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) 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 `WTO + SIZE'. 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) `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 *WTO, const wchar_t *WFROM, size_t SIZE) `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, `memmove' 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) 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) 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) 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) This copies characters from the string FROM (up to and including the terminating null character) 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) This copies wide characters from the 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 * strncpy (char *restrict TO, const char *restrict FROM, size_t SIZE) This function is similar to `strcpy' but always copies exactly SIZE characters into TO. If the length of FROM is more than SIZE, then `strncpy' copies just the first SIZE characters. Note that in this case there is no null terminator written into TO. If the length of FROM is less than SIZE, then `strncpy' copies all of FROM, followed by enough null characters to add up to SIZE characters in all. This behavior is rarely useful, but it is specified by the ISO C standard. The behavior of `strncpy' is undefined if the strings overlap. Using `strncpy' as opposed to `strcpy' is a way to avoid bugs relating to writing past the end of the allocated space for TO. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, SIZE may be large, and when it is, `strncpy' will waste a considerable amount of time copying null characters. - Function: wchar_t * wcsncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) This function is similar to `wcscpy' but always copies exactly SIZE wide characters into WTO. If the length of WFROM is more than SIZE, then `wcsncpy' copies just the first SIZE wide characters. Note that in this case there is no null terminator written into WTO. If the length of WFROM is less than SIZE, then `wcsncpy' 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 specified by the ISO C standard. The behavior of `wcsncpy' is undefined if the strings overlap. Using `wcsncpy' as opposed to `wcscpy' is a way to avoid bugs relating to writing past the end of the allocated space for WTO. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, SIZE may be large, and when it is, `wcsncpy' will waste a considerable amount of time copying null wide characters. - Function: char * strdup (const char *S) This function copies the null-terminated string S into a newly allocated string. The string is allocated using `malloc'; see *Note 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) This function copies the null-terminated wide character string WS into a newly allocated string. The string is allocated using `malloc'; see *Note 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 character string. This function is a GNU extension. - Function: char * strndup (const char *S, size_t SIZE) This function is similar to `strdup' but always copies at most SIZE characters into the newly allocated string. If the length of S is more than SIZE, then `strndup' copies just the first SIZE characters and adds a closing null terminator. Otherwise all characters are copied and the string is terminated. This function is different to `strncpy' in that it always terminates the destination string. `strndup' is a GNU extension. - Function: char * stpcpy (char *restrict TO, const char *restrict FROM) 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 character `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 #include int main (void) { char buffer[10]; char *to = buffer; to = stpcpy (to, "foo"); to = stpcpy (to, "bar"); puts (buffer); return 0; } This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG. 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) 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 character `wto + strlen (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'. - Function: char * stpncpy (char *restrict TO, const char *restrict FROM, size_t SIZE) This function is similar to `stpcpy' but copies always exactly SIZE characters into TO. If the length of FROM is more then SIZE, then `stpncpy' copies just the first SIZE characters and returns a pointer to the character 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 characters to add up to SIZE characters in all. This behaviour is rarely useful, but it is implemented to be useful in contexts where this behaviour of the `strncpy' is used. `stpncpy' returns a pointer to the _first_ written null character. This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. Its behaviour is undefined if the strings overlap. The function is declared in `string.h'. - Function: wchar_t * wcpncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) This function is similar to `wcpcpy' but copies always exactly WSIZE characters into WTO. If the length of WFROM is more then SIZE, then `wcpncpy' copies just the first SIZE wide characters and returns a pointer to the wide character directly following the one 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 characters to add up to SIZE characters in all. This behaviour is rarely useful, but it is implemented to be useful in contexts where this behaviour of the `wcsncpy' is used. `wcpncpy' returns a pointer to the _first_ written null character. This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. Its behaviour is undefined if the strings overlap. `wcpncpy' is a GNU extension and is declared in `wchar.h'. - Macro: char * strdupa (const char *S) This macro is similar to `strdup' but allocates the new string using `alloca' instead of `malloc' (*note 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 #include #include 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' (*note Variable Size Automatic::) can interfere with the parameter passing. This function is only available if GNU CC is used. - Macro: char * strndupa (const char *S, size_t SIZE) This function is similar to `strndup' but like `strdupa' it allocates the new string using `alloca' *note 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. `strndupa' is only available if GNU CC is used. - Function: char * strcat (char *restrict TO, const char *restrict FROM) The `strcat' function is similar to `strcpy', except that the characters from FROM are concatenated or appended to the end of TO, instead of overwriting it. That is, the first character from FROM overwrites the null character 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. - Function: wchar_t * wcscat (wchar_t *restrict WTO, const wchar_t *restrict WFROM) The `wcscat' function is similar to `wcscpy', except that the characters from WFROM are concatenated or appended to the end of WTO, instead of overwriting it. That is, the first character from WFROM overwrites the null 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. Programmers using the `strcat' or `wcscat' function (or the following `strncat' or `wcsncar' functions 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: /* This function concatenates arbitrarily many strings. The last parameter must be `NULL'. */ char * concat (const char *str, ...) { va_list ap, ap2; size_t total = 1; const char *s; char *result; va_start (ap, str); /* Actually `va_copy', but this is the name more gcc versions understand. */ __va_copy (ap2, ap); /* Determine how much space we need. */ 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'; /* Copy the strings. */ 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 efficient: char * concat (const char *str, ...) { va_list ap; size_t allocated = 100; char *result = (char *) malloc (allocated); char *wp; if (allocated != NULL) { char *newp; va_start (ap, atr); wp = result; for (s = str; s != NULL; s = va_arg (ap, const char *)) { size_t len = strlen (s); /* Resize the allocated memory if necessary. */ if (wp + len + 1 > 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); } /* Terminate the result string. */ *wp++ = '\0'; /* Resize memory to the optimal size. */ 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 safe us 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 handle with 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 whether the code cannot be rewritten to take advantage of already calculated results. Again: it is almost always unnecessary to use `strcat'. - Function: char * strncat (char *restrict TO, const char *restrict FROM, size_t SIZE) This function is like `strcat' except that not more than SIZE characters from FROM are appended to the end of TO. A single null character is also always appended to TO, so the total allocated size of TO must be at least `SIZE + 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) { to[strlen (to) + size] = '\0'; strncpy (to + strlen (to), from, size); return to; } The behavior of `strncat' is undefined if the strings overlap. - Function: wchar_t * wcsncat (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) This function is like `wcscat' except that not more than SIZE characters from FROM are appended to the end of TO. A single null character is also always appended to TO, so the total allocated size of TO must be at least `SIZE + 1' bytes 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) { wto[wcslen (to) + size] = L'\0'; wcsncpy (wto + wcslen (wto), wfrom, size); return wto; } The behavior of `wcsncat' is undefined if the strings overlap. Here is an example showing the use of `strncpy' and `strncat' (the wide character version is equivalent). Notice how, in the call to `strncat', the SIZE parameter is computed to avoid overflowing the character array `buffer'. #include #include #define SIZE 10 static char buffer[SIZE]; main () { strncpy (buffer, "hello", SIZE); puts (buffer); strncat (buffer, ", world", SIZE - strlen (buffer) - 1); puts (buffer); } The output produced by this program looks like: hello hello, wo - Function: void bcopy (const void *FROM, void *TO, size_t SIZE) 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) 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. 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. *Note 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 characters in 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) 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 `0'. - Function: int wmemcmp (const wchar_t *A1, const wchar_t *A2, size_t SIZE) 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 character in A2. If the contents of the two blocks are equal, `wmemcmp' returns `0'. 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 characters 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) 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 characters (interpreted as `unsigned char' objects, then promoted to `int'). If the two strings are equal, `strcmp' returns `0'. 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) The `wcscmp' function compares the wide character 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 character in WS2. If the two strings are equal, `wcscmp' returns `0'. 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) This function is like `strcmp', 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 A" and a" 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) 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 A" and a" 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) This function is the similar to `strcmp', 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 wcsncmp (const wchar_t *WS1, const wchar_t *WS2, size_t SIZE) This function is the 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) This function is like `strncmp', except that differences in case are ignored. 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) 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 /* These two strings are the same. */ strcmp ("hello", "Hello") => 32 /* Comparisons are case-sensitive. */ strcmp ("hello", "world") => -15 /* The character `'h'' comes before `'w''. */ strcmp ("hello", "hello, world") => -44 /* Comparing a null character against a comma. */ strncmp ("hello", "hello, world", 5) => 0 /* The initial 5 characters are the same. */ strncmp ("hello, world", "hello, stupid world!!!", 5) => 0 /* The initial 5 characters are the same. */ - Function: int strverscmp (const char *S1, const char *S2) The `strverscmp' function compares the string S1 against S2, considering them as holding indices/version numbers. Return value follows the same conventions as found in the `strverscmp' function. In fact, if S1 and S2 contain no digits, `strverscmp' behaves like `strcmp'. Basically, we compare strings normally (character by character), until we find a digit in each string - then we enter a special comparison mode, where each sequence of digits is taken as a whole. If we reach the end of these two parts without noticing a difference, we return to the standard comparison mode. There are two types of numeric parts: "integral" and "fractional" (those begin with a '0'). The types of the numeric parts affect the way we sort them: * integral/integral: we compare values as you would expect. * fractional/integral: the fractional part is less than the integral one. Again, no surprise. * fractional/fractional: the things become a bit more complex. If the common prefix contains only leading zeroes, the longest part is less than the other one; else the comparison behaves normally. strverscmp ("no digit", "no digit") => 0 /* same behaviour as strcmp. */ strverscmp ("item#99", "item#100") => <0 /* same prefix, but 99 < 100. */ strverscmp ("alpha1", "alpha001") => >0 /* fractional part inferior to integral one. */ strverscmp ("part1_f012", "part1_f01") => >0 /* two fractional parts. */ strverscmp ("foo.009", "foo.0") => <0 /* idem, but with leading zeroes only. */ This function is especially useful when dealing with filename sorting, because filenames frequently hold indices/version numbers. `strverscmp' is a GNU extension. - Function: int bcmp (const void *A1, const void *A2, size_t SIZE) This is an obsolete alias for `memcmp', derived from BSD. 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 *Note 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 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) The `strcoll' function is similar to `strcmp' but uses the collating sequence of the current locale for collation (the `LC_COLLATE' locale). - Function: int wcscoll (const wchar_t *WS1, const wchar_t *WS2) 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' (*note 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'.) /* This is the comparison function used with `qsort'. */ int compare_elements (char **p1, char **p2) { return strcoll (*p1, *p2); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings (char **array, int nstrings) { /* Sort `temp_array' by comparing the strings. */ qsort (array, nstrings, sizeof (char *), compare_elements); } - Function: size_t strxfrm (char *restrict TO, const char *restrict FROM, size_t SIZE) The function `strxfrm' transforms the 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 characters (including a terminating null character) are stored. The behavior is undefined if the strings TO and FROM overlap; see *Note Copying and Concatenation::. 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 characters are stored in TO. In this case, `strxfrm' simply returns the number of characters 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) The function `wcsxfrm' transforms wide character 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 character) are stored. The behavior is undefined if the strings WTO and WFROM overlap; see *Note Copying and Concatenation::. The return value is the length of the entire transformed wide character 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 character string did not entirely fit in the array WTO. In this case, only as much of the wide character string as actually fits was stored. To get the whole transformed wide character string, call `wcsxfrm' again with a bigger output array. The transformed wide character string may be longer than the original wide character string, and it may also be shorter. If SIZE is zero, no 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 character 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; }; /* This is the comparison function used with `qsort' to sort an array of `struct sorter'. */ int compare_elements (struct sorter *p1, struct sorter *p2) { return strcmp (p1->transformed, p2->transformed); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings_fast (char **array, int nstrings) { struct sorter temp_array[nstrings]; int i; /* Set up `temp_array'. Each element contains one input string and its transformed string. */ for (i = 0; i < nstrings; i++) { size_t length = strlen (array[i]) * 2; char *transformed; size_t transformed_length; temp_array[i].input = array[i]; /* First try a buffer perhaps big enough. */ transformed = (char *) xmalloc (length); /* Transform `array[i]'. */ transformed_length = strxfrm (transformed, array[i], length); /* If the buffer was not large enough, resize it and try again. */ if (transformed_length >= length) { /* Allocate the needed space. +1 for terminating `NUL' character. */ transformed = (char *) xrealloc (transformed, transformed_length + 1); /* The return value is not interesting because we know how long the transformed string is. */ (void) strxfrm (transformed, array[i], transformed_length + 1); } temp_array[i].transformed = transformed; } /* Sort `temp_array' by comparing transformed strings. */ qsort (temp_array, sizeof (struct sorter), nstrings, compare_elements); /* Put the elements back in the permanent array in their sorted order. */ for (i = 0; i < nstrings; i++) array[i] = temp_array[i].input; /* Free the strings we allocated. */ for (i = 0; i < 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) { ... /* Transform `array[i]'. */ transformed_length = wcsxfrm (transformed, array[i], length); /* If the buffer was not large enough, resize it and try again. */ if (transformed_length >= length) { /* Allocate the needed space. +1 for terminating `NUL' character. */ transformed = (wchar_t *) xrealloc (transformed, (transformed_length + 1) * sizeof (wchar_t)); /* The return value is not interesting because we know how long the transformed string is. */ (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. 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) 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) 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) 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) 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). - Function: char * strchr (const char *STRING, int C) The `strchr' function finds the first occurrence of the character C (converted to a `char') in the null-terminated string beginning at STRING. The return value is a pointer to the located character, or a null pointer if no match was found. For example, strchr ("hello, world", 'l') => "llo, world" strchr ("hello, world", '?') => NULL The terminating null character 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 a null character as the value of the C argument. It would be better (but less portable) to use `strchrnul' in this case, though. - Function: wchar_t * wcschr (const wchar_t *WSTRING, int WC) The `wcschr' function finds the first occurrence of the wide character WC in the null-terminated wide character 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 character is considered to be part of the wide character string, so you can use this function get a pointer to the end of a wide character string by specifying a null wude 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) `strchrnul' is the same as `strchr' except that if it does not find the character, it returns a pointer to string's terminating null character rather than a null pointer. This function is a GNU extension. - Function: wchar_t * wcschrnul (const wchar_t *WSTRING, wchar_t WC) `wcschrnul' is the same as `wcschr' except that if it does not find the wide character, it returns a pointer to wide character 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 NUL 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 the NUL character. 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) 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) 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) This is like `strchr', except that it searches HAYSTACK for a substring NEEDLE rather than just a single character. It returns a pointer into the string HAYSTACK that is the first character 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) 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) `wcsstr' is an depricated 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) 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. For example, strstr ("hello, world", "L") => "llo, world" strstr ("hello, World", "wo") => "World" - Function: void * memmem (const void *HAYSTACK, size_t HAYSTACK-LEN, const void *NEEDLE, size_t NEEDLE-LEN) This is like `strstr', but NEEDLE and HAYSTACK are byte arrays rather than null-terminated 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) The `strspn' ("string span") function returns the length of the initial substring of STRING that consists entirely of characters that are members of the set specified by the string SKIPSET. The order of the characters in SKIPSET is not important. For example, strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz") => 5 Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: size_t wcsspn (const wchar_t *WSTRING, const wchar_t *SKIPSET) 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) The `strcspn' ("string complement span") function returns the length of the initial substring of STRING that consists entirely of characters that are _not_ members of the set specified by the string STOPSET. (In other words, it returns the offset of the first character in STRING that is a member of the set STOPSET.) For example, strcspn ("hello, world", " \t\n,.;!?") => 5 Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: size_t wcscspn (const wchar_t *WSTRING, const wchar_t *STOPSET) 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 character in STRING that is a member of the set STOPSET.) - Function: char * strpbrk (const char *STRING, const char *STOPSET) The `strpbrk' ("string pointer break") function is related to `strcspn', except that it returns a pointer to the first character 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 character from STOPSET is found. For example, strpbrk ("hello, world", " \t\n,.;!?") => ", world" Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: wchar_t * wcspbrk (const wchar_t *WSTRING, const wchar_t *STOPSET) 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 character from STOPSET is found. Compatibility String Search Functions ------------------------------------- - Function: char * index (const char *STRING, int C) `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) `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. 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) 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 characters that are members of this set are discarded. The first 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 character that is a member of the delimiter set. This character in the original string NEWSTRING is overwritten by a null character, 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 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 `strtok'. If the end of the string NEWSTRING is reached, or if the remainder of string consists only of delimiter characters, `strtok' returns a null pointer. Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: wchar_t * wcstok (wchar_t *NEWSTRING, const char *DELIMITERS) 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 character string are indicated by passing a null pointer as the NEWSTRING argument. Calling `wcstok' with another non-null NEWSTRING argument reinitializes the state information. It is guaranteed that no other library function ever calls `wcstok' behind your back (which would mess up this internal state information). The DELIMITERS argument is a wide character 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 character string NEWSTRING is overwritten by a null wide character, 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 character string NEWSTRING is reached, or if the remainder of string consists only of delimiter wide characters, `wcstok' returns a null pointer. Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. *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' (*note Copying and Concatenation::). 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. *Note 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 libc 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 functions `strtok' and `wcstok' are not reentrant. *Note Nonreentrancy::, for a discussion of where and why reentrancy is important. Here is a simple example showing the use of `strtok'. #include #include ... 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 => "words" */ token = strtok (NULL, delimiters); /* token => "separated" */ token = strtok (NULL, delimiters); /* token => "by" */ token = strtok (NULL, delimiters); /* token => "spaces" */ token = strtok (NULL, delimiters); /* token => "and" */ token = strtok (NULL, delimiters); /* token => "punctuation" */ token = strtok (NULL, delimiters); /* token => NULL */ The GNU C library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy. They are only available for multibyte character strings. - Function: char * strtok_r (char *NEWSTRING, const char *DELIMITERS, char **SAVE_PTR) 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 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) 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 character from DELIMITER in a row `strsep' returns an empty string for each pair of characters 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 #include ... const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *running; char *token; ... running = strdupa (string); token = strsep (&running, delimiters); /* token => "words" */ token = strsep (&running, delimiters); /* token => "separated" */ token = strsep (&running, delimiters); /* token => "by" */ token = strsep (&running, delimiters); /* token => "spaces" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "and" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "punctuation" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => NULL */ - Function: char * basename (const char *FILENAME) The GNU version of the `basename' function returns the last component of the path in FILENAME. This function is the prefered 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 overriden by the XPG version, if `libgen.h' is included. Example of using GNU `basename': #include int main (int argc, char *argv[]) { char *prog = basename (argv[0]); if (argc < 2) { fprintf (stderr, "Usage %s \n", prog); exit (1); } ... } *Portability Note:* This function may produce different results on different systems. - Function: char * basename (char *PATH) This is the standard XPG defined `basename'. It is similar in spirit to the GNU version, but may modify the PATH by removing trailing '/' characters. If the PATH is made up entirely of '/' characters, 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 int main (int argc, char *argv[]) { char *prog; char *path = strdupa (argv[0]); prog = basename (path); if (argc < 2) { fprintf (stderr, "Usage %s \n", prog); exit (1); } ... } - Function: char * dirname (char *PATH) 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 '/' characters, then "." is returned. The prototype for this function can be found in `libgen.h'. 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) `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. 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, *Note Cryptographic Functions::. This function is declared in `string.h'. - Function: void * memfrob (void *MEM, size_t LENGTH) `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 *Note Cryptographic Functions::. *Portability Note:* This function is unique to the GNU C library. 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 characters in the range allowed for storing or transfering. SVID systems (and nowadays XPG compliant systems) provide minimal support for this task. - Function: char * l64a (long int N) This function encodes a 32-bit input value using characters from the basic character set. It returns a pointer to a 6 character 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) { /* We know in advance how long the buffer has to be. */ unsigned char *in = (unsigned char *) buf; char *out = malloc (6 + ((len + 3) / 4) * 6 + 1); char *cp = out; /* Encode the length. */ /* Using `htonl' is necessary so that the data can be decoded even on machines with different byte order. */ cp = mempcpy (cp, l64a (htonl (len)), 6); while (len > 3) { unsigned long int n = *in++; n = (n << 8) | *in++; n = (n << 8) | *in++; n = (n << 8) | *in++; len -= 4; if (n) cp = mempcpy (cp, l64a (htonl (n)), 6); else /* `l64a' returns the empty string for n==0, so we must generate its encoding ("......") by hand. */ cp = stpcpy (cp, "......"); } if (len > 0) { unsigned long int n = *in++; if (--len > 0) { n = (n << 8) | *in++; if (--len > 0) n = (n << 8) | *in; } memcpy (cp, l64a (htonl (n)), 6); cp += 6; } *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) The parameter STRING should contain a string which was produced by a call to `l64a'. The function processes at least 6 characters of this string, and decodes the characters it finds according to the table below. It stops decoding when it finds a character 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 characters. The decoded number is returned as a `long int' value. The `l64a' and `a64l' functions use a base 64 encoding, in which each character of an encoded string represents six bits of an input word. These symbols are used for the base 64 digits: 0 1 2 3 4 5 6 7 0 `.' `/' `0' `1' `2' `3' `4' `5' 8 `6' `7' `8' `9' `A' `B' `C' `D' 16 `E' `F' `G' `H' `I' `J' `K' `L' 24 `M' `N' `O' `P' `Q' `R' `S' `T' 32 `U' `V' `W' `X' `Y' `Z' `a' `b' 40 `c' `d' `e' `f' `g' `h' `i' `j' 48 `k' `l' `m' `n' `o' `p' `q' `r' 56 `s' `t' `u' `v' `w' `x' `y' `z' 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. Argz and Envz Vectors ===================== "argz vectors" are vectors of strings in a contiguous block of memory, each element separated from its neighbors by null-characters (`'\0''). "Envz vectors" are an extension of argz vectors where each element is a name-value pair, separated by a `'='' character (as in a Unix environment). 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 `0' 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 creating 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). *Note Unconstrained Allocation::. All argz functions that do memory allocation have a return type of `error_t', and return `0' 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) The `argz_create' function converts the Unix-style argument vector ARGV (a vector of pointers to normal C strings, terminated by `(char *)0'; *note 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) The `argz_create_sep' function converts the null-terminated string STRING into an argz vector (returned in ARGZ and ARGZ_LEN) by splitting it into elements at every occurrence of the character SEP. - Function: size_t argz_count (const char *ARGZ, size_t ARG_LEN) Returns the number of elements in the argz vector ARGZ and ARGZ_LEN. - Function: void argz_extract (char *ARGZ, size_t ARGZ_LEN, char **ARGV) 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 `0'. ARGV must be pre-allocated with enough space to hold all the elements in ARGZ plus the terminating `(char *)0' (`(argz_count (ARGZ, ARGZ_LEN) + 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 (*note Executing a File::). - Function: void argz_stringify (char *ARGZ, size_t LEN, int SEP) The `argz_stringify' converts ARGZ into a normal string with the elements separated by the character 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) The `argz_add' function adds the string STR to the end of the argz vector `*ARGZ', and updates `*ARGZ' and `*ARGZ_LEN' accordingly. - Function: error_t argz_add_sep (char **ARGZ, size_t *ARGZ_LEN, const char *STR, int DELIM) The `argz_add_sep' function is similar to `argz_add', but STR is split into separate elements in the result at occurrences of the character 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) The `argz_append' function appends BUF_LEN bytes starting at BUF to the argz vector `*ARGZ', reallocating `*ARGZ' to accommodate it, and adding BUF_LEN to `*ARGZ_LEN'. - Function: error_t argz_delete (char **ARGZ, size_t *ARGZ_LEN, char *ENTRY) If ENTRY points to the beginning of one of the elements in the argz vector `*ARGZ', the `argz_delete' function will remove this entry and reallocate `*ARGZ', modifying `*ARGZ' and `*ARGZ_LEN' 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) The `argz_insert' function inserts the string ENTRY into the argz vector `*ARGZ' at a point just before the existing element pointed to by BEFORE, reallocating `*ARGZ' and updating `*ARGZ' and `*ARGZ_LEN'. If BEFORE is `0', ENTRY is added to the end instead (as if by `argz_add'). Since the first element is in fact the same as `*ARGZ', passing in `*ARGZ' as the value of BEFORE will result in ENTRY being inserted at the beginning. - Function: char * argz_next (char *ARGZ, size_t ARGZ_LEN, const char *ENTRY) 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 `0' if there are no elements following ENTRY. If ENTRY is `0', the first element of ARGZ is returned. This behavior suggests two styles of iteration: char *entry = 0; while ((entry = argz_next (ARGZ, ARGZ_LEN, entry))) ACTION; (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 = ARGZ; entry; entry = argz_next (ARGZ, ARGZ_LEN, entry)) ACTION; Note that the latter depends on ARGZ having a value of `0' 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 **ARGZ, size_t *ARGZ_LEN, const char *STR, const char *WITH, unsigned *REPLACE_COUNT) Replace any occurrences of the string STR in ARGZ with WITH, reallocating ARGZ as necessary. If REPLACE_COUNT is non-zero, `*REPLACE_COUNT' will be incremented by number of replacements performed. 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 `'='' character; if multiple `'='' characters are present in an element, those after the first are considered part of the value, and treated like all other non-`'\0'' characters. If _no_ `'='' characters 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 `0' 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 `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 `0' 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) 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 `'='' character. If there is no entry with that name, `0' is returned. - Function: char * envz_get (const char *ENVZ, size_t ENVZ_LEN, const char *NAME) 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), `0' is returned. - Function: error_t envz_add (char **ENVZ, size_t *ENVZ_LEN, const char *NAME, const char *VALUE) The `envz_add' function adds an entry to `*ENVZ' (updating `*ENVZ' and `*ENVZ_LEN') 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 `0', then the new entry will 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) The `envz_merge' function adds each entry in ENVZ2 to ENVZ, as if with `envz_add', updating `*ENVZ' and `*ENVZ_LEN'. 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) The `envz_strip' function removes any null entries from ENVZ, updating `*ENVZ' and `*ENVZ_LEN'. 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 which was added to the C library to support multiple character sets. Introduction to Extended Characters =================================== A variety of solutions 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 exist. 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 whatever communication channel. Examples of external representations include files lying in a directory that are going 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 changes 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 which 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, 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, there exists a completely new family of functions which can handle texts of this kind 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 which 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 which take a single wide character. - Data type: wchar_t This data type is used as the base type for wide character strings. I.e., 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 this type 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'. This might make sense for embedded systems. But for GNU systems this type is always 32 bits wide. It is 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 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 this contradicts the purpose of the `wchar_t' type. - Data type: wint_t `wint_t' is a data type used for parameters and variables which contain a single wide character. As the name already suggests it is the equivalent to `int' when using the normal `char' strings. The types `wchar_t' and `wint_t' have often the same representation if their size if 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 got introduced in Amendment 1 to ISO C90. As there are for the `char' data type there also exist macros 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 got 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 got 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. I.e., sloppy code like { int c; ... while ((c = getc (fp)) < 0) ... } has to be rewritten to explicitly use `WEOF' 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 storing and transmittal, since a single wide character consists of more than one byte they are effected by byte-ordering. I.e., machines with different endianesses would see different value accessing the same data. This also applies for communication protocols which are all byte-based and therefore 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 an customized byte oriented character set. For all the above reasons, an external encoding which 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. There exist a variety of different character sets which can be used for this external encoding. Information which will not be exhaustively presented here-instead, a description of the major groups will suffice. All of the ASCII-based character sets [_bkoz_: do you mean Roman character sets? If not, what do you mean here?] 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 operation 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. Another reason to choose this is because of constraints from interaction with other programs (which might not be 8-bit clean). * 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. Embedded in the text can be characters which can be used to change the state. 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 which 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 operations systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN) or SJIS (Shift-JIS, a Japanese encoding). But there are also character sets using a state which 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. E.g., the byte sequence `0xc2 0x61' (non-spacing acute accent, following 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). This type of character set is 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 10464 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 which should be used. In fact, UTF-8 will hopefully soon be the only external encoding that has to be supported. It proves to be universally usable and the 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. I.e., if a project is expected to only be used in, 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 which 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 as much as possible avoid making assumption about the wide character representation. If the programmer uses only the functions provided by the C library to handle wide character strings there should not be any compatibility problems with other systems. Overview about Character Handling Functions =========================================== A Unix C library contains three different sets of functions in two families to handle character set conversion. The one function family is specified in the ISO C standard and therefore is portable even beyond the Unix world. The most commonly known set of functions, coming from the ISO C90 standard, is unfortunately the least useful one. In fact, 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. 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 *Note 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 the standard which allow specifying a size but basically they also expect terminated strings. Despite these limitations the ISO C functions can very well be used in many contexts. In graphical user interfaces, for instance, it is not uncommon to have functions which require text to be displayed in a wide character string if it 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 (*note Generic Charset Conversion::). Selecting the conversion and its properties ------------------------------------------- We already said above that the currently selected locale for the `LC_CTYPE' category decides about the conversion which 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 character set always is UCS-4, at least on GNU systems. A characteristic of each multibyte character set is the maximum number of bytes which can be necessary to represent one character. This information is quite important when writing code which uses the conversion functions. In the examples below we will see some examples. The ISO C standard defines two macros which provide this information. - Macro: int MB_LEN_MAX This macro 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 it 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 fact, 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 (&buf[len], 1, MB_CUR_MAX - len, fp); /* ... process 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 always `MB_CUR_MAX' bytes are available in BUF. Note that it isn't a problem if `MB_CUR_MAX' is not a compile-time constant. Representing the state of the conversion ---------------------------------------- In the introduction of this chapter it was said that certain character sets use a "stateful" encoding. I.e., 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. This type is defined in `wchar.h'. It got introduced in Amendment 1 to ISO C90. To use objects of this type 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 (&state, '\0', sizeof (state)); /* from now on STATE can be used. */ ... } 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 or not 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) This function determines whether the state object pointed to by PS is in the initial state or not. If PS is a null pointer or the object is in the initial state the return value is nonzero. Otherwise it is zero. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Code using this function often looks similar to this: { mbstate_t state; memset (&state, '\0', sizeof (state)); /* Use STATE. */ ... if (! mbsinit (&state)) { /* Emit code to return to initial state. */ const wchar_t empty[] = L""; const wchar_t *srcp = empty; wcsrtombs (outbuf, &srcp, outbuflen, &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 (*note Converting Strings::). Please note that on GNU systems 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 which prohibits making `wchar_t' using a stateful encoding. 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 which consists of single byte sequences there are functions to help with converting bytes. One very important and often applicable scenario is where ASCII is a subpart of the multibyte character set. I.e., all ASCII characters stand for itself and all other characters have at least a first byte which is beyond the range 0 to 127. - Function: wint_t btowc (int C) 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) C' 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. There is no `mbstate_t' object used from which the state information is taken and the function also does not use any static state. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Despite the limitation that the single byte value always is 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 extension 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 = &buf[29]; *wcp = L'\0'; while (val != 0) { *--wcp = btowc ('0' + val % 10); val /= 10; } if (wcp == &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 it is necessary `btowc'. There also is a function for the conversion in the other direction. - Function: int wctob (wint_t C) 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'. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. There are more general functions to convert single character 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) 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 which 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 pointer to by PWC iff 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 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 also 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. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Using this function is straight forward. A function which copies a multibyte string into a wide character string while at the same time converting all lowercase character into uppercase could look like this (this is not the final version, just an example; it has no error checking, and leaks sometimes 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 (&state, '\0', sizeof (state)); while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0) { if (nbytes >= (size_t) -2) /* Invalid input string. */ return NULL; *result++ = towupper (tmp[0]); len -= nbytes; s += nbytes; } return result; } The use of `mbrtowc' should be clear. A single wide character is stored in `TMP[0]' and the number of consumed bytes is stored in the variable NBYTES. In case the the conversion was 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 the function 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 results than there are bytes in the multibyte input string. This method yields to a pessimistic guess about the size of the result and if many wide character strings have to be constructed this way or the strings are long, the extra memory required allocated because the input string contains multibyte characters might be significant. It would be possible to resize the allocated memory block to the correct size before returning it. 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. But there is a function which does part of the work. - Function: size_t mbrlen (const char *restrict S, size_t N, mbstate_t *PS) 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 the first N bytes possibly form a valid multibyte character but it 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. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. The tentative reader now will of course note that `mbrlen' can be implemented as mbrtowc (NULL, s, n, ps != NULL ? ps : &internal) This is true and in fact is mentioned in the official specification. Now, 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 (&state, '\0', sizeof (state)); while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0) { if (nbytes >= (size_t) -2) /* Something is wrong. */ 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 OK since a) this value is larger then the length of the longest multibyte character sequence and b) because 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 character 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' implemented using `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) 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 the state stored in the objects pointer 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 is stored in the string S. Otherwise a byte sequence (possibly including shift sequences) is written into the string S. This of 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 byte 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. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Using this function 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 (and 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; /* Find the end of the existing string. */ char *wp = strchr (s, '\0'); len -= wp - s; memset (&state, '\0', sizeof (state)); do { size_t nbytes; if (len < MB_CUR_LEN) { /* We cannot guarantee that the next character fits into the buffer, so return an error. */ errno = E2BIG; return NULL; } nbytes = wcrtomb (wp, *ws, &state); if (nbytes == (size_t) -1) /* Error in the conversion. */ 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 never is 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 or not. If this solution is really unsuitable there is a very slow but more accurate solution. ... if (len < MB_CUR_LEN) { mbstate_t temp_state; memcpy (&temp_state, &state, sizeof (state)); if (wcrtomb (NULL, *ws, &temp_state) > len) { /* We cannot guarantee that the next character fits into the buffer, so return an error. */ errno = E2BIG; return NULL; } } ... Here we do perform the conversion which 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 think about this: 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 meant for educational purposes. 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, thus the GNU C library contains a few extensions which 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) The `mbsrtowcs' function ("multibyte string restartable to wide character string") converts an NUL terminated multibyte character string at `*SRC' 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 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 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. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. The definition of this function has one limitation which has to be understood. The requirement that DST has to be a NUL terminated string provides problems if one wants to convert buffers with text. A buffer is normally no collection of NUL terminated strings but instead a continuous collection of lines, separated by newline characters. Now assume 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 always could do only a 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 is 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 never should be used in generally used code. The generic conversion interface (*note 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 which take additional parameters specifying the maximal number of bytes which 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) The `wcsrtombs' function ("wide character string restartable to multibyte string") converts the NUL terminated wide character string at `*SRC' 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 locally 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 DEST 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 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. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. The restriction mentions above for the `mbsrtowcs' function applies also here. There is no possibility to directly control 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) 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. I.e., the multibyte character string `*SRC' need not be NUL terminated. But if a NUL byte is found within the NMC first bytes of the string the conversion stops here. This function is a GNU extensions. It is meant to work around the problems mentioned above. Now it is possible to convert buffer with multibyte character text piece for 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 (&state, '\0', sizeof (state)); while (1) { wchar_t linebuf[100]; const char *endp = strchr (src, '\n'); size_t n; /* Exit if there is no more line. */ if (endp == NULL) break; n = mbsnrtowcs (linebuf, &src, endp - src, 99, &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 which 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) The `wcsnrtombs' function implements the conversion from wide character strings to multibyte character strings. It is similar to `wcsrtombs' but it takes, just like `mbsnrtowcs', an extra parameter which specifies the length of the input string. No more than NWC wide characters from the input string `*SRC' are converted. If the input string contains a NUL wide character in the first NWC character to conversion stops at this place. This function is a GNU extension and just like `mbsnrtowcs' is helps in situations where no NUL terminated input strings are available. 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) { /* Note the use of `MB_LEN_MAX'. `MB_CUR_MAX' cannot portably be used here. */ char buffer[BUFSIZ + MB_LEN_MAX]; mbstate_t state; int filled = 0; int eof = 0; /* Initialize the state. */ memset (&state, '\0', sizeof (state)); while (!eof) { ssize_t nread; ssize_t nwrite; char *inp = buffer; wchar_t outbuf[BUFSIZ]; wchar_t *outp = outbuf; /* Fill up the buffer from the input file. */ nread = read (input, buffer + filled, BUFSIZ); if (nread < 0) { perror ("read"); return 0; } /* If we reach end of file, make a note to read no more. */ if (nread == 0) eof = 1; /* `filled' is now the number of bytes in `buffer'. */ filled += nread; /* Convert those bytes to wide characters-as many as we can. */ while (1) { size_t thislen = mbrtowc (outp, inp, filled, &state); /* Stop converting at invalid character; this can mean we have read just the first part of a valid character. */ if (thislen == (size_t) -1) break; /* We want to handle embedded NUL bytes but the return value is 0. Correct this. */ if (thislen == 0) thislen = 1; /* Advance past this character. */ inp += thislen; filled -= thislen; ++outp; } /* Write the wide characters we just made. */ nwrite = write (output, outbuf, (outp - outbuf) * sizeof (wchar_t)); if (nwrite < 0) { perror ("write"); return 0; } /* See if we have a _real_ invalid character. */ if ((eof && filled > 0) || filled >= MB_CUR_MAX) { error (0, 0, "invalid multibyte character"); return 0; } /* If any characters must be carried forward, put them at the beginning of `buffer'. */ if (filled > 0) memmove (inp, buffer, filled); } return 1; } Non-reentrant Conversion Function ================================= The functions described in the last 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 they are not described in the first place is that they are almost entirely useless. The problem is that all the functions for conversion defined in ISO C90 use a local state. This 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 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 from the last section are used in place of non-reentrant conversion functions.* Non-reentrant Conversion of Single Characters --------------------------------------------- - Function: int mbtowc (wchar_t *restrict RESULT, const char *restrict STRING, size_t SIZE) 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 `*RESULT'. `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 character, 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 `*RESULT', 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 `*RESULT'. 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. *Note Shift State::. - Function: int wctomb (char *STRING, wchar_t WCHAR) 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 `0', also storing `'\0'' in `*STRING'. 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. *Note 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 which computes the length of a multibyte character. It can be defined in terms of `mbtowc'. - Function: int mblen (const char *STRING, size_t SIZE) 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 character, 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. *Note Shift State::. The function `mblen' is declared in `stdlib.h'. Non-reentrant Conversion of Strings ----------------------------------- For convenience reasons the ISO C90 standard defines also 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 *Note Converting Strings::. - Function: size_t mbstowcs (wchar_t *WSTRING, const char *STRING, size_t SIZE) 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, this 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) 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, this 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. 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 which 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); /* Initialize shift state. */ mblen (NULL, 0); while (1) { int thischar = mblen (s, length); /* Deal with end of string and invalid characters. */ if (thischar == 0) break; if (thischar == -1) { error ("invalid multibyte character"); break; } /* Advance past this character. */ 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. Generic Charset Conversion ========================== The conversion functions mentioned so far in this chapter all had in common that they operate on character sets which 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 GNU C library it always is 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 or 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'. This introduces major problems for the rest of the programs since several more functions (e.g., the character classification functions, *note 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 but no constraints on the character sets selected for source and destination. Only the set of available conversions is limiting them. The standard does not specify that any conversion at all must be available. It is a measure of the quality of the implementation. In the following text first the interface to `iconv', the conversion function, will be described. Comparisons with other implementations will show what pitfalls lie on the way of portable applications. At last, the implementation is described as far as interesting to the advanced user who wants to extend the conversion capabilities. 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 implement one step. Before the interfaces are described it is necessary to introduce a datatype. Just like other open-use-close interface the functions introduced here work using a 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 get 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) 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 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 file 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. Within the data structures associated with the descriptor there is 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 arbitrarily many directories. How this extension has to be written will be explained below (*note glibc iconv Implementation::). Here it is only important to say that all directories mentioned in the `GCONV_PATH' environment variable are considered 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. This function got 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 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) 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 error are: `EBADF' The conversion descriptor is invalid. This function was introduced together with the rest of the `iconv' functions in XPG2 and it 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) 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 `*INBUF' and it contains `*INBYTESLEFT' 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. `*OUTBUF' points to the beginning of the buffer with at least `*OUTBYTESLEFT' bytes room for the result. The buffer pointer again is of type `char' and the length is measured in bytes. If OUTBUF or `*OUTBUF' 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 on whether the conversion has to deal with states or not. 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 three reasons. The first is that all characters from the input buffer are converted. This actually can mean two things: really all bytes from the input buffer are consumed or there are some bytes at the end of the buffer which possibly can form a complete character but the input is incomplete. The second reason for a stop is when the output buffer is full. And the third reason is that the input contains invalid characters. In all these cases the buffer pointers after the last successful conversion, for input and output buffer, 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 this case the value pointed to by INBYTESLEFT is nonzero. `EILSEQ' The conversion stopped because of an invalid byte sequence in the input. After the call `*INBUF' 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. This 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 is demonstrated below. 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) { /* Something went wrong. */ if (errno == EINVAL) error (0, 0, "conversion from '%s' to wchar_t not available", charset); else perror ("iconv_open"); /* Terminate the output string. */ *outbuf = L'\0'; return -1; } while (avail > 0) { size_t nread; size_t nconv; char *inptr = inbuf; /* Read more input. */ nread = read (fd, inbuf + insize, sizeof (inbuf) - insize); if (nread == 0) { /* When we come here the file is completely read. This still could mean there are some unused characters in the `inbuf'. Put them back. */ if (lseek (fd, -insize, SEEK_CUR) == -1) result = -1; /* Now write out the byte sequence to get into the initial state if this is necessary. */ iconv (cd, NULL, NULL, &wrptr, &avail); break; } insize += nread; /* Do the conversion. */ nconv = iconv (cd, &inptr, &insize, &wrptr, &avail); if (nconv == (size_t) -1) { /* Not everything went right. It might only be an unfinished byte sequence at the end of the buffer. Or it is a real problem. */ if (errno == EINVAL) /* This is harmless. Simply move the unused bytes to the beginning of the buffer so that they can be used in the next round. */ memmove (inbuf, inptr, insize); else { /* It is a real problem. Maybe we ran out of space in the output buffer or we have invalid input. In any case back the file pointer to the position of the last processed byte. */ lseek (fd, -insize, SEEK_CUR); result = -1; break; } } } /* Terminate the output string. */ if (avail >= 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' return 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 than can 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 it 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 which 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. 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 which 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. On top of this the generic nature of these functions make them slower than specifically implemented functions. * The C library only contains a framework which can dynamically load object files and execute the therein contained conversion functions. 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 these possibilities, the majority 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 his 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 in these situations and therefore this point is moot. The danger is that one gets acquainted with this 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. E.g., conversions from one character set to a set of, say, 10 others counts as 10 conversion. Together with the other direction this makes already 20. One can imagine the thin coverage these platform 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 system 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 which has to convert from A to C. A call like cd = iconv_open ("C", "A"); does fail according to the assumption above. But what does the program do now? The conversion is really necessary and therefore simply giving up is no possibility. This is a nuisance. The `iconv' function should take care of this. But how should the program proceed from here on? If it would try to convert to character set B first the two `iconv_open' calls cd1 = iconv_open ("B", "A"); and cd2 = iconv_open ("C", "B"); 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. Beside 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 shows one of the design errors of `iconv' mentioned above. It should at least be possible to determine the list of available conversion programmatically so that if `iconv_open' says there is no such conversion, one could make sure this also is true for indirect routes. 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 this means that static linking needs not to 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 easily be added. 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' this really means there is no known way, directly or indirectly, to perform the wanted conversion. This 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., converting 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 currently existing set of conversions is simply meant to cover all conversions which might be of interest. All currently available conversions use the triangulation method above, making conversion run unnecessarily slow. If, e.g., 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 can easy 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. 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. There are two more words expected on the line. The first one 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 always an official name but this need not correspond to the most popular name. Beside this many character sets have special names which are somehow constructed. E.g., all character sets specified by the ISO have an alias of the form `ISO-IR-NNN' where NNN is the registration number. This allows programs which 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. 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 what has to be done is to put the new module, be its name 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 which contains the information about the conversions and which shared object implements them. 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 which 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 to EUC-JP) the cost is 1+1 = 2. But the above `gconv-modules' file specifies that the new conversion modules can perform this conversion with only the cost of 1. A mysterious piece about the `gconv-modules' file above (and also the file coming with the GNU C library) are the names of the character sets specified in the `module' lines. Why do almost all the names end in `//'? And this is not all: the names can actually be regular expressions. At this point of time this mystery should not be revealed, unless you have the relevant spell-casting materials: ashes from an original DOS 6.2 boot disk burnt in effigy, a crucifix blessed by St. Emacs, assorted herbal roots from Central America, sand from Cebu, etc. Sorry! *The part of the implementation where this is used is not yet finished. For now please simply follow the existing examples. It'll become clearer once it is. -drepper* A last remark about the `gconv-modules' is about the names not ending with `//'. There often is a character set named `INTERNAL' mentioned. From the discussion above and the chosen name it should have become clear that this is the name for the representation used in the intermediate step of the triangulation. We have said that this is UCS-4 but actually it is not quite right. The UCS-4 specification also includes the specification of the byte ordering used. Since a UCS-4 value consists of four bytes a stored value is effected by byte ordering. The internal representation is _not_ the same as UCS-4 in case the byte ordering of the processor (or at least the running process) is not the same as the one required for UCS-4. This is done for performance reasons as one does not want to perform unnecessary byte-swapping operations if one is not interested in actually seeing the result in UCS-4. To avoid trouble with endianess the internal representation consistently is named `INTERNAL' even on big-endian systems where the representations are identical. `iconv' module data structures .............................. So far this section described how modules are located and considered to be used. What remains to be described is the interface of the modules so that one can write new ones. This section describes the interface as it is in use in January 1999. The interface will change in future a bit but hopefully only in an upward compatible way. The definitions necessary to write new modules are publicly available in the non-standard header `gconv.h'. The following text will therefore describe the definitions from this header file. But first it is necessary to get an overview. From the perspective of the user of `iconv' the interface is quite simple: the `iconv_open' function returns a handle which can be used in calls to `iconv' and finally the handle is freed with a call to `iconv_close'. The problem is: the handle has to be able to represent the possibly long sequences of conversion steps and also the state of each conversion since the handle is all which is passed to the `iconv' function. Therefore the data structures are really the elements to understanding the implementation. We need two different kinds of data structures. The first describes the conversion and the second describes the state etc. There are really two type definitions like this in `gconv.h'. - Data type: struct __gconv_step This data structure describes one conversion a module can perform. For each function in a loaded module with conversion functions there is exactly one object of this type. This object is shared by all users of the conversion. I.e., this object does not contain any information corresponding to an actual conversion. It only describes the conversion itself. `struct __gconv_loaded_object *__shlib_handle' `const char *__modname' `int __counter' All these elements of the structure are used internally in the C library to coordinate loading and unloading the shared. One must not expect any of the other elements be available or initialized. `const char *__from_name' `const char *__to_name' `__from_name' and `__to_name' contain the names of the source and destination character sets. They can be used to identify the actual conversion to be carried out since one module might implement conversions for more than one character set and/or direction. `gconv_fct __fct' `gconv_init_fct __init_fct' `gconv_end_fct __end_fct' These elements contain pointers to the functions in the loadable module. The interface will be explained below. `int __min_needed_from' `int __max_needed_from' `int __min_needed_to' `int __max_needed_to;' These values have to be filled in the init function of the module. The `__min_needed_from' value specifies how many bytes a character of the source character set at least needs. The `__max_needed_from' specifies the maximum value which also includes possible shift sequences. The `__min_needed_to' and `__max_needed_to' values serve the same purpose but this time for the destination character set. It is crucial that these values are accurate since otherwise the conversion functions will have problems or not work at all. `int __stateful' This element must also be initialized by the init function. It is nonzero if the source character set is stateful. Otherwise it is zero. `void *__data' This element can be used freely by the conversion functions in the module. It can be used to communicate extra information from one call to another. It need not be initialized if not needed at all. If this element gets assigned a pointer to dynamically allocated memory (presumably in the init function) it has to be made sure that the end function deallocates the memory. Otherwise the application will leak memory. It is important to be aware that this data structure is shared by all users of this specification conversion and therefore the `__data' element must not contain data specific to one specific use of the conversion function. - Data type: struct __gconv_step_data This is the data structure which contains the information specific to each use of the conversion functions. `char *__outbuf' `char *__outbufend' These elements specify the output buffer for the conversion step. The `__outbuf' element points to the beginning of the buffer and `__outbufend' points to the byte following the last byte in the buffer. The conversion function must not assume anything about the size of the buffer but it can be safely assumed the there is room for at least one complete character in the output buffer. Once the conversion is finished and the conversion is the last step the `__outbuf' element must be modified to point after last last byte written into the buffer to signal how much output is available. If this conversion step is not the last one the element must not be modified. The `__outbufend' element must not be modified. `int __is_last' This element is nonzero if this conversion step is the last one. This information is necessary for the recursion. See the description of the conversion function internals below. This element must never be modified. `int __invocation_counter' The conversion function can use this element to see how many calls of the conversion function already happened. Some character sets require when generating output a certain prolog and by comparing this value with zero one can find out whether it is the first call and therefore the prolog should be emitted or not. This element must never be modified. `int __internal_use' This element is another one rarely used but needed in certain situations. It got assigned a nonzero value in case the conversion functions are used to implement `mbsrtowcs' et.al. I.e., the function is not used directly through the `iconv' interface. This sometimes makes a difference as it is expected that the `iconv' functions are used to translate entire texts while the `mbsrtowcs' functions are normally only used to convert single strings and might be used multiple times to convert entire texts. But in this situation we would have problem complying with some rules of the character set specification. Some character sets require a prolog which must appear exactly once for an entire text. If a number of `mbsrtowcs' calls are used to convert the text only the first call must add the prolog. But since there is no communication between the different calls of `mbsrtowcs' the conversion functions have no possibility to find this out. The situation is different for sequences of `iconv' calls since the handle allows access to the needed information. This element is mostly used together with `__invocation_counter' in a way like this: if (!data->__internal_use && data->__invocation_counter == 0) /* Emit prolog. */ ... This element must never be modified. `mbstate_t *__statep' The `__statep' element points to an object of type `mbstate_t' (*note Keeping the state::). The conversion of an stateful character set must use the object pointed to by this element to store information about the conversion state. The `__statep' element itself must never be modified. `mbstate_t __state' This element _never_ must be used directly. It is only part of this structure to have the needed space allocated. `iconv' module interfaces ......................... With the knowledge about the data structures we now can describe the conversion functions itself. To understand the interface a bit of knowledge about the functionality in the C library which loads the objects with the conversions is necessary. It is often the case that one conversion is used more than once. I.e., there are several `iconv_open' calls for the same set of character sets during one program run. The `mbsrtowcs' et.al. functions in the GNU C library also use the `iconv' functionality which increases the number of uses of the same functions even more. For this reason the modules do not get loaded exclusively for one conversion. Instead a module once loaded can be used by arbitrarily many `iconv' or `mbsrtowcs' calls at the same time. The splitting of the information between conversion function specific information and conversion data makes this possible. The last section showed the two data structures used to do this. This is of course also reflected in the interface and semantics of the functions the modules must provide. There are three functions which must have the following names: `gconv_init' The `gconv_init' function initializes the conversion function specific data structure. This very same object is shared by all conversion which use this conversion and therefore no state information about the conversion itself must be stored in here. If a module implements more than one conversion the `gconv_init' function will be called multiple times. `gconv_end' The `gconv_end' function is responsible to free all resources allocated by the `gconv_init' function. If there is nothing to do this function can be missing. Special care must be taken if the module implements more than one conversion and the `gconv_init' function does not allocate the same resources for all conversions. `gconv' This is the actual conversion function. It is called to convert one block of text. It gets passed the conversion step information initialized by `gconv_init' and the conversion data, specific to this use of the conversion functions. There are three data types defined for the three module interface function and these define the interface. - Data type: int (*__gconv_init_fct) (struct __gconv_step *) This specifies the interface of the initialization function of the module. It is called exactly once for each conversion the module implements. As explained int the description of the `struct __gconv_step' data structure above the initialization function has to initialize parts of it. `__min_needed_from' `__max_needed_from' `__min_needed_to' `__max_needed_to' These elements must be initialized to the exact numbers of the minimum and maximum number of bytes used by one character in the source and destination character set respectively. If the characters all have the same size the minimum and maximum values are the same. `__stateful' This element must be initialized to an nonzero value if the source character set is stateful. Otherwise it must be zero. If the initialization function needs to communication some information to the conversion function this can happen using the `__data' element of the `__gconv_step' structure. But since this data is shared by all the conversion is must not be modified by the conversion function. How this can be used is shown in the example below. #define MIN_NEEDED_FROM 1 #define MAX_NEEDED_FROM 4 #define MIN_NEEDED_TO 4 #define MAX_NEEDED_TO 4 int gconv_init (struct __gconv_step *step) { /* Determine which direction. */ struct iso2022jp_data *new_data; enum direction dir = illegal_dir; enum variant var = illegal_var; int result; if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0) { dir = from_iso2022jp; var = iso2022jp; } else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0) { dir = to_iso2022jp; var = iso2022jp; } else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0) { dir = from_iso2022jp; var = iso2022jp2; } else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0) { dir = to_iso2022jp; var = iso2022jp2; } result = __GCONV_NOCONV; if (dir != illegal_dir) { new_data = (struct iso2022jp_data *) malloc (sizeof (struct iso2022jp_data)); result = __GCONV_NOMEM; if (new_data != NULL) { new_data->dir = dir; new_data->var = var; step->__data = new_data; if (dir == from_iso2022jp) { step->__min_needed_from = MIN_NEEDED_FROM; step->__max_needed_from = MAX_NEEDED_FROM; step->__min_needed_to = MIN_NEEDED_TO; step->__max_needed_to = MAX_NEEDED_TO; } else { step->__min_needed_from = MIN_NEEDED_TO; step->__max_needed_from = MAX_NEEDED_TO; step->__min_needed_to = MIN_NEEDED_FROM; step->__max_needed_to = MAX_NEEDED_FROM + 2; } /* Yes, this is a stateful encoding. */ step->__stateful = 1; result = __GCONV_OK; } } return result; } The function first checks which conversion is wanted. The module from which this function is taken implements four different conversion and which one is selected can be determined by comparing the names. The comparison should always be done without paying attention to the case. Then a data structure is allocated which contains the necessary information about which conversion is selected. The data structure `struct iso2022jp_data' is locally defined since outside the module this data is not used at all. Please note that if all four conversions this modules supports are requested there are four data blocks. One interesting thing is the initialization of the `__min_' and `__max_' elements of the step data object. A single ISO-2022-JP character can consist of one to four bytes. Therefore the `MIN_NEEDED_FROM' and `MAX_NEEDED_FROM' macros are defined this way. The output is always the `INTERNAL' character set (aka UCS-4) and therefore each character consists of exactly four bytes. For the conversion from `INTERNAL' to ISO-2022-JP we have to take into account that escape sequences might be necessary to switch the character sets. Therefore the `__max_needed_to' element for this direction gets assigned `MAX_NEEDED_FROM + 2'. This takes into account the two bytes needed for the escape sequences to single the switching. The asymmetry in the maximum values for the two directions can be explained easily: when reading ISO-2022-JP text escape sequences can be handled alone. I.e., it is not necessary to process a real character since the effect of the escape sequence can be recorded in the state information. The situation is different for the other direction. Since it is in general not known which character comes next one cannot emit escape sequences to change the state in advance. This means the escape sequences which have to be emitted together with the next character. Therefore one needs more room then only for the character itself. The possible return values of the initialization function are: `__GCONV_OK' The initialization succeeded `__GCONV_NOCONV' The requested conversion is not supported in the module. This can happen if the `gconv-modules' file has errors. `__GCONV_NOMEM' Memory required to store additional information could not be allocated. The functions called before the module is unloaded is significantly easier. It often has nothing at all to do in which case it can be left out completely. - Data type: void (*__gconv_end_fct) (struct gconv_step *) The task of this function is it to free all resources allocated in the initialization function. Therefore only the `__data' element of the object pointed to by the argument is of interest. Continuing the example from the initialization function, the finalization function looks like this: void gconv_end (struct __gconv_step *data) { free (data->__data); } The most important function is the conversion function itself. It can get quite complicated for complex character sets. But since this is not of interest here we will only describe a possible skeleton for the conversion function. - Data type: int (*__gconv_fct) (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int) The conversion function can be called for two basic reason: to convert text or to reset the state. From the description of the `iconv' function it can be seen why the flushing mode is necessary. What mode is selected is determined by the sixth argument, an integer. If it is nonzero it means that flushing is selected. Common to both mode is where the output buffer can be found. The information about this buffer is stored in the conversion step data. A pointer to this is passed as the second argument to this function. The description of the `struct __gconv_step_data' structure has more information on this. What has to be done for flushing depends on the source character set. If it is not stateful nothing has to be done. Otherwise the function has to emit a byte sequence to bring the state object in the initial state. Once this all happened the other conversion modules in the chain of conversions have to get the same chance. Whether another step follows can be determined from the `__is_last' element of the step data structure to which the first parameter points. The more interesting mode is when actually text has to be converted. The first step in this case is to convert as much text as possible from the input buffer and store the result in the output buffer. The start of the input buffer is determined by the third argument which is a pointer to a pointer variable referencing the beginning of the buffer. The fourth argument is a pointer to the byte right after the last byte in the buffer. The conversion has to be performed according to the current state if the character set is stateful. The state is stored in an object pointed to by the `__statep' element of the step data (second argument). Once either the input buffer is empty or the output buffer is full the conversion stops. At this point the pointer variable referenced by the third parameter must point to the byte following the last processed byte. I.e., if all of the input is consumed this pointer and the fourth parameter have the same value. What now happens depends on whether this step is the last one or not. If it is the last step the only thing which has to be done is to update the `__outbuf' element of the step data structure to point after the last written byte. This gives the caller the information on how much text is available in the output buffer. Beside this the variable pointed to by the fifth parameter, which is of type `size_t', must be incremented by the number of characters (_not bytes_) which were converted in a non-reversible way. Then the function can return. In case the step is not the last one the later conversion functions have to get a chance to do their work. Therefore the appropriate conversion function has to be called. The information about the functions is stored in the conversion data structures, passed as the first parameter. This information and the step data are stored in arrays so the next element in both cases can be found by s