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5.5.2 Particular Function Checks

These macros check for particular C functions—whether they exist, and in some cases how they respond when given certain arguments.

— Macro: AC_FUNC_ALLOCA

Check how to get alloca. Tries to get a builtin version by checking for alloca.h or the predefined C preprocessor macros __GNUC__ and _AIX. If this macro finds alloca.h, it defines HAVE_ALLOCA_H.

If those attempts fail, it looks for the function in the standard C library. If any of those methods succeed, it defines HAVE_ALLOCA. Otherwise, it sets the output variable ALLOCA to ‘${LIBOBJDIR}alloca.o’ and defines C_ALLOCA (so programs can periodically call ‘alloca (0)’ to garbage collect). This variable is separate from LIBOBJS so multiple programs can share the value of ALLOCA without needing to create an actual library, in case only some of them use the code in LIBOBJS. The ‘${LIBOBJDIR}’ prefix serves the same purpose as in LIBOBJS (see AC_LIBOBJ vs LIBOBJS).

This macro does not try to get alloca from the System V R3 libPW or the System V R4 libucb because those libraries contain some incompatible functions that cause trouble. Some versions do not even contain alloca or contain a buggy version. If you still want to use their alloca, use ar to extract alloca.o from them instead of compiling alloca.c.

Source files that use alloca should start with a piece of code like the following, to declare it properly. 

          #ifdef HAVE_ALLOCA_H
          # include <alloca.h>
          #elif defined __GNUC__
          # define alloca __builtin_alloca
          #elif defined _AIX
          # define alloca __alloca
          #elif defined _MSC_VER
          # include <malloc.h>
          # define alloca _alloca
          #else
          # include <stddef.h>
          # ifdef  __cplusplus
          extern "C"
          # endif
          void *alloca (size_t);
          #endif
— Macro: AC_FUNC_CHOWN

If the chown function is available and works (in particular, it should accept -1 for uid and gid), define HAVE_CHOWN. The result of this macro is cached in the ac_cv_func_chown_works variable.

— Macro: AC_FUNC_CLOSEDIR_VOID

If the closedir function does not return a meaningful value, define CLOSEDIR_VOID. Otherwise, callers ought to check its return value for an error indicator.

Currently this test is implemented by running a test program. When cross compiling the pessimistic assumption that closedir does not return a meaningful value is made.

The result of this macro is cached in the ac_cv_func_closedir_void variable.

This macro is obsolescent, as closedir returns a meaningful value on current systems. New programs need not use this macro.

— Macro: AC_FUNC_ERROR_AT_LINE

If the error_at_line function is not found, require an AC_LIBOBJ replacement of ‘error’.

The result of this macro is cached in the ac_cv_lib_error_at_line variable.

— Macro: AC_FUNC_FNMATCH

If the fnmatch function conforms to Posix, define HAVE_FNMATCH. Detect common implementation bugs, for example, the bugs in Solaris 2.4.

Unlike the other specific AC_FUNC macros, AC_FUNC_FNMATCH does not replace a broken/missing fnmatch. This is for historical reasons. See AC_REPLACE_FNMATCH below.

The result of this macro is cached in the ac_cv_func_fnmatch_works variable.

This macro is obsolescent. New programs should use Gnulib's fnmatch-posix module. See Gnulib.

— Macro: AC_FUNC_FNMATCH_GNU

Behave like AC_REPLACE_FNMATCH (replace) but also test whether fnmatch supports GNU extensions. Detect common implementation bugs, for example, the bugs in the GNU C Library 2.1.

The result of this macro is cached in the ac_cv_func_fnmatch_gnu variable.

This macro is obsolescent. New programs should use Gnulib's fnmatch-gnu module. See Gnulib.

— Macro: AC_FUNC_FORK

This macro checks for the fork and vfork functions. If a working fork is found, define HAVE_WORKING_FORK. This macro checks whether fork is just a stub by trying to run it.

If vfork.h is found, define HAVE_VFORK_H. If a working vfork is found, define HAVE_WORKING_VFORK. Otherwise, define vfork to be fork for backward compatibility with previous versions of autoconf. This macro checks for several known errors in implementations of vfork and considers the system to not have a working vfork if it detects any of them. It is not considered to be an implementation error if a child's invocation of signal modifies the parent's signal handler, since child processes rarely change their signal handlers.

Since this macro defines vfork only for backward compatibility with previous versions of autoconf you're encouraged to define it yourself in new code: 

          #ifndef HAVE_WORKING_VFORK
          # define vfork fork
          #endif
— Macro: AC_FUNC_FSEEKO

If the fseeko function is available, define HAVE_FSEEKO. Define _LARGEFILE_SOURCE if necessary to make the prototype visible on some systems (e.g., glibc 2.2). Otherwise linkage problems may occur when compiling with AC_SYS_LARGEFILE on largefile-sensitive systems where off_t does not default to a 64bit entity. All systems with fseeko also supply ftello.

— Macro: AC_FUNC_GETGROUPS

If the getgroups function is available and works (unlike on Ultrix 4.3, where ‘getgroups (0, 0)’ always fails), define HAVE_GETGROUPS. Set GETGROUPS_LIBS to any libraries needed to get that function. This macro runs AC_TYPE_GETGROUPS.

— Macro: AC_FUNC_GETLOADAVG

Check how to get the system load averages. To perform its tests properly, this macro needs the file getloadavg.c; therefore, be sure to set the AC_LIBOBJ replacement directory properly (see Generic Functions, AC_CONFIG_LIBOBJ_DIR).

If the system has the getloadavg function, define HAVE_GETLOADAVG, and set GETLOADAVG_LIBS to any libraries necessary to get that function. Also add GETLOADAVG_LIBS to LIBS. Otherwise, require an AC_LIBOBJ replacement for ‘getloadavg’ with source code in dir/getloadavg.c, and possibly define several other C preprocessor macros and output variables:

  1. Define C_GETLOADAVG.
  2. Define SVR4, DGUX, UMAX, or UMAX4_3 if on those systems.
  3. If nlist.h is found, define HAVE_NLIST_H.
  4. If ‘struct nlist’ has an ‘n_un.n_name’ member, define HAVE_STRUCT_NLIST_N_UN_N_NAME. The obsolete symbol NLIST_NAME_UNION is still defined, but do not depend upon it.
  5. Programs may need to be installed set-group-ID (or set-user-ID) for getloadavg to work. In this case, define GETLOADAVG_PRIVILEGED, set the output variable NEED_SETGID to ‘true’ (and otherwise to ‘false’), and set KMEM_GROUP to the name of the group that should own the installed program.

The AC_FUNC_GETLOADAVG macro is obsolescent. New programs should use Gnulib's getloadavg module. See Gnulib.

— Macro: AC_FUNC_GETMNTENT

Check for getmntent in the standard C library, and then in the sun, seq, and gen libraries, for UNICOS, IRIX 4, PTX, and UnixWare, respectively. Then, if getmntent is available, define HAVE_GETMNTENT and set ac_cv_func_getmntent to yes. Otherwise set ac_cv_func_getmntent to no.

The result of this macro can be overridden by setting the cache variable ac_cv_search_getmntent.

— Macro: AC_FUNC_GETPGRP

Define GETPGRP_VOID if it is an error to pass 0 to getpgrp; this is the Posix behavior. On older BSD systems, you must pass 0 to getpgrp, as it takes an argument and behaves like Posix's getpgid. 

          #ifdef GETPGRP_VOID
            pid = getpgrp ();
          #else
            pid = getpgrp (0);
          #endif

This macro does not check whether getpgrp exists at all; if you need to work in that situation, first call AC_CHECK_FUNC for getpgrp.

The result of this macro is cached in the ac_cv_func_getpgrp_void variable.

This macro is obsolescent, as current systems have a getpgrp whose signature conforms to Posix. New programs need not use this macro.

— Macro: AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK

If link is a symbolic link, then lstat should treat link/ the same as link/.. However, many older lstat implementations incorrectly ignore trailing slashes.

It is safe to assume that if lstat incorrectly ignores trailing slashes, then other symbolic-link-aware functions like unlink also incorrectly ignore trailing slashes.

If lstat behaves properly, define LSTAT_FOLLOWS_SLASHED_SYMLINK, otherwise require an AC_LIBOBJ replacement of lstat.

The result of this macro is cached in the ac_cv_func_lstat_dereferences_slashed_symlink variable.

— Macro: AC_FUNC_MALLOC

If the malloc function is compatible with the GNU C library malloc (i.e., ‘malloc (0)’ returns a valid pointer), define HAVE_MALLOC to 1. Otherwise define HAVE_MALLOC to 0, ask for an AC_LIBOBJ replacement for ‘malloc’, and define malloc to rpl_malloc so that the native malloc is not used in the main project.

Typically, the replacement file malloc.c should look like (note the ‘#undef malloc’): 

     #include <config.h>
     #undef malloc
     
     #include <sys/types.h>
     
     void *malloc ();
     
     /* Allocate an N-byte block of memory from the heap.
        If N is zero, allocate a 1-byte block.  */
     
     void *
     rpl_malloc (size_t n)
     {
       if (n == 0)
         n = 1;
       return malloc (n);
     }

The result of this macro is cached in the ac_cv_func_malloc_0_nonnull variable.

— Macro: AC_FUNC_MBRTOWC

Define HAVE_MBRTOWC to 1 if the function mbrtowc and the type mbstate_t are properly declared.

The result of this macro is cached in the ac_cv_func_mbrtowc variable.

— Macro: AC_FUNC_MEMCMP

If the memcmp function is not available, or does not work on 8-bit data (like the one on SunOS 4.1.3), or fails when comparing 16 bytes or more and with at least one buffer not starting on a 4-byte boundary (such as the one on NeXT x86 OpenStep), require an AC_LIBOBJ replacement for ‘memcmp’.

The result of this macro is cached in the ac_cv_func_memcmp_working variable.

This macro is obsolescent, as current systems have a working memcmp. New programs need not use this macro.

— Macro: AC_FUNC_MKTIME

If the mktime function is not available, or does not work correctly, require an AC_LIBOBJ replacement for ‘mktime’. For the purposes of this test, mktime should conform to the Posix standard and should be the inverse of localtime.

The result of this macro is cached in the ac_cv_func_working_mktime variable.

— Macro: AC_FUNC_MMAP

If the mmap function exists and works correctly, define HAVE_MMAP. This checks only private fixed mapping of already-mapped memory.

The result of this macro is cached in the ac_cv_func_mmap_fixed_mapped variable.

— Macro: AC_FUNC_OBSTACK

If the obstacks are found, define HAVE_OBSTACK, else require an AC_LIBOBJ replacement for ‘obstack’.

The result of this macro is cached in the ac_cv_func_obstack variable.

— Macro: AC_FUNC_REALLOC

If the realloc function is compatible with the GNU C library realloc (i.e., ‘realloc (NULL, 0)’ returns a valid pointer), define HAVE_REALLOC to 1. Otherwise define HAVE_REALLOC to 0, ask for an AC_LIBOBJ replacement for ‘realloc’, and define realloc to rpl_realloc so that the native realloc is not used in the main project. See AC_FUNC_MALLOC for details.

The result of this macro is cached in the ac_cv_func_realloc_0_nonnull variable.

— Macro: AC_FUNC_SELECT_ARGTYPES

Determines the correct type to be passed for each of the select function's arguments, and defines those types in SELECT_TYPE_ARG1, SELECT_TYPE_ARG234, and SELECT_TYPE_ARG5 respectively. SELECT_TYPE_ARG1 defaults to ‘int’, SELECT_TYPE_ARG234 defaults to ‘int *’, and SELECT_TYPE_ARG5 defaults to ‘struct timeval *’.

This macro is obsolescent, as current systems have a select whose signature conforms to Posix. New programs need not use this macro.

— Macro: AC_FUNC_SETPGRP

If setpgrp takes no argument (the Posix version), define SETPGRP_VOID. Otherwise, it is the BSD version, which takes two process IDs as arguments. This macro does not check whether setpgrp exists at all; if you need to work in that situation, first call AC_CHECK_FUNC for setpgrp.

The result of this macro is cached in the ac_cv_func_setpgrp_void variable.

This macro is obsolescent, as current systems have a setpgrp whose signature conforms to Posix. New programs need not use this macro.

— Macro: AC_FUNC_STAT
— Macro: AC_FUNC_LSTAT

Determine whether stat or lstat have the bug that it succeeds when given the zero-length file name as argument. The stat and lstat from SunOS 4.1.4 and the Hurd (as of 1998-11-01) do this.

If it does, then define HAVE_STAT_EMPTY_STRING_BUG (or HAVE_LSTAT_EMPTY_STRING_BUG) and ask for an AC_LIBOBJ replacement of it.

The results of these macros are cached in the ac_cv_func_stat_empty_string_bug and the ac_cv_func_lstat_empty_string_bug variables, respectively.

These macros are obsolescent, as no current systems have the bug. New programs need not use these macros.

— Macro: AC_FUNC_STRCOLL

If the strcoll function exists and works correctly, define HAVE_STRCOLL. This does a bit more than ‘AC_CHECK_FUNCS(strcoll)’, because some systems have incorrect definitions of strcoll that should not be used.

The result of this macro is cached in the ac_cv_func_strcoll_works variable.

— Macro: AC_FUNC_STRERROR_R

If strerror_r is available, define HAVE_STRERROR_R, and if it is declared, define HAVE_DECL_STRERROR_R. If it returns a char * message, define STRERROR_R_CHAR_P; otherwise it returns an int error number. The Thread-Safe Functions option of Posix requires strerror_r to return int, but many systems (including, for example, version 2.2.4 of the GNU C Library) return a char * value that is not necessarily equal to the buffer argument.

The result of this macro is cached in the ac_cv_func_strerror_r_char_p variable.

— Macro: AC_FUNC_STRFTIME

Check for strftime in the intl library, for SCO Unix. Then, if strftime is available, define HAVE_STRFTIME.

This macro is obsolescent, as no current systems require the intl library for strftime. New programs need not use this macro.

— Macro: AC_FUNC_STRTOD

If the strtod function does not exist or doesn't work correctly, ask for an AC_LIBOBJ replacement of ‘strtod’. In this case, because strtod.c is likely to need ‘pow’, set the output variable POW_LIB to the extra library needed.

This macro caches its result in the ac_cv_func_strtod variable and depends upon the result in the ac_cv_func_pow variable.

— Macro: AC_FUNC_STRTOLD

If the strtold function exists and conforms to C99, define HAVE_STRTOLD.

This macro caches its result in the ac_cv_func_strtold variable.

— Macro: AC_FUNC_STRNLEN

If the strnlen function is not available, or is buggy (like the one from AIX 4.3), require an AC_LIBOBJ replacement for it.

This macro caches its result in the ac_cv_func_strnlen_working variable.

— Macro: AC_FUNC_UTIME_NULL

If ‘utime (file, NULL)’ sets file's timestamp to the present, define HAVE_UTIME_NULL.

This macro caches its result in the ac_cv_func_utime_null variable.

This macro is obsolescent, as all current systems have a utime that behaves this way. New programs need not use this macro.

— Macro: AC_FUNC_VPRINTF

If vprintf is found, define HAVE_VPRINTF. Otherwise, if _doprnt is found, define HAVE_DOPRNT. (If vprintf is available, you may assume that vfprintf and vsprintf are also available.)

This macro is obsolescent, as all current systems have vprintf. New programs need not use this macro.

— Macro: AC_REPLACE_FNMATCH

If the fnmatch function does not conform to Posix (see AC_FUNC_FNMATCH), ask for its AC_LIBOBJ replacement.

The files fnmatch.c, fnmatch_loop.c, and fnmatch_.h in the AC_LIBOBJ replacement directory are assumed to contain a copy of the source code of GNU fnmatch. If necessary, this source code is compiled as an AC_LIBOBJ replacement, and the fnmatch_.h file is linked to fnmatch.h so that it can be included in place of the system <fnmatch.h>.

This macro caches its result in the ac_cv_func_fnmatch_works variable.

This macro is obsolescent, as it assumes the use of particular source files. New programs should use Gnulib's fnmatch-posix module, which provides this macro along with the source files. See Gnulib.