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 for the alloca function. Define HAVE_ALLOCA_H if alloca.h defines a working alloca. If not, look for a builtin alternative. If either method succeeds, define HAVE_ALLOCA. Otherwise, set the output variable ALLOCA to ‘${LIBOBJDIR}alloca.o’ and define C_ALLOCA (so programs can periodically call ‘alloca (0)’ to garbage collect). This variable is separate from LIBOBJS so multiple programs can share the value of ALLOCA without needing to create an actual library, in case only some of them use the code in LIBOBJS. The ‘${LIBOBJDIR}’ prefix serves the same purpose as in LIBOBJS (see AC_LIBOBJ vs. LIBOBJS).

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

#include <stdlib.h>
#include <stddef.h>
#ifdef HAVE_ALLOCA_H
# include <alloca.h>
#elif !defined alloca
# ifdef __GNUC__
#  define alloca __builtin_alloca
# elif defined _MSC_VER
#  include <malloc.h>
#  define alloca _alloca
# elif !defined HAVE_ALLOCA
#  ifdef  __cplusplus
extern "C"
#  endif
void *alloca (size_t);
# endif
#endif

If you don’t want to maintain this piece of code in your package manually, you can instead use the Gnulib module alloca-opt or alloca. See Gnulib.

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.

If you want a workaround, that is, a chown function that is available and works, you can use the Gnulib module chown. See Gnulib.

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.

The AC_FUNC_ERROR_AT_LINE macro is obsolescent. New programs should use Gnulib’s error module. See Gnulib.

Macro: AC_FUNC_FNMATCH

If the fnmatch function conforms to Posix, define HAVE_FNMATCH.

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.

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

The results of this macro are cached in the ac_cv_func_fork_works and ac_cv_func_vfork_works variables. In order to override the test, you also need to set the ac_cv_func_fork and ac_cv_func_vfork variables.

Macro: AC_FUNC_FSEEKO

If the fseeko and ftello functions are available, define HAVE_FSEEKO. Define _LARGEFILE_SOURCE if necessary to make the prototype visible.

Configure scripts that use AC_FUNC_FSEEKO should normally also use AC_SYS_LARGEFILE to ensure that off_t can represent all supported file sizes. See AC_SYS_LARGEFILE.

The Gnulib module fseeko invokes AC_FUNC_FSEEKO and also contains workarounds for other portability problems of fseeko. See Gnulib.

Macro: AC_FUNC_GETGROUPS

Perform all the checks performed by AC_TYPE_GETGROUPS (see AC_TYPE_GETGROUPS). Then, if the getgroups function is available and known to work correctly, define HAVE_GETGROUPS. Set the output variable GETGROUPS_LIB to any libraries needed to get that function.

This macro relies on a list of systems with known, serious bugs in getgroups. If this list mis-identifies your system’s getgroups as buggy, or as not buggy, you can override it by setting the cache variable ac_cv_func_getgroups_works in a config.site file (see Setting Site Defaults). Please also report the error to .

The Gnulib module getgroups provides workarounds for additional, less severe portability problems with this function.

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 Function Checks, 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’ 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.

The AC_FUNC_GETMNTENT macro is obsolescent. New programs should use Gnulib’s mountlist module. See Gnulib.

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.

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.

The AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK macro is obsolescent. New programs should use Gnulib’s lstat module. See Gnulib.

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 <stdlib.h>

/* 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.

If you don’t want to maintain a malloc.c file in your package manually, you can instead use the Gnulib module malloc-gnu.

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.

The Gnulib module mbrtowc not only ensures that the function is declared, but also works around other portability problems of this function.

Macro: AC_FUNC_MEMCMP

If the memcmp function is not available or does not work, 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.

The AC_FUNC_MKTIME macro is obsolescent. New programs should use Gnulib’s mktime module. See Gnulib.

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.

Note: This macro asks for more than what an average program needs from mmap. In particular, the use of MAP_FIXED fails on HP-UX 11, whereas mmap otherwise works fine on this platform.

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.

The AC_FUNC_OBSTACK macro is obsolescent. New programs should use Gnulib’s obstack module. See Gnulib.

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.

If you don’t want to maintain a realloc.c file in your package manually, you can instead use the Gnulib module realloc-gnu.

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. This macro also does not check for the Solaris variant of setpgrp, which returns a pid_t instead of an int; portable code should only use the return value by comparing it against -1 to check for errors.

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

This macro is obsolescent, as all forms of setpgrp are also obsolescent. New programs should use the Posix function setpgid, which takes two process IDs as arguments (like the BSD setpgrp).

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.

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. But it does not check against a known bug of this function on Solaris 10.

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.

The Gnulib module strerror_r not only ensures that the function has the return type specified by Posix, but also works around other portability problems of this function.

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.

The AC_FUNC_STRTOD macro is obsolescent. New programs should use Gnulib’s strtod module. See Gnulib.

Macro: AC_FUNC_STRTOLD

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

This macro caches its result in the ac_cv_func_strtold variable.

The Gnulib module strtold not only ensures that the function exists, but also works around other portability problems of this function.

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.

The AC_FUNC_STRNLEN macro is obsolescent. New programs should use Gnulib’s strnlen module. See Gnulib.

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.