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Particular Function Checks

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

AC_FUNC_ALLOCA Macro
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 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.

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. In some versions of AIX, the declaration of alloca must precede everything else except for comments and preprocessor directives. The #pragma directive is indented so that pre-ANSI C compilers will ignore it, rather than choke on it. 

/* AIX requires this to be the first thing in the file.  */
#ifndef __GNUC__
# if HAVE_ALLOCA_H
#  include <alloca.h>
# else
#  ifdef _AIX
 #pragma alloca
#  else
#   ifndef alloca /* predefined by HP cc +Olibcalls */
char *alloca ();
#   endif
#  endif
# endif
#endif

AC_FUNC_CHOWN Macro
If the chown function is available and works (in particular, it should accept -1 for uid and gid), define HAVE_CHOWN.

AC_FUNC_CLOSEDIR_VOID Macro
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.

AC_FUNC_ERROR_AT_LINE Macro
If the error_at_line function is not found, require an AC_LIBOBJ replacement of error.

AC_FUNC_FNMATCH Macro
If the fnmatch function is available and works (unlike the one on Solaris 2.4), define HAVE_FNMATCH.

AC_FUNC_FORK Macro
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: 

#if !HAVE_WORKING_VFORK
# define vfork fork
#endif

AC_FUNC_FSEEKO Macro
If the fseeko function is available, define HAVE_FSEEKO. Define _LARGEFILE_SOURCE if necessary.

AC_FUNC_GETGROUPS Macro
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.

AC_FUNC_GETLOADAVG Macro
Check how to get the system load averages. If the system has the getloadavg function, define HAVE_GETLOADAVG, and set GETLOADAVG_LIBS to any libraries needed to get that function. Also add GETLOADAVG_LIBS to LIBS.

Otherwise, require an AC_LIBOBJ replacement (getloadavg.c) of 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 NLIST_STRUCT.
  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 setgid (or setuid) 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.

AC_FUNC_GETMNTENT Macro
Check for getmntent in the sun, seq, and gen libraries, for Irix 4, PTX, and Unixware, respectively. Then, if getmntent is available, define HAVE_GETMNTENT.

AC_FUNC_GETPGRP Macro
Define GETPGRP_VOID if it is an error to pass 0 to getpgrp; this is the POSIX.1 behavior. On older BSD systems, you must pass 0 to getpgrp, as it takes an argument and behaves like POSIX.1's getpgid. 
#if 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.

AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK 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 and unlink also incorrectly ignore trailing slashes.

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

AC_FUNC_MALLOC Macro
If the malloc works correctly (malloc (0) returns a valid pointer), define HAVE_MALLOC.

AC_FUNC_MEMCMP Macro
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.

AC_FUNC_MKTIME Macro
If the mktime function is not available, or does not work correctly, require an AC_LIBOBJ replacement for mktime.

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

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

AC_FUNC_SELECT_ARGTYPES Macro
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 *.

AC_FUNC_SETPGRP Macro
If setpgrp takes no argument (the POSIX.1 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.

AC_FUNC_STAT Macro
AC_FUNC_LSTAT Macro
Determine whether stat or lstat have the bug that it succeeds when given the zero-length file name 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.

AC_FUNC_SETVBUF_REVERSED Macro
If setvbuf takes the buffering type as its second argument and the buffer pointer as the third, instead of the other way around, define SETVBUF_REVERSED.

AC_FUNC_STRCOLL Macro
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.

AC_FUNC_STRTOD Macro
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.

AC_FUNC_STRERROR_R Macro
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-200X 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.

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

AC_FUNC_STRNLEN Macro
Check for a working strnlen, and ask for its replacement. Some architectures are know to provide broken versions of strnlen, such as AIX 4.3.

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

AC_FUNC_VPRINTF Macro
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.)