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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.

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__
#  include <alloca.h>
# else
#  ifdef _AIX
 #pragma alloca
#  else
#   ifndef alloca /* predefined by HP cc +Olibcalls */
char *alloca ();
#   endif
#  endif
# endif

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

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.

If the error_at_line function is not found, require an AC_LIBOBJ replacement of `error'.

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

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:
# define vfork fork

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

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.

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.

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.

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.

  pid = getpgrp ();
  pid = getpgrp (0);

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.

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.

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

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'.

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

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

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

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 *'.

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.

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.

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.

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.

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.

If strerror_r is available, define HAVE_STRERROR_R. If its implementation correctly returns a char *, define HAVE_WORKING_STRERROR_R. On at least DEC UNIX 4.0[A-D] and HP-UX B.10.20, strerror_r returns int. Actually, this tests only whether it returns a scalar or an array, but that should be enough. This is used by the common `error.c'.

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

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

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.)

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