( Particular Headers

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 4.4.1 Particular Header Checks
 These macros check for particular system header files--whether they
 exist, and in some cases whether they declare certain symbols.
      Define `SYS_SIGLIST_DECLARED' if the variable `sys_siglist' is
      declared in a system header file, either `signal.h' or `unistd.h'.
  -- Macro: AC_DIR_HEADER
      Like calling `AC_HEADER_DIRENT' and `AC_FUNC_CLOSEDIR_VOID', but
      defines a different set of C preprocessor macros to indicate which
      header file is found.  This macro and the names it defines are
      considered obsolete.  The names it defines are:
      In addition, if the `closedir' function does not return a
      meaningful value, define `VOID_CLOSEDIR'.
      Check for the following header files, and for the first one that is
      found and defines `DIR', define the listed C preprocessor macro:
      The directory library declarations in the source code should look
      something like the following:
           #if HAVE_DIRENT_H
           # include <dirent.h>
           # define NAMLEN(dirent) strlen((dirent)->d_name)
           # define dirent direct
           # define NAMLEN(dirent) (dirent)->d_namlen
           # if HAVE_SYS_NDIR_H
           #  include <sys/ndir.h>
           # endif
           # if HAVE_SYS_DIR_H
           #  include <sys/dir.h>
           # endif
           # if HAVE_NDIR_H
           #  include <ndir.h>
           # endif
      Using the above declarations, the program would declare variables
      to be type `struct dirent', not `struct direct', and would access
      the length of a directory entry name by passing a pointer to a
      `struct dirent' to the `NAMLEN' macro.
      This macro also checks for the SCO Xenix `dir' and `x' libraries.
      If `sys/types.h' does not define `major', `minor', and `makedev',
      but `sys/mkdev.h' does, define `MAJOR_IN_MKDEV'; otherwise, if
      `sys/sysmacros.h' does, define `MAJOR_IN_SYSMACROS'.
  -- Macro: AC_HEADER_STDC
      Define `STDC_HEADERS' if the system has ANSI C header files.
      Specifically, this macro checks for `stdlib.h', `stdarg.h',
      `string.h', and `float.h'; if the system has those, it probably
      has the rest of the ANSI C header files.  This macro also checks
      whether `string.h' declares `memchr' (and thus presumably the
      other `mem' functions), whether `stdlib.h' declare `free' (and
      thus presumably `malloc' and other related functions), and whether
      the `ctype.h' macros work on characters with the high bit set, as
      ANSI C requires.
      Use `STDC_HEADERS' instead of `__STDC__' to determine whether the
      system has ANSI-compliant header files (and probably C library
      functions) because many systems that have GCC do not have ANSI C
      header files.
      On systems without ANSI C headers, there is so much variation that
      it is probably easier to declare the functions you use than to
      figure out exactly what the system header files declare.  Some
      systems contain a mix of functions ANSI and BSD; some are mostly
      ANSI but lack `memmove'; some define the BSD functions as macros in
      `string.h' or `strings.h'; some have only the BSD functions but
      `string.h'; some declare the memory functions in `memory.h', some
      in `string.h'; etc.  It is probably sufficient to check for one
      string function and one memory function; if the library has the
      ANSI versions of those then it probably has most of the others.
      If you put the following in `':
           AC_CHECK_FUNCS(strchr memcpy)
      then, in your code, you can put declarations like this:
           #if STDC_HEADERS
           # include <string.h>
           # ifndef HAVE_STRCHR
           #  define strchr index
           #  define strrchr rindex
           # endif
           char *strchr (), *strrchr ();
           # ifndef HAVE_MEMCPY
           #  define memcpy(d, s, n) bcopy ((s), (d), (n))
           #  define memmove(d, s, n) bcopy ((s), (d), (n))
           # endif
      If you use a function like `memchr', `memset', `strtok', or
      `strspn', which have no BSD equivalent, then macros won't suffice;
      you must provide an implementation of each function.  An easy way
      to incorporate your implementations only when needed (since the
      ones in system C libraries may be hand optimized) is to, taking
      `memchr' for example, put it in `memchr.c' and use
      If `sys/wait.h' exists and is compatible with POSIX.1, define
      `HAVE_SYS_WAIT_H'.  Incompatibility can occur if `sys/wait.h' does
      not exist, or if it uses the old BSD `union wait' instead of `int'
      to store a status value.  If `sys/wait.h' is not POSIX.1
      compatible, then instead of including it, define the POSIX.1
      macros with their usual interpretations.  Here is an example:
           #include <sys/types.h>
           #if HAVE_SYS_WAIT_H
           # include <sys/wait.h>
           #ifndef WEXITSTATUS
           # define WEXITSTATUS(stat_val) ((unsigned)(stat_val) >> 8)
           #ifndef WIFEXITED
           # define WIFEXITED(stat_val) (((stat_val) & 255) == 0)
  -- Macro: AC_MEMORY_H
      Define `NEED_MEMORY_H' if `memcpy', `memcmp', etc. are not
      declared in `string.h' and `memory.h' exists.  This macro is
      obsolete; instead, use `AC_CHECK_HEADERS(memory.h)'.  See the
      example for `AC_HEADER_STDC'.
  -- Macro: AC_UNISTD_H
      Define `HAVE_UNISTD_H' if the system has `unistd.h'.  This macro
      is obsolete; instead, use `AC_CHECK_HEADERS(unistd.h)'.
      The way to check if the system supports POSIX.1 is:
           #if HAVE_UNISTD_H
           # include <sys/types.h>
           # include <unistd.h>
           #ifdef _POSIX_VERSION
           /* Code for POSIX.1 systems.  */
      `_POSIX_VERSION' is defined when `unistd.h' is included on POSIX.1
      systems.  If there is no `unistd.h', it is definitely not a
      POSIX.1 system.  However, some non-POSIX.1 systems do have
  -- Macro: AC_USG
      Define `USG' if the system does not have `strings.h', `rindex',
      `bzero', etc.  This implies that it has `string.h', `strrchr',
      `memset', etc.
      The symbol `USG' is obsolete.  Instead of this macro, see the
      example for `AC_HEADER_STDC'.
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