File: autoconf.info,  Node: Default Includes,  Prev: Standard Symbols,  Up: Common Behavior

5.1.2 Default Includes
----------------------

Test programs frequently need to include headers that may or may not be
available on the system whose features are being tested.  Each test can
use all the preprocessor macros that have been ‘AC_DEFINE’d by previous
tests, so for example one may write

     #include 
     #ifdef HAVE_SYS_TIME_H
     # include 
     #endif

if ‘sys/time.h’ has already been tested for.

   All hosted environments that are still of interest for portable code
provide all of the headers specified in C89 (as amended in 1995):
‘assert.h’, ‘ctype.h’, ‘errno.h’, ‘float.h’, ‘iso646.h’, ‘limits.h’,
‘locale.h’, ‘math.h’, ‘setjmp.h’, ‘signal.h’, ‘stdarg.h’, ‘stddef.h’,
‘stdio.h’, ‘stdlib.h’, ‘string.h’, ‘time.h’, ‘wchar.h’, and ‘wctype.h’.
Most programs can safely include these headers unconditionally.  A
program not intended to be portable to C89 can also safely include the
C99-specified header ‘stdbool.h’.  Other headers, including headers from
C99 and later revisions of the C standard, might need to be tested for
(*note Header Files::) or their bugs may need to be worked around (*note
Gnulib::).

   If your program needs to be portable to a _freestanding_ environment,
such as an embedded OS that doesn't provide all of the facilities of the
C89 standard library, you may need to test for some of the above headers
as well.  Note that many Autoconf macros internally assume that the
complete set of C89 headers are available.

   Most generic macros use the following macro to provide a default set
of includes:

 -- Macro: AC_INCLUDES_DEFAULT ([INCLUDE-DIRECTIVES])
     Expand to INCLUDE-DIRECTIVES if present and nonempty, otherwise to:

          #include 
          #ifdef HAVE_STDIO_H
          # include 
          #endif
          #ifdef HAVE_STDLIB_H
          # include 
          #endif
          #ifdef HAVE_STRING_H
          # include 
          #endif
          #ifdef HAVE_INTTYPES_H
          # include 
          #endif
          #ifdef HAVE_STDINT_H
          # include 
          #endif
          #ifdef HAVE_STRINGS_H
          # include 
          #endif
          #ifdef HAVE_SYS_TYPES_H
          # include 
          #endif
          #ifdef HAVE_SYS_STAT_H
          # include 
          #endif
          #ifdef HAVE_UNISTD_H
          # include 
          #endif

     Using this macro without INCLUDE-DIRECTIVES has the side effect of
     checking for ‘stdio.h’, ‘stdlib.h’, ‘string.h’, ‘inttypes.h’,
     ‘stdint.h’, ‘strings.h’, ‘sys/types.h’, ‘sys/stat.h’, and
     ‘unistd.h’, as if by ‘AC_CHECK_HEADERS_ONCE’.  For backward
     compatibility, the macro ‘STDC_HEADERS’ will be defined when both
     ‘stdlib.h’ and ‘string.h’ are available.

     *Portability Note:* It is safe for most programs to assume the
     presence of all of the headers required by the original 1990 C
     standard.  ‘AC_INCLUDES_DEFAULT’ checks for ‘stdio.h’, ‘stdlib.h’,
     and ‘string.h’, even though they are in that list, because they
     might not be available when compiling for a "freestanding
     environment" (in which most of the features of the C library are
     optional).  You probably do not need to write ‘#ifdef HAVE_STDIO_H’
     in your own code.

     ‘inttypes.h’ and ‘stdint.h’ were added to C in the 1999 revision of
     the standard, and ‘strings.h’, ‘sys/types.h’, ‘sys/stat.h’, and
     ‘unistd.h’ are POSIX extensions.  You _should_ guard uses of these
     headers with appropriate conditionals.

 -- Macro: AC_CHECK_INCLUDES_DEFAULT
     Check for all the headers that ‘AC_INCLUDES_DEFAULT’ would check
     for as a side-effect, if this has not already happened.

     This macro mainly exists so that ‘autoupdate’ can replace certain
     obsolete constructs with it.  You should not need to use it
     yourself; in fact, it is likely to be safe to delete it from any
     script in which it appears.  (‘autoupdate’ does not know whether
     preprocessor macros such as ‘HAVE_STDINT_H’ are used in the
     program, nor whether they would get defined as a side-effect of
     other checks.)