(autoconf.info.gz) Default Includes

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 Default Includes
 ----------------
 
    Several tests depend upon a set of header files.  Since these headers
 are not universally available, tests actually have to provide a set of
 protected includes, such as:
 
      #if TIME_WITH_SYS_TIME
      # include <sys/time.h>
      # include <time.h>
      #else
      # if HAVE_SYS_TIME_H
      #  include <sys/time.h>
      # else
      #  include <time.h>
      # endif
      #endif
 
 Unless you know exactly what you are doing, you should avoid using
 unconditional includes, and check the existence of the headers you
 include beforehand ( Header Files).
 
    Most generic macros provide the following default set of includes:
 
      #include <stdio.h>
      #if HAVE_SYS_TYPES_H
      # include <sys/types.h>
      #endif
      #if HAVE_SYS_STAT_H
      # include <sys/stat.h>
      #endif
      #if STDC_HEADERS
      # include <stdlib.h>
      # include <stddef.h>
      #else
      # if HAVE_STDLIB_H
      #  include <stdlib.h>
      # endif
      #endif
      #if HAVE_STRING_H
      # if !STDC_HEADERS && HAVE_MEMORY_H
      #  include <memory.h>
      # endif
      # include <string.h>
      #endif
      #if HAVE_STRINGS_H
      # include <strings.h>
      #endif
      #if HAVE_INTTYPES_H
      # include <inttypes.h>
      #else
      # if HAVE_STDINT_H
      #  include <stdint.h>
      # endif
      #endif
      #if HAVE_UNISTD_H
      # include <unistd.h>
      #endif
 
    If the default includes are used, then Autoconf will automatically
 check for the presence of these headers and their compatibility, i.e.,
 you don't need to run `AC_HEADERS_STDC', nor check for `stdlib.h' etc.
 
    These headers are checked for in the same order as they are included.
 For instance, on some systems `string.h' and `strings.h' both exist,
 but conflict.  Then `HAVE_STRING_H' will be defined, but
 `HAVE_STRINGS_H' won't.
 

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