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5.1.2 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: 

     #ifdef TIME_WITH_SYS_TIME
     # include <sys/time.h>
     # include <time.h>
     #else
     # ifdef 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 (see Header Files).

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

— Macro: AC_INCLUDES_DEFAULT ([include-directives])

Expand to include-directives if defined, otherwise to: 

          #include <stdio.h>
          #ifdef HAVE_SYS_TYPES_H
          # include <sys/types.h>
          #endif
          #ifdef HAVE_SYS_STAT_H
          # include <sys/stat.h>
          #endif
          #ifdef STDC_HEADERS
          # include <stdlib.h>
          # include <stddef.h>
          #else
          # ifdef HAVE_STDLIB_H
          #  include <stdlib.h>
          # endif
          #endif
          #ifdef HAVE_STRING_H
          # if !defined STDC_HEADERS && defined HAVE_MEMORY_H
          #  include <memory.h>
          # endif
          # include <string.h>
          #endif
          #ifdef HAVE_STRINGS_H
          # include <strings.h>
          #endif
          #ifdef HAVE_INTTYPES_H
          # include <inttypes.h>
          #endif
          #ifdef HAVE_STDINT_H
          # include <stdint.h>
          #endif
          #ifdef HAVE_UNISTD_H
          # include <unistd.h>
          #endif

If the default includes are used, then check for the presence of these headers and their compatibility, i.e., you don't need to run AC_HEADER_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 is defined, not HAVE_STRINGS_H.