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(autoconf.info)Generic Headers

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Generic Header Checks
---------------------

   These macros are used to find system header files not covered by the
"particular" test macros.  If you need to check the contents of a header
as well as find out whether it is present, you have to write your own
test for it (Note: Writing Tests).

 - Macro: AC_CHECK_HEADER (HEADER-FILE, [ACTION-IF-FOUND],
          [ACTION-IF-NOT-FOUND], [INCLUDES = `default-includes'])
     If the system header file HEADER-FILE is usable, execute shell
     commands ACTION-IF-FOUND, otherwise execute ACTION-IF-NOT-FOUND.
     If you just want to define a symbol if the header file is
     available, consider using `AC_CHECK_HEADERS' instead.

     The meaning of "usable" depends upon the content of INCLUDES:

    if INCLUDES is empty
          check whether

               HEADER-FILE

          can be _preprocessed_ without error.

    if INCLUDE is set
          Check whether

               INCLUDES
               #include <HEADER-FILE>

          can be _compiled_ without error.  You may use
          `AC_CHECK_HEADER' (and `AC_CHECK_HEADERS') to check whether
          two headers are compatible.

     You may pass any kind of dummy content for INCLUDES, such as a
     single space, a comment, to check whether HEADER-FILE compiles
     with success.

 - Macro: AC_CHECK_HEADERS (HEADER-FILE..., [ACTION-IF-FOUND],
          [ACTION-IF-NOT-FOUND], [INCLUDES = `default-includes'])
     For each given system header file HEADER-FILE in the
     whitespace-separated argument list that exists, define
     `HAVE_HEADER-FILE' (in all capitals).  If ACTION-IF-FOUND is
     given, it is additional shell code to execute when one of the
     header files is found.  You can give it a value of `break' to
     break out of the loop on the first match.  If ACTION-IF-NOT-FOUND
     is given, it is executed when one of the header files is not found.

     Be sure to read the documentation of `AC_CHECK_HEADER' to
     understand the influence of INCLUDES.