(autoconf.info)Configuration Headers

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3.4 Configuration Header Files
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When a package tests more than a few C preprocessor symbols, the command
lines to pass '-D' options to the compiler can get quite long.  This
causes two problems.  One is that the 'make' output is hard to visually
scan for errors.  More seriously, the command lines can exceed the
length limits of some operating systems.  As an alternative to passing
'-D' options to the compiler, 'configure' scripts can create a C header
file containing '#define' directives.  The 'AC_CONFIG_HEADER' macro
selects this kind of output.  It should be called right after 'AC_INIT'.

   The package should '#include' the configuration header file before
any other header files, to prevent inconsistencies in declarations (for
example, if it redefines 'const').  Use '#include <config.h>' instead of
'#include "config.h"', and pass the C compiler a '-I.' option (or
'-I..'; whichever directory contains 'config.h').  That way, even if the
source directory is configured itself (perhaps to make a distribution),
other build directories can also be configured without finding the
'config.h' from the source directory.

 -- Macro: AC_CONFIG_HEADER (HEADER-TO-CREATE ...)
     Make 'AC_OUTPUT' create the file(s) in the whitespace-separated
     list HEADER-TO-CREATE containing C preprocessor '#define'
     statements, and replace '@DEFS@' in generated files with
     '-DHAVE_CONFIG_H' instead of the value of 'DEFS'.  The usual name
     for HEADER-TO-CREATE is 'config.h'.

     If HEADER-TO-CREATE already exists and its contents are identical
     to what 'AC_OUTPUT' would put in it, it is left alone.  Doing this
     allows some changes in configuration without needlessly causing
     object files that depend on the header file to be recompiled.

     Usually the input file is named 'HEADER-TO-CREATE.in'; however, you
     can override the input file name by appending to HEADER-TO-CREATE,
     a colon-separated list of input files.  Examples:
          AC_CONFIG_HEADER(defines.h:defines.hin)
          AC_CONFIG_HEADER(defines.h:defs.pre:defines.h.in:defs.post)
     Doing this allows you to keep your file names acceptable to MS-DOS,
     or to prepend and/or append boilerplate to the file.

Header Templates

Input for the configuration headers.

Invoking autoheader

How to create configuration templates.