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Configuration Header Files

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_HEADERS 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.

AC_CONFIG_HEADERS (header ..., [cmds], [init-cmds]) Macro
This macro is one of the instantiating macros, see Configuration Actions. Make AC_OUTPUT create the file(s) in the whitespace-separated list header 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 is config.h.

If header 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.in; however, you can override the input file name by appending to header, a colon-separated list of input files. Examples: 

AC_CONFIG_HEADERS([config.h:config.hin])
AC_CONFIG_HEADERS([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.

See Configuration Actions, for more details on header.