When a package contains more than a few tests that define 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. Though it can be called anywhere
between AC_INIT and AC_OUTPUT, it is customary to call
it 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, or if it defines a macro like
_FILE_OFFSET_BITS that affects the behavior of system
headers). Note that it is okay to only include config.h from
.c files; the project’s .h files can rely on
config.h already being included first by the corresponding
.c file.
To provide for VPATH builds, remember to pass the C compiler a -I. option (or -I..; whichever directory contains config.h). Even if you use ‘#include "config.h"’, the preprocessor searches only the directory of the currently read file, i.e., the source directory, not the build directory.
With the appropriate -I option, you can use ‘#include <config.h>’. Actually, it’s a good habit to use it, because in the rare case when the source directory contains another config.h, the build directory should be searched first.
This macro is one of the instantiating macros; see Performing Configuration Actions. Make AC_OUTPUT create the file(s) in the
blank-or-newline-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;
header should not contain shell metacharacters.
See Special Characters in Output Variables.
If header already exists and its contents are identical to what
AC_OUTPUT would put in it, it is left alone. Doing this allows
making some changes in the 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. For example, you might need to make the input file name acceptable to DOS variants:
AC_CONFIG_HEADERS([config.h:config.hin])
This macro is defined as the name of the first declared config header and undefined if no config headers have been declared up to this point. A third-party macro may, for example, require use of a config header without invoking AC_CONFIG_HEADERS twice, like this:
AC_CONFIG_COMMANDS_PRE(
[m4_ifndef([AH_HEADER], [AC_CONFIG_HEADERS([config.h])])])
See Performing Configuration Actions, for more details on header.