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
macro selects this kind of output. Though it can be called anywhere
AC_OUTPUT, it is customary to call
it right after
The package should ‘#include’ the configuration header file before
any other header files, to prevent inconsistencies in declarations (for
example, if it redefines
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 Configuration Actions. Make
AC_OUTPUTcreate the file(s) in the blank-or-newline-separated list header containing C preprocessor
#definestatements, 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_OUTPUTwould 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 Configuration Actions, for more details on header.