The autoheader program can create a template file of C
‘#define’ statements for configure to use.
It searches for the first invocation of AC_CONFIG_HEADERS in
configure sources to determine the name of the template.
(If the first call of AC_CONFIG_HEADERS specifies more than one
input file name, autoheader uses the first one.)
It is recommended that only one input file is used. If you want to append
a boilerplate code, it is preferable to use
‘AH_BOTTOM([#include <conf_post.h>])’.
File conf_post.h is not processed during the configuration then,
which make things clearer. Analogically, AH_TOP can be used to
prepend a boilerplate code.
In order to do its job, autoheader needs you to document all
of the symbols that you might use. Typically this is done via an
AC_DEFINE or AC_DEFINE_UNQUOTED call whose first argument
is a literal symbol and whose third argument describes the symbol
(see Defining Symbols). Alternatively, you can use
AH_TEMPLATE (see Autoheader Macros), or you can supply a
suitable input file for a subsequent configuration header file.
Symbols defined by Autoconf's builtin tests are already documented properly;
you need to document only those that you
define yourself.
You might wonder why autoheader is needed: after all, why would configure need to “patch” a config.h.in to produce a config.h instead of just creating config.h from scratch? Well, when everything rocks, the answer is just that we are wasting our time maintaining autoheader: generating config.h directly is all that is needed. When things go wrong, however, you'll be thankful for the existence of autoheader.
The fact that the symbols are documented is important in order to check that config.h makes sense. The fact that there is a well-defined list of symbols that should be defined (or not) is also important for people who are porting packages to environments where configure cannot be run: they just have to fill in the blanks.
But let's come back to the point: the invocation of autoheader...
If you give autoheader an argument, it uses that file instead of configure.ac and writes the header file to the standard output instead of to config.h.in. If you give autoheader an argument of -, it reads the standard input instead of configure.ac and writes the header file to the standard output.
autoheader accepts the following options: