4.8.2 Using autoheader to Create config.h.in
The autoheader program can create a template file of C
‘#define’ statements for configure to use. If
autoheader creates file.in; if multiple file
arguments are given, the first one is used. Otherwise,
autoheader creates config.h.in.
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_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
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
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:
- Print a summary of the command line options and exit.
- Print the version number of Autoconf and exit.
- Report processing steps.
- Don't remove the temporary files.
- Remake the template file even if newer than its input files.
- -I dir
- Append dir to the include path. Multiple invocations accumulate.
- -B dir
- Prepend dir to the include path. Multiple invocations accumulate.
- -W category
- Report the warnings related to category (which can actually be a
comma separated list). Current categories include:
- report the uses of obsolete constructs
- report all the warnings
- report none
- treats warnings as errors
- disable warnings falling into category