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 configure.ac invokes AC_CONFIG_HEADERS(file), 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 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:

--help

-h

Print a summary of the command line options and exit.

--version

-V

Print the version number of Autoconf and exit.

--verbose

-v

Report processing steps.

--debug

-d

Don't remove the temporary files.

--force

-f

Remake the template file even if newer than its input files.

--include=dir

-I dir

Append dir to the include path. Multiple invocations accumulate.

--prepend-include=dir

-B dir

Prepend dir to the include path. Multiple invocations accumulate.

--warnings=category

-W category

Report the warnings related to category (which can actually be a comma separated list). Current categories include:

‘obsolete’

report the uses of obsolete constructs

‘all’

report all the warnings

‘none’

report none

‘error’

treats warnings as errors

‘no-category’

disable warnings falling into category