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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; i.e., there must be at least one AC_DEFINE or one AC_DEFINE_UNQUOTED call with a third argument for each symbol (see section 7.1 Defining C Preprocessor Symbols). An additional constraint is that the first argument of AC_DEFINE must be a literal. Note that all symbols defined by Autoconf's builtin tests are already documented properly; you only need to document 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 #define'd (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: autoheader's invocation...

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 include path. Multiple invocations accumulate.

`-B dir'
Prepend dir to 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

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