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4.9.2 Using autoheader to Create ‘config.h.in

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 section Defining C Preprocessor Symbols). Alternatively, you can use AH_TEMPLATE (see section 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


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