4.6 Performing Configuration Actions

configure is designed so that it appears to do everything itself, but there is actually a hidden slave: config.status. configure is in charge of examining your system, but it is config.status that actually takes the proper actions based on the results of configure. The most typical task of config.status is to instantiate files.

This section describes the common behavior of the four standard instantiating macros: AC_CONFIG_FILES, AC_CONFIG_HEADERS, AC_CONFIG_COMMANDS and AC_CONFIG_LINKS. They all have this prototype:

     AC_CONFIG_ITEMS(tag..., [commands], [init-cmds])

where the arguments are:

tag...

A blank-or-newline-separated list of tags, which are typically the names of the files to instantiate.

You are encouraged to use literals as tags. In particular, you should avoid

          ... && my_foos="$my_foos fooo"
          ... && my_foos="$my_foos foooo"
          AC_CONFIG_ITEMS([$my_foos])

and use this instead:

          ... && AC_CONFIG_ITEMS([fooo])
          ... && AC_CONFIG_ITEMS([foooo])

The macros AC_CONFIG_FILES and AC_CONFIG_HEADERS use special tag values: they may have the form ‘output’ or ‘output:inputs’. The file output is instantiated from its templates, inputs (defaulting to ‘output.in’).

‘AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk)]’, for example, asks for the creation of the file Makefile that contains the expansion of the output variables in the concatenation of boiler/top.mk and boiler/bot.mk.

The special value ‘-’ might be used to denote the standard output when used in output, or the standard input when used in the inputs. You most probably don't need to use this in configure.ac, but it is convenient when using the command line interface of ./config.status, see config.status Invocation, for more details.

The inputs may be absolute or relative file names. In the latter case they are first looked for in the build tree, and then in the source tree. Input files should be text files, and a line length below 2000 bytes should be safe.

commands

Shell commands output literally into config.status, and associated with a tag that the user can use to tell config.status which commands to run. The commands are run each time a tag request is given to config.status, typically each time the file tag is created.

The variables set during the execution of configure are not available here: you first need to set them via the init-cmds. Nonetheless the following variables are precomputed:

srcdir

The name of the top source directory, assuming that the working directory is the top build directory. This is what the configure option --srcdir sets.

ac_top_srcdir

The name of the top source directory, assuming that the working directory is the current build directory.

ac_top_build_prefix

The name of the top build directory, assuming that the working directory is the current build directory. It can be empty, or else ends with a slash, so that you may concatenate it.

ac_srcdir

The name of the corresponding source directory, assuming that the working directory is the current build directory.

The current directory refers to the directory (or pseudo-directory) containing the input part of tags. For instance, running

          AC_CONFIG_COMMANDS([deep/dir/out:in/in.in], [...], [...])

with --srcdir=../package produces the following values:

          # Argument of --srcdir
          srcdir='../package'
          # Reversing deep/dir
          ac_top_build_prefix='../../'
          # Concatenation of $ac_top_build_prefix and srcdir
          ac_top_srcdir='../../../package'
          # Concatenation of $ac_top_srcdir and deep/dir
          ac_srcdir='../../../package/deep/dir'

independently of ‘in/in.in’.

init-cmds

Shell commands output unquoted near the beginning of config.status, and executed each time config.status runs (regardless of the tag). Because they are unquoted, for example, ‘$var’ is output as the value of var. init-cmds is typically used by configure to give config.status some variables it needs to run the commands.

You should be extremely cautious in your variable names: all the init-cmds share the same name space and may overwrite each other in unpredictable ways. Sorry...

All these macros can be called multiple times, with different tag values, of course!