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4.5 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_FOOS(tag..., [commands], [init-cmds])

where the arguments are:

A whitespace-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"

and use this instead:

... && AC_CONFIG_FOOS(fooo)
... && AC_CONFIG_FOOS(foooo)

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

For instance `AC_CONFIG_FILES(Makefile:boiler/top.mk:boiler/bot.mk)' asks for the creation of `Makefile' that will be 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 14. Recreating a Configuration, for more details.

The inputs may be absolute or relative filenames. In the latter case they are first looked for in the build tree, and then in the source tree.

Shell commands output literally into `config.status', and associated with a tag that the user can use to tell `config.status' which the 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:

The path from the top build directory to the top source directory. This is what configure's option `--srcdir' sets.

The path from the current build directory to the top source directory.

The path from the current build directory to the top build directory. It can be empty, or else ends with a slash, so that you may concatenate it.

The path from the current build directory to the corresponding source 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
# Reversing deep/dir
# Concatenation of $ac_top_builddir and srcdir
# Concatenation of $ac_top_srcdir and deep/dir

independently of `in/in.in'.

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' will be 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 tags, of course!

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