7.1 How Configuration Should Work

Each GNU distribution should come with a shell script named configure. This script is given arguments which describe the kind of machine and system you want to compile the program for.

The configure script must record the configuration options so that they affect compilation.

One way to do this is to make a link from a standard name such as ‘config.h’ to the proper configuration file for the chosen system. If you use this technique, the distribution should not contain a file named ‘config.h’. This is so that people won't be able to build the program without configuring it first.

Another thing that configure can do is to edit the Makefile. If you do this, the distribution should not contain a file named ‘Makefile’. Instead, it should include a file ‘Makefile.in’ which contains the input used for editing. Once again, this is so that people won't be able to build the program without configuring it first.

If configure does write the ‘Makefile’, then ‘Makefile’ should have a target named ‘Makefile’ which causes configure to be rerun, setting up the same configuration that was set up last time. The files that configure reads should be listed as dependencies of ‘Makefile’.

All the files which are output from the configure script should have comments at the beginning explaining that they were generated automatically using configure. This is so that users won't think of trying to edit them by hand.

The configure script should write a file named ‘config.status’ which describes which configuration options were specified when the program was last configured. This file should be a shell script which, if run, will recreate the same configuration.

The configure script should accept an option of the form ‘--srcdir=dirname’ to specify the directory where sources are found (if it is not the current directory). This makes it possible to build the program in a separate directory, so that the actual source directory is not modified.

If the user does not specify ‘--srcdir’, then configure should check both ‘.’ and ‘..’ to see if it can find the sources. If it finds the sources in one of these places, it should use them from there. Otherwise, it should report that it cannot find the sources, and should exit with nonzero status.

Usually the easy way to support ‘--srcdir’ is by editing a definition of VPATH into the Makefile. Some rules may need to refer explicitly to the specified source directory. To make this possible, configure can add to the Makefile a variable named srcdir whose value is precisely the specified directory.

The configure script should also take an argument which specifies the type of system to build the program for. This argument should look like this:

cpu-company-system

For example, an Athlon-based GNU/Linux system might be ‘i686-pc-linux-gnu’.

The configure script needs to be able to decode all plausible alternatives for how to describe a machine. Thus, ‘athlon-pc-gnu/linux’ would be a valid alias. There is a shell script called ‘config.sub’ that you can use as a subroutine to validate system types and canonicalize aliases.

The configure script should also take the option ‘--build=buildtype’, which should be equivalent to a plain buildtype argument. For example, ‘configure --build=i686-pc-linux-gnu’ is equivalent to ‘configure i686-pc-linux-gnu’. When the build type is not specified by an option or argument, the configure script should normally guess it using the shell script ‘config.guess’.

Other options are permitted to specify in more detail the software or hardware present on the machine, to include or exclude optional parts of the package, or to adjust the name of some tools or arguments to them:

‘--enable-feature[=parameter]’

Configure the package to build and install an optional user-level facility called feature. This allows users to choose which optional features to include. Giving an optional parameter of ‘no’ should omit feature, if it is built by default.

No ‘--enable’ option should ever cause one feature to replace another. No ‘--enable’ option should ever substitute one useful behavior for another useful behavior. The only proper use for ‘--enable’ is for questions of whether to build part of the program or exclude it.

‘--with-package’

The package package will be installed, so configure this package to work with package.

Possible values of package include ‘gnu-as’ (or ‘gas’), ‘gnu-ld’, ‘gnu-libc’, ‘gdb’, ‘x’, and ‘x-toolkit’.

Do not use a ‘--with’ option to specify the file name to use to find certain files. That is outside the scope of what ‘--with’ options are for.

‘variable=value’

Set the value of the variable variable to value. This is used to override the default values of commands or arguments in the build process. For example, the user could issue ‘configure CFLAGS=-g CXXFLAGS=-g’ to build with debugging information and without the default optimization.

Specifying variables as arguments to configure, like this:

./configure CC=gcc

is preferable to setting them in environment variables:

CC=gcc ./configure

as it helps to recreate the same configuration later with ‘config.status’.

All configure scripts should accept all of the “detail” options and the variable settings, whether or not they make any difference to the particular package at hand. In particular, they should accept any option that starts with ‘--with-’ or ‘--enable-’. This is so users will be able to configure an entire GNU source tree at once with a single set of options.

You will note that the categories ‘--with-’ and ‘--enable-’ are narrow: they do not provide a place for any sort of option you might think of. That is deliberate. We want to limit the possible configuration options in GNU software. We do not want GNU programs to have idiosyncratic configuration options.

Packages that perform part of the compilation process may support cross-compilation. In such a case, the host and target machines for the program may be different.

The configure script should normally treat the specified type of system as both the host and the target, thus producing a program which works for the same type of machine that it runs on.

To compile a program to run on a host type that differs from the build type, use the configure option ‘--host=hosttype’, where hosttype uses the same syntax as buildtype. The host type normally defaults to the build type.

To configure a cross-compiler, cross-assembler, or what have you, you should specify a target different from the host, using the configure option ‘--target=targettype’. The syntax for targettype is the same as for the host type. So the command would look like this:

./configure --host=hosttype --target=targettype

The target type normally defaults to the host type. Programs for which cross-operation is not meaningful need not accept the ‘--target’ option, because configuring an entire operating system for cross-operation is not a meaningful operation.

Some programs have ways of configuring themselves automatically. If your program is set up to do this, your configure script can simply ignore most of its arguments.