5.2 Format of configure.ac

configure.ac is a file which is converted into a configure script for a software package. This file contains invocations of the Autoconf macros that test the system features your package needs or can use. Macros already exist to check for many features; see Existing Tests for their descriptions. For most other features, you can use Autoconf template macros to produce custom checks; see Writing Tests for information about them. For especially tricky or specialized features, configure.ac might need to contain some hand-crafted shell commands; see Portable Shell Programming. The autoscan program can give you a start in writing configure.ac (see autoscan Invocation for more information).

Automake scans the package’s configure.ac to determine certain information about the package. Some autoconf macros are required, and some variables must be defined in configure.ac. Automake will also use information from configure.ac to tailor its output.

Previous versions of Autoconf used the name configure.in instead of configure.ac. This name is still supported but is no longer recommended, and will cause warnings from autoconf. The name was changed because the tool needed to process this file is not described by its extension, and there is possible confusion with the extension of config.h.in and so on (for which ‘.in’ means “to be processed by configure”).