libbuild2-autoconf

GNU Autoconf emulation build system module for build2.

Specifically, this module provides an in-based rule for processing config.h.in files. Besides the Autoconf special line flavor (#undef), it also supports the CMake (#cmakedefine) and Meson (#mesondefine) variants.

Similar to Autoconf, this module provides built-in support for a number of common HAVE_* configuration options. However, the values of these options are not discovered by dynamic probing, such as trying to compile a test program to check if the feature is present. Instead, they are set to static expected values based on the platform/compiler macro checks (see note at the beginning of Project Configuration for rationale).

See libbuild2/autoconf/checks/ for the list of available build-in checks.

Using in your projects

This module is part of the standard pre-installed build2 modules and no extra integration steps are required other than the using directive in your buildfile. For example, for Autoconf config.h.in:

using autoconf

h{config}: in{config}

Or for CMake config.h.cmake:

using autoconf

h{config}: in{config.h.cmake}

The default falvor is autoconf but if the input file has the .cmake or .meson extension, then the cmake or meson flavors are selected automatically. If, however, the standard config.h.in file is re-used for CMake/Meson, then the flavor must be specified explicitly with the autoconf.flavor variable, for example:

using autoconf

h{config}: in{config}
{
  autoconf.flavor = meson
}

Besides the built-in configuration options, custom substitutions can be specified as buildfile variables in the same way as with the in module. For example:

/* config.h.in */

#define PACKAGE_NAME @PACKAGE_NAME@
#define PACKAGE_VERSION @PACKAGE_VERSION@

#undef HAVE_STRLCPY
#undef HAVE_STRLCAT

h{config}: in{config}
{
  PACKAGE_NAME = $project
  PACKAGE_VERSION = $version
}

This mechanism can also be used to override the build-in checks, for example:

h{config}: in{config}
{
  HAVE_STRLCPY = true
}

The build-in checks can be prefixed in order to avoid clashes with similarly named macros in other headers. This is an especially good idea if the resulting header is public. To enable this, we specify the prefix with the autoconf.prefix variable and then use the prefixed versions of the options in the config.h.in file. For example:

/* config.h.in */

#undef LIBFOO_HAVE_STRLCPY
#undef LIBFOO_HAVE_STRLCAT

h{config}: in{config}
{
  autoconf.prefix = LIBFOO_
}

Note that autoconf.prefix only affects the lookup of the built-in checks. Custom substitutions and overrides of build-in checks must include the prefix. For example:

h{config}: in{config}
{
  autoconf.prefix = LIBFOO_

  LIBFOO_HAVE_STRLCPY = true
}

Adding new checks

To add a check for a new configuration option <NAME> simply create the <NAME>.h header file with the corresponding check and place it into libbuild2/autoconf/checks/ (use existing checks for inspiration).

The first line in this header file must be in the form:

// <NAME>

Subsequent lines should be C-style comments or preprocessor directives that #define or #undef <NAME> depending on whether the feature is available (though there can be idiosyncrasies; see const.h, for example). Note that there should be no double-quotes or backslashes except for line continuations. For example:

// HAVE_BAR

#undef HAVE_BAR

/* No bar on Windows except with MinGW. */
#if !defined(_WIN32) || \
     defined(__MINGW32__)
#  define HAVE_BAR 1
#endif

Note also that the module implementation may need to replace <NAME> with its prefixed version if the autoconf.prefix functionality is in use (see above). This is done by textually substituting every occurrence of <NAME> that is separated on both left and right hand sides (that is, both characters immediately before and after <NAME> are not [A-Za-z0-9_]).