libbuild2-autoconf/libbuild2-autoconf

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 built-in checks. Submit requests for new checks as issues. Submit implementations of new checks (or any other improvements) as PRs or patches.

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 or key-value pairs in the same way as with the in module. For example, as buildfile variables:

/* 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
}

As key-value pairs in the autoconf.substitutions map (which is an alias for the in.substitutions variable; see the in module for details):

/* config.h.in */

#undef _GNU_SOURCE
#undef _POSIX_SOURCE

gnu_source = ($c.stdlib == 'glibc')
posix_source = ($c.target.class != 'windows' && !$gnu_source)

h{config}: in{config}
{
  autoconf.substitutions  = _GNU_SOURCE@$gnu_source
  autoconf.substitutions += _POSIX_SOURCE@$posix_source
}

In particular, the autoconf.substitutions mechanism is the only way to have substitutions that cannot be specified as buildfile variables because they start with an underscore (and thus are reserved, as in the above example) or refer to one of the predefined variables.

The custom substitutions can also be used to override the built-in checks, for example:

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

Note that an implementation of a check may depend on another check. As a result, substitutions should not be conditional at the preprocessor level (unless all the checks are part of the same condition). Nor should the results of checks be adjusted until after the last check. For example:

#ifndef _WIN32
#  cmakedefine HAVE_EXPLICIT_BZERO // Conditional substitution.
#endif

#cmakedefine HAVE_EXPLICIT_MEMSET  // Shares implementation with BZERO.

#cmakedefine BYTE_ORDER

#if BYTE_ORDER == LITTLE_ENDIAN
#  undef BYTE_ORDER               // Adjusting the result.
#endif

#cmakedefine WORDS_BIGENDIAN      // Based on BYTE_ORDER.

Below is the correct way to achieve the above semantics:

#cmakedefine HAVE_EXPLICIT_BZERO
#cmakedefine HAVE_EXPLICIT_MEMSET

#cmakedefine BYTE_ORDER
#cmakedefine WORDS_BIGENDIAN

#ifdef _WIN32
#  undef HAVE_EXPLICIT_BZERO
#endif

#if BYTE_ORDER == LITTLE_ENDIAN
#  undef BYTE_ORDER
#endif

The built-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 built-in checks must include the prefix. For example:

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

  LIBFOO_HAVE_STRLCPY = true
}

Note also that some built-in check names are unprefixable, usually because they are standard macro names (for example, BYTE_ORDER) that on some platforms come from system headers (for example, <sys/endian.h> on FreeBSD). Such checks have ! after their names on the first line of their implementation files (for example, // BYTE_ORDER!).

Adding new checks

To add a check for a new configuration option <NAME> simply create the <NAME>.h header file (preserving the case) 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>[!] [: <BASE>...]

If the name is followed by the ! modifier, then it is unprefixable (see the previous section for details). The name can also be followed by : and a list of base checks. Such checks are automatically inserted before the rest of the lines in the resulting substitution. One notable check that you may want to use as a base is BUILD2_AUTOCONF_LIBC_VERSION (see comments for details).

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, to add a check for option HAVE_BAR, we could create the HAVE_BAR.h header file with the following content:

// 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 (unless it is unprefixable) 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_]).

Within a file duplicate checks are automatically suppressed. And if multiple files are involved, then the user is expected to employ the autoconf.prefix functionality to avoid clashes across files. However, this does not help unprefixable names and, as a result, such checks should be implemented in ways that deal with duplication (for example, include guards).

The duplicate suppression is incompatible with conditional (at the preprocessor level) checks, for example, assuming both HAVE_EXPLICIT_* checks are based on BUILD2_AUTOCONF_LIBC_VERSION:

#ifndef _WIN32
#  undef HAVE_EXPLICIT_BZERO
#endif

#undef HAVE_EXPLICIT_MEMSET

In this example, the autoconf module will omit the second copy of the BUILD2_AUTOCONF_LIBC_VERSION check as part of the HAVE_EXPLICIT_MEMSET substitution because it was already inserted as part of the HAVE_EXPLICIT_BZERO substitution. But the first copy will not be preprocessed on Windows.

While there is no bulletproof way to detect such situations (because the unconditional check could be BUILD2_AUTOCONF_LIBC_VERSION itself; perhaps we should only have private bases that are only accessed by the user via derived public checks), it is a good idea for checks that are based on other checks to verify that the base macros are in fact defined, for example:

// HAVE_EXPLICIT_BZERO : BUILD2_AUTOCONF_LIBC_VERSION

#ifndef BUILD2_AUTOCONF_LIBC_VERSION
#  error BUILD2_AUTOCONF_LIBC_VERSION appears to be conditionally included
#endif

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