# libbuild2-autoconf

GNU Autoconf emulation build system module for `build2`.

Specifically, this module provides an [`in`][module-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][proj-config] for rationale).

See [`libbuild2/autoconf/checks/`][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-in] module. For example:

```
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
}
```

## 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/`][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_FOO
#ifndef _WIN32
#  define HAVE_FOO 1
#else
#  undef HAVE_FOO /* No foo on Windows. */
#endif
```

[module-in]: https://build2.org/build2/doc/build2-build-system-manual.xhtml#module-in
[proj-config]: https://build2.org/build2/doc/build2-build-system-manual.xhtml#proj-config
[checks]: https://github.com/build2/libbuild2-autoconf/tree/master/libbuild2-autoconf/libbuild2/autoconf/checks/