2021-10-31 11:36:24 +00:00
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# libbuild2-autoconf
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GNU Autoconf emulation build system module for `build2`.
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Specifically, this module provides an [`in`][module-in]-based rule for
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processing `config.h.in` files. Besides the Autoconf special line flavor
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(`#undef`), it also supports the CMake (`#cmakedefine`) and Meson
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(`#mesondefine`) variants.
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Similar to Autoconf, this module provides built-in support for a number of
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common `HAVE_*` configuration options. However, the values of these options
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are not discovered by dynamic probing, such as trying to compile a test
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program to check if the feature is present. Instead, they are set to static
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expected values based on the platform/compiler macro checks (see note at the
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beginning of [Project Configuration][proj-config] for rationale).
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See [`libbuild2/autoconf/checks/`][checks] for the list of available built-in
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checks. Submit requests for new checks as issues. Submit implementations of
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new checks (or any other improvements) as PRs or patches.
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## Using in your projects
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This module is part of the standard pre-installed `build2` modules and no
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extra integration steps are required other than the `using` directive in
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your `buildfile`. For example, for Autoconf `config.h.in`:
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```
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using autoconf
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h{config}: in{config}
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```
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Or for CMake `config.h.cmake`:
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```
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using autoconf
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h{config}: in{config.h.cmake}
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```
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The default falvor is `autoconf` but if the input file has the `.cmake` or
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`.meson` extension, then the `cmake` or `meson` flavors are selected
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automatically. If, however, the standard `config.h.in` file is re-used for
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CMake/Meson, then the flavor must be specified explicitly with the
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`autoconf.flavor` variable, for example:
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```
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using autoconf
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h{config}: in{config}
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{
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autoconf.flavor = meson
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}
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```
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Besides the built-in configuration options, custom substitutions can be
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specified as `buildfile` variables in the same way as with the
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[`in`][module-in] module. For example:
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```
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/* config.h.in */
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#define PACKAGE_NAME @PACKAGE_NAME@
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#define PACKAGE_VERSION @PACKAGE_VERSION@
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#undef HAVE_STRLCPY
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#undef HAVE_STRLCAT
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```
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```
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h{config}: in{config}
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{
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PACKAGE_NAME = $project
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PACKAGE_VERSION = $version
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}
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```
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This mechanism can also be used to override the built-in checks, for example:
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```
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h{config}: in{config}
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{
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HAVE_STRLCPY = true
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}
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```
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The built-in checks can be prefixed in order to avoid clashes with similarly
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named macros in other headers. This is an especially good idea if the
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resulting header is public. To enable this, we specify the prefix with
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the `autoconf.prefix` variable and then use the prefixed versions of
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the options in the `config.h.in` file. For example:
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```
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/* config.h.in */
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#undef LIBFOO_HAVE_STRLCPY
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#undef LIBFOO_HAVE_STRLCAT
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```
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```
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h{config}: in{config}
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{
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autoconf.prefix = LIBFOO_
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}
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```
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Note that `autoconf.prefix` only affects the lookup of the built-in checks.
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Custom substitutions and overrides of built-in checks must include the
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prefix. For example:
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```
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h{config}: in{config}
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{
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autoconf.prefix = LIBFOO_
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LIBFOO_HAVE_STRLCPY = true
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}
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```
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Note also that some built-in check names are *unprefixable*, usually because
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they are standard macro names (for example, `BYTE_ORDER`) that on some
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platforms come from system headers (for example, `<sys/endian.h>` on FreeBSD).
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Such checks have `!` after their names on the first line of their
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implementation files (for example, `// BYTE_ORDER!`).
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## Adding new checks
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To add a check for a new configuration option `<NAME>` simply create the
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`<NAME>.h` header file (preserving the case) with the corresponding check and
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place it into [`libbuild2/autoconf/checks/`][checks] (use existing checks for
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inspiration).
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The first line in this header file must be in the form:
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```
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// <NAME>[!] [: <BASE>...]
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```
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If the name is followed by the `!` modifier, then it is *unprefixable* (see
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the previous section for details). The name can also be followed by `:` and a
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list of base checks. Such checks are automatically inserted before the rest of
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the lines in the resulting substitution. One notable check that you may
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want to use as a base is [`LIBC_VERSION`][libc-version] (see comments for
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details).
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Subsequent lines should be C-style comments or preprocessor directives that
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`#define` or `#undef` `<NAME>` depending on whether the feature is available
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(though there can be idiosyncrasies; see `const.h`, for example). Note that
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there should be no double-quotes or backslashes except for line
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continuations. For example, to add a check for option `HAVE_BAR`, we could
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create the `HAVE_BAR.h` header file with the following content:
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```
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// HAVE_BAR
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#undef HAVE_BAR
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/* No bar on Windows except with MinGW. */
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#if !defined(_WIN32) || \
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defined(__MINGW32__)
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# define HAVE_BAR 1
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#endif
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```
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Note also that the module implementation may need to replace `<NAME>` with its
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prefixed version (unless it is unprefixable) if the `autoconf.prefix`
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functionality is in use (see above). This is done by textually substituting
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every occurrence of `<NAME>` that is separated on both left and right hand
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sides (that is, both characters immediately before and after `<NAME>` are not
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`[A-Za-z0-9_]`).
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Within a file duplicate checks are automatically suppressed. And if multiple
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files are involved, then the user is expected to employ the `autoconf.prefix`
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functionality to avoid clashes across files. However, this does not help
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unprefixable names and, as a result, such checks should be implemented in
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ways that deal with duplication (for example, include guards).
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[module-in]: https://build2.org/build2/doc/build2-build-system-manual.xhtml#module-in
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[proj-config]: https://build2.org/build2/doc/build2-build-system-manual.xhtml#proj-config
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[checks]: https://github.com/build2/libbuild2-autoconf/tree/master/libbuild2-autoconf/libbuild2/autoconf/checks/
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[libc-version]: https://github.com/build2/libbuild2-autoconf/tree/master/libbuild2-autoconf/libbuild2/autoconf/checks/LIBC_VERSION.h
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