// -*- mode:doc; -*-
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=== Infrastructure for CMake-based packages

[[cmake-package-tutorial]]

==== +cmake-package+ tutorial

First, let's see how to write a +.mk+ file for a CMake-based package,
with an example :

------------------------
01: ################################################################################
02: #
03: # libfoo
04: #
05: ################################################################################
06:
07: LIBFOO_VERSION = 1.0
08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
09: LIBFOO_SITE = http://www.foosoftware.org/download
10: LIBFOO_INSTALL_STAGING = YES
11: LIBFOO_INSTALL_TARGET = NO
12: LIBFOO_CONF_OPTS = -DBUILD_DEMOS=ON
13: LIBFOO_DEPENDENCIES = libglib2 host-pkgconf
14:
15: $(eval $(cmake-package))
------------------------

On line 7, we declare the version of the package.

On line 8 and 9, we declare the name of the tarball (xz-ed tarball recommended)
and the location of the tarball on the Web. Buildroot will automatically
download the tarball from this location.

On line 10, we tell Buildroot to install the package to the staging
directory. The staging directory, located in +output/staging/+
is the directory where all the packages are installed, including their
development files, etc. By default, packages are not installed to the
staging directory, since usually, only libraries need to be installed in
the staging directory: their development files are needed to compile
other libraries or applications depending on them. Also by default, when
staging installation is enabled, packages are installed in this location
using the +make install+ command.

On line 11, we tell Buildroot to not install the package to the
target directory. This directory contains what will become the root
filesystem running on the target. For purely static libraries, it is
not necessary to install them in the target directory because they will
not be used at runtime. By default, target installation is enabled; setting
this variable to NO is almost never needed. Also by default, packages are
installed in this location using the +make install+ command.

On line 12, we tell Buildroot to pass custom options to CMake when it is
configuring the package.

On line 13, we declare our dependencies, so that they are built
before the build process of our package starts.

Finally, on line line 15, we invoke the +cmake-package+
macro that generates all the Makefile rules that actually allows the
package to be built.

[[cmake-package-reference]]

==== +cmake-package+ reference

The main macro of the CMake package infrastructure is
+cmake-package+. It is similar to the +generic-package+ macro. The ability to
have target and host packages is also available, with the
+host-cmake-package+ macro.

Just like the generic infrastructure, the CMake infrastructure works
by defining a number of variables before calling the +cmake-package+
macro.

First, all the package metadata information variables that exist in
the generic infrastructure also exist in the CMake infrastructure:
+LIBFOO_VERSION+, +LIBFOO_SOURCE+, +LIBFOO_PATCH+, +LIBFOO_SITE+,
+LIBFOO_SUBDIR+, +LIBFOO_DEPENDENCIES+, +LIBFOO_INSTALL_STAGING+,
+LIBFOO_INSTALL_TARGET+.

A few additional variables, specific to the CMake infrastructure, can
also be defined. Many of them are only useful in very specific cases,
typical packages will therefore only use a few of them.

* +LIBFOO_SUBDIR+ may contain the name of a subdirectory inside the
  package that contains the main CMakeLists.txt file. This is useful,
  if for example, the main CMakeLists.txt file is not at the root of
  the tree extracted by the tarball. If +HOST_LIBFOO_SUBDIR+ is not
  specified, it defaults to +LIBFOO_SUBDIR+.

* +LIBFOO_CMAKE_BACKEND+ specifies the cmake backend to use, one of
  `make` (to use the GNU Makefiles generator, the default) or `ninja`
  (to use the Ninja generator).

* +LIBFOO_CONF_ENV+, to specify additional environment variables to
  pass to CMake. By default, empty.

* +LIBFOO_CONF_OPTS+, to specify additional configure options to pass
  to CMake. By default, empty. A number of common CMake options are
  set by the +cmake-package+ infrastructure; so it is normally not
  necessary to set them in the package's +*.mk+ file unless you want
  to override them:

** +CMAKE_BUILD_TYPE+ is driven by +BR2_ENABLE_RUNTIME_DEBUG+;
** +CMAKE_INSTALL_PREFIX+;
** +BUILD_SHARED_LIBS+ is driven by +BR2_STATIC_LIBS+;
** +BUILD_DOC+, +BUILD_DOCS+ are disabled;
** +BUILD_EXAMPLE+, +BUILD_EXAMPLES+ are disabled;
** +BUILD_TEST+, +BUILD_TESTS+, +BUILD_TESTING+ are disabled.

* +LIBFOO_BUILD_ENV+ and +LIBFOO_BUILD_OPTS+ to specify additional
  environment variables, or command line options, to pass to the backend
  at build time.

* +LIBFOO_SUPPORTS_IN_SOURCE_BUILD = NO+ should be set when the package
  cannot be built inside the source tree but needs a separate build
  directory.

* +LIBFOO_MAKE+, to specify an alternate +make+ command. This is
  typically useful when parallel make is enabled in the configuration
  (using +BR2_JLEVEL+) but that this feature should be disabled for
  the given package, for one reason or another. By default, set to
  +$(MAKE)+. If parallel building is not supported by the package,
  then it should be set to +LIBFOO_MAKE=$(MAKE1)+.

* +LIBFOO_MAKE_ENV+, to specify additional environment variables to
  pass to make in the build step. These are passed before the +make+
  command. By default, empty.

* +LIBFOO_MAKE_OPTS+, to specify additional variables to pass to make
  in the build step. These are passed after the +make+ command. By
  default, empty.

* +LIBFOO_INSTALL_OPTS+ contains the make options used to
  install the package to the host directory. By default, the value
  is +install+, which is correct for most CMake packages. It is still
  possible to override it.

* +LIBFOO_INSTALL_STAGING_OPTS+ contains the make options used to
  install the package to the staging directory. By default, the value
  is +DESTDIR=$(STAGING_DIR) install/fast+, which is correct for most
  CMake packages. It is still possible to override it.

* +LIBFOO_INSTALL_TARGET_OPTS+ contains the make options used to
  install the package to the target directory. By default, the value
  is +DESTDIR=$(TARGET_DIR) install/fast+. The default value is correct
  for most CMake packages, but it is still possible to override it if
  needed.

With the CMake infrastructure, all the steps required to build and
install the packages are already defined, and they generally work well
for most CMake-based packages. However, when required, it is still
possible to customize what is done in any particular step:

* By adding a post-operation hook (after extract, patch, configure,
  build or install). See xref:hooks[] for details.

* By overriding one of the steps. For example, even if the CMake
  infrastructure is used, if the package +.mk+ file defines its own
  +LIBFOO_CONFIGURE_CMDS+ variable, it will be used instead of the
  default CMake one. However, using this method should be restricted
  to very specific cases. Do not use it in the general case.
