View source for CMake/Tutorials/How to create a ProjectConfig.cmake file

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== Introduction ==

Native CMake projects that are intended to be used by other projects (e.g. libraries, but also tools that could be useful as a build-utility, such as documentation generators, wrapper generators, etc.) should provide at a minimum a <tt><name>Config.cmake</tt> or a <tt><lower-name>-config.cmake</tt> file. This file can then be used by the [http://www.cmake.org/cmake/help/v2.8.10/cmake.html#command:find_package <tt>find_package()</tt>] command in ''config-mode'' to provide information about include-directories, libraries and their dependencies, required compile-flags or locations of executables. You are advised to carefully read the documentation of the <tt>find_package()</tt> command before proceeding. This short article will show you how to a very simple project usable for other CMake-based projects.

The full example is available as a download in the zip-file [[Media:FooBar.zip|FooBar.zip]].

=== The FooBar project ===

Let's assume the project contains a simple shared library, ''foo'' and a utility that uses the library, ''bar''. The source tree could have the following layout:

<pre>
FooBar/
|-- CMakeLists.txt
|-- FooBarConfig.cmake.in
|-- FooBarConfigVersion.cmake.in
|-- foo/
|   |-- CMakeLists.txt
|   |-- config.h.in
|   |-- foo.h
|   `-- foo.c
`-- bar/
    |-- CMakeLists.txt
    `-- bar.c
</pre>

The files <tt>FooBar/foo/{config.h.in,foo.h,foo.c}</tt> and <tt>FooBar/bar/bar.c</tt> are of little interest here and their contents are left to the imagination of the reader.

=== The main <tt>FooBar/CMakeLists.txt</tt> file ===

A simple <tt>FooBar/CMakeLists.txt</tt> could look like the following, where the really interesting stuff starts after the respective comment.

<source lang="cmake">
cmake_minimum_required(VERSION 2.8)
project(FooBar C)

set(FOOBAR_MAJOR_VERSION 0)
set(FOOBAR_MINOR_VERSION 1)
set(FOOBAR_PATCH_VERSION 0)
set(FOOBAR_VERSION
  ${FOOBAR_MAJOR_VERSION}.${FOOBAR_MINOR_VERSION}.${FOOBAR_PATCH_VERSION})

# Offer the user the choice of overriding the installation directories
set(INSTALL_LIB_DIR lib CACHE PATH "Installation directory for libraries")
set(INSTALL_BIN_DIR bin CACHE PATH "Installation directory for executables")
set(INSTALL_INCLUDE_DIR include CACHE PATH
  "Installation directory for header files")
if(WIN32 AND NOT CYGWIN)
  set(DEF_INSTALL_CMAKE_DIR CMake)
else()
  set(DEF_INSTALL_CMAKE_DIR lib/CMake/FooBar)
endif()
set(INSTALL_CMAKE_DIR ${DEF_INSTALL_CMAKE_DIR} CACHE PATH
  "Installation directory for CMake files")

# Make relative paths absolute (needed later on)
foreach(p LIB BIN INCLUDE CMAKE)
  set(var INSTALL_${p}_DIR)
  if(NOT IS_ABSOLUTE "${${var}}")
    set(${var} "${CMAKE_INSTALL_PREFIX}/${${var}}")
  endif()
endforeach()

# set up include-directories
include_directories(
  "${PROJECT_SOURCE_DIR}"   # to find foo/foo.h
  "${PROJECT_BINARY_DIR}")  # to find foo/config.h

# Add sub-directories
add_subdirectory(foo)
add_subdirectory(bar)

# The interesting stuff goes here
# ===============================

# Add all targets to the build-tree export set
export(TARGETS foo bar
  FILE "${PROJECT_BINARY_DIR}/FooBarTargets.cmake")

# Export the package for use from the build-tree
# (this registers the build-tree with a global CMake-registry)
export(PACKAGE FooBar)

# Create the FooBarConfig.cmake and FooBarConfigVersion files
file(RELATIVE_PATH REL_INCLUDE_DIR "${INSTALL_CMAKE_DIR}"
   "${INSTALL_INCLUDE_DIR}")
# ... for the build tree
set(CONF_INCLUDE_DIRS "${PROJECT_SOURCE_DIR}" "${PROJECT_BINARY_DIR}")
configure_file(FooBarConfig.cmake.in
  "${PROJECT_BINARY_DIR}/FooBarConfig.cmake" @ONLY)
# ... for the install tree
set(CONF_INCLUDE_DIRS "\${FOOBAR_CMAKE_DIR}/${REL_INCLUDE_DIR}")
configure_file(FooBarConfig.cmake.in
  "${PROJECT_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/FooBarConfig.cmake" @ONLY)
# ... for both
configure_file(FooBarConfigVersion.cmake.in
  "${PROJECT_BINARY_DIR}/FooBarConfigVersion.cmake" @ONLY)

# Install the FooBarConfig.cmake and FooBarConfigVersion.cmake
install(FILES
  "${PROJECT_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/FooBarConfig.cmake"
  "${PROJECT_BINARY_DIR}/FooBarConfigVersion.cmake"
  DESTINATION "${INSTALL_CMAKE_DIR}" COMPONENT dev)

# Install the export set for use with the install-tree
install(EXPORT FooBarTargets DESTINATION
  "${INSTALL_CMAKE_DIR}" COMPONENT dev)
</source>

=== The files <tt>FooBar/{foo,bar}/CMakeLists.txt</tt> ===

The file <tt>FooBar/foo/CMakeLists.txt</tt> is pretty simple and looks like expected:

<source lang="cmake">
configure_file(config.h.in "${CMAKE_CURRENT_BINARY_DIR}/config.h" @ONLY)

add_library(foo SHARED foo.c foo.h config.h.in)

set_target_properties(foo PROPERTIES
  PUBLIC_HEADER "foo.h;${CMAKE_CURRENT_BINARY_DIR}/config.h")

install(TARGETS foo
  # IMPORTANT: Add the foo library to the "export-set"
  EXPORT FooBarTargets
  RUNTIME DESTINATION "${INSTALL_BIN_DIR}" COMPONENT bin
  LIBRARY DESTINATION "${INSTALL_LIB_DIR}" COMPONENT shlib
  PUBLIC_HEADER DESTINATION "${INSTALL_INCLUDE_DIR}/foo"
    COMPONENT dev)
</source>

The file <tt>FooBar/bar/CMakeLists.txt</tt> is even shorter:

<source lang="cmake">
add_executable(bar bar.c)

target_link_libraries(bar foo)

install(TARGETS bar
  # IMPORTANT: Add the bar executable to the "export-set"
  EXPORT FooBarTargets
  RUNTIME DESTINATION "${INSTALL_BIN_DIR}" COMPONENT bin)
</source>

=== The <tt>FooBar/FooBarConfig.cmake.in</tt> file ===

The really interesting file is <tt>FooBar/FooBarConfig.cmake.in</tt>. Although it usually can be quite simple, it seems to cause considerable confusion to new CMake-users. For the FooBar project the following is a plausible implementation:

<source lang="cmake">
# - Config file for the FooBar package
# It defines the following variables
#  FOOBAR_INCLUDE_DIRS - include directories for FooBar
#  FOOBAR_LIBRARIES    - libraries to link against
#  FOOBAR_EXECUTABLE   - the bar executable

# Compute paths
get_filename_component(FOOBAR_CMAKE_DIR "${CMAKE_CURRENT_LIST_FILE}" PATH)
set(FOOBAR_INCLUDE_DIRS "@CONF_INCLUDE_DIRS@")

# Our library dependencies (contains definitions for IMPORTED targets)
if(NOT TARGET foo AND NOT FooBar_BINARY_DIR)
  include("${FOOBAR_CMAKE_DIR}/FooBarTargets.cmake")
endif()

# These are IMPORTED targets created by FooBarTargets.cmake
set(FOOBAR_LIBRARIES foo)
set(FOOBAR_EXECUTABLE bar)
</source>

If your package also provides CMake macros or functions, you might want to put them in a file <tt>FooBarUse.cmake</tt> (or similar), install it alongside <tt>FooBarConfig.cmake</tt> and define the variable <tt>FOOBAR_USE_FILE</tt> in above code and set it to the location of the <tt>FooBarUse.cmake</tt> file.

=== The <tt>FooBar/FooBarConfigVersion.cmake.in</tt> file ===

The last file to discuss is the <tt>FooBar/FooBarConfigVersion.cmake.in</tt>. It is important because it allows client projects to determine the version of FooBar they found using the <tt>find_package</tt> command, but more importantly, it allows the same command to automatically determine whether the detected version is suitable if the client-project requested a minimum (or even exact) version of FooBar. The file is also straightforward and usually takes the following form:

<source lang="cmake">
set(PACKAGE_VERSION "@FOOBAR_VERSION@")

# Check whether the requested PACKAGE_FIND_VERSION is compatible
if("${PACKAGE_VERSION}" VERSION_LESS "${PACKAGE_FIND_VERSION}")
  set(PACKAGE_VERSION_COMPATIBLE FALSE)
else()
  set(PACKAGE_VERSION_COMPATIBLE TRUE)
  if ("${PACKAGE_VERSION}" VERSION_EQUAL "${PACKAGE_FIND_VERSION}")
    set(PACKAGE_VERSION_EXACT TRUE)
  endif()
endif()
</source>

=== Closing Remarks ===

In the above approach the <tt>FooBar/FooBarConfig.cmake.in</tt> file uses a relative path to locate the include directory. This has the advantage of making the installed package ''relocatable'', i.e. the user can relocate the whole installation tree and as long as the relative paths remain the same, the package will continue to work with no need for manual tweaking of the installed files. However, the approach also has a serious drawback: On Windows it requires that the variables <tt>INSTALL_INCLUDE_DIR</tt> and <tt>INSTALL_CMAKE_DIR</tt> are set to paths with the same drive letter. Otherwise there is no valid relative path connecting the two. If such an installation is a realistic scenario for your project, you might need to hard-code the full <tt>INSTALL_INCLUDE_DIR</tt> path when configuring <tt>FooBar/FooBarConfig.cmake.in</tt> at the cost of your package being no longer relocatable.

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