Config-file Packages

A config-file package is a set of files provided by upstreams for downstreams to use. CMake searches in a number of locations for package configuration files, as described in the find_package() documentation. The most simple way for a CMake user to tell cmake(1) to search in a non-standard prefix for a package is to set the CMAKE_PREFIX_PATH cache variable.

Config-file packages are provided by upstream vendors as part of development packages, that is, they belong with the header files and any other files provided to assist downstreams in using the package.

A set of variables which provide package status information are also set automatically when using a config-file package. The <PackageName>_FOUND variable is set to true or false, depending on whether the package was found. The <PackageName>_DIR cache variable is set to the location of the package configuration file.

Find-module Packages

A find module is a file with a set of rules for finding the required pieces of a dependency, primarily header files and libraries. Typically, a find module is needed when the upstream is not built with CMake, or is not CMake-aware enough to otherwise provide a package configuration file. Unlike a package configuration file, it is not shipped with upstream, but is used by downstream to find the files by guessing locations of files with platform-specific hints.

Unlike the case of an upstream-provided package configuration file, no single point of reference identifies the package as being found, so the <PackageName>_FOUND variable is not automatically set by the find_package() command. It can still be expected to be set by convention however and should be set by the author of the Find-module. Similarly there is no <PackageName>_DIR variable, but each of the artifacts such as library locations and header file locations provide a separate cache variable.

See the cmake-developer(7) manual for more information about creating Find-module files.

Package Configuration File

Consider a project Foo that installs the following files:

<prefix>/include/foo-1.2/foo.h
<prefix>/lib/foo-1.2/libfoo.a

It may also provide a CMake package configuration file:

<prefix>/lib/cmake/foo-1.2/FooConfig.cmake

with content defining IMPORTED targets, or defining variables, such as:

# ...
# (compute PREFIX relative to file location)
# ...
set(Foo_INCLUDE_DIRS ${PREFIX}/include/foo-1.2)
set(Foo_LIBRARIES ${PREFIX}/lib/foo-1.2/libfoo.a)

If another project wishes to use Foo it need only to locate the FooConfig.cmake file and load it to get all the information it needs about package content locations. Since the package configuration file is provided by the package installation it already knows all the file locations.

The find_package() command may be used to search for the package configuration file. This command constructs a set of installation prefixes and searches under each prefix in several locations. Given the name Foo, it looks for a file called FooConfig.cmake or foo-config.cmake. The full set of locations is specified in the find_package() command documentation. One place it looks is:

<prefix>/lib/cmake/Foo*/

where Foo* is a case-insensitive globbing expression. In our example the globbing expression will match <prefix>/lib/cmake/foo-1.2 and the package configuration file will be found.

Once found, a package configuration file is immediately loaded. It, together with a package version file, contains all the information the project needs to use the package.

Package Version File

When the find_package() command finds a candidate package configuration file it looks next to it for a version file. The version file is loaded to test whether the package version is an acceptable match for the version requested. If the version file claims compatibility the configuration file is accepted. Otherwise it is ignored.

The name of the package version file must match that of the package configuration file but has either -version or Version appended to the name before the .cmake extension. For example, the files:

<prefix>/lib/cmake/foo-1.3/foo-config.cmake
<prefix>/lib/cmake/foo-1.3/foo-config-version.cmake

and:

<prefix>/lib/cmake/bar-4.2/BarConfig.cmake
<prefix>/lib/cmake/bar-4.2/BarConfigVersion.cmake

are each pairs of package configuration files and corresponding package version files.

When the find_package() command loads a version file it first sets the following variables:

The version file must use these variables to check whether it is compatible or an exact match for the requested version and set the following variables with results:

Version files are loaded in a nested scope so they are free to set any variables they wish as part of their computation. The find_package command wipes out the scope when the version file has completed and it has checked the output variables. When the version file claims to be an acceptable match for the requested version the find_package command sets the following variables for use by the project:

The variables report the version of the package that was actually found. The <PackageName> part of their name matches the argument given to the find_package() command.

Creating a Package Configuration File

In this case, the ClimbingStatsConfig.cmake file could be as simple as:

include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")

As this allows downstreams to use the IMPORTED targets. If any macros should be provided by the ClimbingStats package, they should be in a separate file which is installed to the same location as the ClimbingStatsConfig.cmake file, and included from there.

This can also be extended to cover dependencies:

# ...
add_library(ClimbingStats SHARED climbingstats.cpp)
generate_export_header(ClimbingStats)
find_package(Stats 2.6.4 REQUIRED)
target_link_libraries(ClimbingStats PUBLIC Stats::Types)

As the Stats::Types target is a PUBLIC dependency of ClimbingStats, downstreams must also find the Stats package and link to the Stats::Types library. The Stats package should be found in the ClimbingStatsConfig.cmake file to ensure this. The find_dependency macro from the CMakeFindDependencyMacro helps with this by propagating whether the package is REQUIRED, or QUIET etc. All REQUIRED dependencies of a package should be found in the Config.cmake file:

include(CMakeFindDependencyMacro)
find_dependency(Stats 2.6.4)
include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsMacros.cmake")

The find_dependency macro also sets ClimbingStats_FOUND to False if the dependency is not found, along with a diagnostic that the ClimbingStats package can not be used without the Stats package.

If COMPONENTS are specified when the downstream uses find_package(), they are listed in the <PackageName>_FIND_COMPONENTS variable. If a particular component is non-optional, then the <PackageName>_FIND_REQUIRED_<comp> will be true. This can be tested with logic in the package configuration file:

include(CMakeFindDependencyMacro)
find_dependency(Stats 2.6.4)
include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsMacros.cmake")
set(_ClimbingStats_supported_components Plot Table)
foreach(_comp ${ClimbingStats_FIND_COMPONENTS})


  if (NOT ";${_ClimbingStats_supported_components};" MATCHES ";${_comp};")


    set(ClimbingStats_FOUND False)


    set(ClimbingStats_NOT_FOUND_MESSAGE "Unsupported component: ${_comp}")


  endif()


  include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStats${_comp}Targets.cmake")
endforeach()

Here, the ClimbingStats_NOT_FOUND_MESSAGE is set to a diagnosis that the package could not be found because an invalid component was specified. This message variable can be set for any case where the _FOUND variable is set to False, and will be displayed to the user.

Creating a Package Configuration File for the Build Tree

The export(EXPORT) command creates an IMPORTED targets definition file which is specific to the build-tree, and is not relocatable. This can similarly be used with a suitable package configuration file and package version file to define a package for the build tree which may be used without installation. Consumers of the build tree can simply ensure that the CMAKE_PREFIX_PATH contains the build directory, or set the ClimbingStats_DIR to <build_dir>/ClimbingStats in the cache.

Creating Relocatable Packages

A relocatable package must not reference absolute paths of files on the machine where the package is built that will not exist on the machines where the package may be installed.

Packages created by install(EXPORT) are designed to be relocatable, using paths relative to the location of the package itself. When defining the interface of a target for EXPORT, keep in mind that the include directories should be specified as relative paths which are relative to the CMAKE_INSTALL_PREFIX:

target_include_directories(tgt INTERFACE


  # Wrong, not relocatable:


  $<INSTALL_INTERFACE:${CMAKE_INSTALL_PREFIX}/include/TgtName>
)
target_include_directories(tgt INTERFACE


  # Ok, relocatable:


  $<INSTALL_INTERFACE:include/TgtName>
)

The $<INSTALL_PREFIX> generator expression may be used as a placeholder for the install prefix without resulting in a non-relocatable package. This is necessary if complex generator expressions are used:

target_include_directories(tgt INTERFACE


  # Ok, relocatable:


  $<INSTALL_INTERFACE:$<$<CONFIG:Debug>:$<INSTALL_PREFIX>/include/TgtName>>
)

This also applies to paths referencing external dependencies. It is not advisable to populate any properties which may contain paths, such as INTERFACE_INCLUDE_DIRECTORIES and INTERFACE_LINK_LIBRARIES, with paths relevant to dependencies. For example, this code may not work well for a relocatable package:

target_link_libraries(ClimbingStats INTERFACE


  ${Foo_LIBRARIES} ${Bar_LIBRARIES}


  )
target_include_directories(ClimbingStats INTERFACE


  "$<INSTALL_INTERFACE:${Foo_INCLUDE_DIRS};${Bar_INCLUDE_DIRS}>"


  )

The referenced variables may contain the absolute paths to libraries and include directories as found on the machine the package was made on. This would create a package with hard-coded paths to dependencies and not suitable for relocation.

Ideally such dependencies should be used through their own IMPORTED targets that have their own IMPORTED_LOCATION and usage requirement properties such as INTERFACE_INCLUDE_DIRECTORIES populated appropriately. Those imported targets may then be used with the target_link_libraries() command for ClimbingStats:

target_link_libraries(ClimbingStats INTERFACE Foo::Foo Bar::Bar)

With this approach the package references its external dependencies only through the names of IMPORTED targets. When a consumer uses the installed package, the consumer will run the appropriate find_package() commands (via the find_dependency macro described above) to find the dependencies and populate the imported targets with appropriate paths on their own machine.

Unfortunately many modules shipped with CMake do not yet provide IMPORTED targets because their development pre-dated this approach. This may improve incrementally over time. Workarounds to create relocatable packages using such modules include:

User Package Registry

The User Package Registry is stored in a per-user location. The export(PACKAGE) command may be used to register a project build tree in the user package registry. CMake currently provides no interface to add install trees to the user package registry. Installers must be manually taught to register their packages if desired.

On Windows the user package registry is stored in the Windows registry under a key in HKEY_CURRENT_USER.

A <PackageName> may appear under registry key:

HKEY_CURRENT_USER\Software\Kitware\CMake\Packages\<PackageName>

as a REG_SZ value, with arbitrary name, that specifies the directory containing the package configuration file.

On UNIX platforms the user package registry is stored in the user home directory under ~/.cmake/packages. A <PackageName> may appear under the directory:

~/.cmake/packages/<PackageName>

as a file, with arbitrary name, whose content specifies the directory containing the package configuration file.

System Package Registry

The System Package Registry is stored in a system-wide location. CMake currently provides no interface to add to the system package registry. Installers must be manually taught to register their packages if desired.

On Windows the system package registry is stored in the Windows registry under a key in HKEY_LOCAL_MACHINE. A <PackageName> may appear under registry key:

HKEY_LOCAL_MACHINE\Software\Kitware\CMake\Packages\<PackageName>

as a REG_SZ value, with arbitrary name, that specifies the directory containing the package configuration file.

There is no system package registry on non-Windows platforms.

Disabling the Package Registry

In some cases using the Package Registries is not desirable. CMake allows one to disable them using the following variables:

Package Registry Example

A simple convention for naming package registry entries is to use content hashes. They are deterministic and unlikely to collide (export(PACKAGE) uses this approach). The name of an entry referencing a specific directory is simply the content hash of the directory path itself.

If a project arranges for package registry entries to exist, such as:

> reg query HKCU\Software\Kitware\CMake\Packages\MyPackage
HKEY_CURRENT_USER\Software\Kitware\CMake\Packages\MyPackage


 45e7d55f13b87179bb12f907c8de6fc4 REG_SZ c:/Users/Me/Work/lib/cmake/MyPackage


 7b4a9844f681c80ce93190d4e3185db9 REG_SZ c:/Users/Me/Work/MyPackage-build

or:

$ cat ~/.cmake/packages/MyPackage/7d1fb77e07ce59a81bed093bbee945bd
/home/me/work/lib/cmake/MyPackage
$ cat ~/.cmake/packages/MyPackage/f92c1db873a1937f3100706657c63e07
/home/me/work/MyPackage-build

then the CMakeLists.txt code:

find_package(MyPackage)

will search the registered locations for package configuration files (MyPackageConfig.cmake). The search order among package registry entries for a single package is unspecified and the entry names (hashes in this example) have no meaning. Registered locations may contain package version files (MyPackageConfigVersion.cmake) to tell find_package() whether a specific location is suitable for the version requested.

Package Registry Ownership

Package registry entries are individually owned by the project installations that they reference. A package installer is responsible for adding its own entry and the corresponding uninstaller is responsible for removing it.

The export(PACKAGE) command populates the user package registry with the location of a project build tree. Build trees tend to be deleted by developers and have no "uninstall" event that could trigger removal of their entries. In order to keep the registries clean the find_package() command automatically removes stale entries it encounters if it has sufficient permissions. CMake provides no interface to remove an entry referencing an existing build tree once export(PACKAGE) has been invoked. However, if the project removes its package configuration file from the build tree then the entry referencing the location will be considered stale.