CMake/Tutorials/Package Registry

Introduction

CMake 2.8.5 and later provide two central locations to register packages that have been built or installed anywhere on a system:

• User Package Registry
• System Package Registry

The registries are especially useful to help project find packages in non-standard install locations or directly in their own build trees. A project may populate either the user or system registry (using its own means, see below) to refer to its location. In either case the package should store at the registered location a package configuration file (<package>Config.cmake) and optionally a package version file (<package>ConfigVersion.cmake).

The find_package command searches the two package registries as two of the search steps specified in its documentation. If it has sufficient permissions it also removes stale package registry entries that refer to directories that do not exist or do not contain a matching package configuration file.

User

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 <package> may appear under registry key

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

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 <package> may appear under the directory

  ~/.cmake/packages/<package>

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

System

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 <package> may appear under registry key

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

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.

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.

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.

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