cmake-developer(7)

Introduction

This manual is intended for reference by developers working with cmake-language(7) code, whether writing their own modules, authoring their own build systems, or working on CMake itself.

See https://cmake.org/get-involved/ to get involved in development of CMake upstream. It includes links to contribution instructions, which in turn link to developer guides for CMake itself.

Find Modules

A “find module” is a Find<PackageName>.cmake file to be loaded by the find_package() command when invoked for <PackageName>.

The primary task of a find module is to determine whether a package exists on the system, set the <PackageName>_FOUND variable to reflect this and provide any variables, macros and imported targets required to use the package. A find module is useful in cases where an upstream library does not provide a config file package.

The traditional approach is to use variables for everything, including libraries and executables: see the Standard Variable Names section below. This is what most of the existing find modules provided by CMake do.

The more modern approach is to behave as much like config file packages files as possible, by providing imported target. This has the advantage of propagating Transitive Usage Requirements to consumers.

In either case (or even when providing both variables and imported targets), find modules should provide backwards compatibility with old versions that had the same name.

A FindFoo.cmake module will typically be loaded by the command:

find_package(Foo [major[.minor[.patch[.tweak]]]]
             [EXACT] [QUIET] [REQUIRED]
             [[COMPONENTS] [components...]]
             [OPTIONAL_COMPONENTS components...]
             [NO_POLICY_SCOPE])

See the find_package() documentation for details on what variables are set for the find module. Most of these are dealt with by using FindPackageHandleStandardArgs.

Briefly, the module should only locate versions of the package compatible with the requested version, as described by the Foo_FIND_VERSION family of variables. If Foo_FIND_QUIETLY is set to true, it should avoid printing messages, including anything complaining about the package not being found. If Foo_FIND_REQUIRED is set to true, the module should issue a FATAL_ERROR if the package cannot be found. If neither are set to true, it should print a non-fatal message if it cannot find the package.

Packages that find multiple semi-independent parts (like bundles of libraries) should search for the components listed in Foo_FIND_COMPONENTS if it is set , and only set Foo_FOUND to true if for each searched-for component <c> that was not found, Foo_FIND_REQUIRED_<c> is not set to true. The HANDLE_COMPONENTS argument of find_package_handle_standard_args() can be used to implement this.

If Foo_FIND_COMPONENTS is not set, which modules are searched for and required is up to the find module, but should be documented.

For internal implementation, it is a generally accepted convention that variables starting with underscore are for temporary use only.

Standard Variable Names

For a FindXxx.cmake module that takes the approach of setting variables (either instead of or in addition to creating imported targets), the following variable names should be used to keep things consistent between find modules. Note that all variables start with Xxx_ to make sure they do not interfere with other find modules; the same consideration applies to macros, functions and imported targets.

Xxx_INCLUDE_DIRS

The final set of include directories listed in one variable for use by client code. This should not be a cache entry.

Xxx_LIBRARIES

The libraries to link against to use Xxx. These should include full paths. This should not be a cache entry.

Xxx_DEFINITIONS

Definitions to use when compiling code that uses Xxx. This really shouldn’t include options such as -DHAS_JPEG that a client source-code file uses to decide whether to #include <jpeg.h>

Xxx_EXECUTABLE

Where to find the Xxx tool.

Xxx_Yyy_EXECUTABLE

Where to find the Yyy tool that comes with Xxx.

Xxx_LIBRARY_DIRS

Optionally, the final set of library directories listed in one variable for use by client code. This should not be a cache entry.

Xxx_ROOT_DIR

Where to find the base directory of Xxx.

Xxx_VERSION_Yy

Expect Version Yy if true. Make sure at most one of these is ever true.

Xxx_WRAP_Yy

If False, do not try to use the relevant CMake wrapping command.

Xxx_Yy_FOUND

If False, optional Yy part of Xxx system is not available.

Xxx_FOUND

Set to false, or undefined, if we haven’t found, or don’t want to use Xxx.

Xxx_NOT_FOUND_MESSAGE

Should be set by config-files in the case that it has set Xxx_FOUND to FALSE. The contained message will be printed by the find_package() command and by find_package_handle_standard_args() to inform the user about the problem.

Xxx_RUNTIME_LIBRARY_DIRS

Optionally, the runtime library search path for use when running an executable linked to shared libraries. The list should be used by user code to create the PATH on windows or LD_LIBRARY_PATH on UNIX. This should not be a cache entry.

Xxx_VERSION

The full version string of the package found, if any. Note that many existing modules provide Xxx_VERSION_STRING instead.

Xxx_VERSION_MAJOR

The major version of the package found, if any.

Xxx_VERSION_MINOR

The minor version of the package found, if any.

Xxx_VERSION_PATCH

The patch version of the package found, if any.

The following names should not usually be used in CMakeLists.txt files, but are typically cache variables for users to edit and control the behaviour of find modules (like entering the path to a library manually)

Xxx_LIBRARY

The path of the Xxx library (as used with find_library(), for example).

Xxx_Yy_LIBRARY

The path of the Yy library that is part of the Xxx system. It may or may not be required to use Xxx.

Xxx_INCLUDE_DIR

Where to find headers for using the Xxx library.

Xxx_Yy_INCLUDE_DIR

Where to find headers for using the Yy library of the Xxx system.

To prevent users being overwhelmed with settings to configure, try to keep as many options as possible out of the cache, leaving at least one option which can be used to disable use of the module, or locate a not-found library (e.g. Xxx_ROOT_DIR). For the same reason, mark most cache options as advanced. For packages which provide both debug and release binaries, it is common to create cache variables with a _LIBRARY_<CONFIG> suffix, such as Foo_LIBRARY_RELEASE and Foo_LIBRARY_DEBUG.

While these are the standard variable names, you should provide backwards compatibility for any old names that were actually in use. Make sure you comment them as deprecated, so that no-one starts using them.

A Sample Find Module

We will describe how to create a simple find module for a library Foo.

The top of the module should begin with a license notice, followed by a blank line, and then followed by a Bracket Comment. The comment should begin with .rst: to indicate that the rest of its content is reStructuredText-format documentation. For example:

# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
# file Copyright.txt or https://cmake.org/licensing for details.

#[=======================================================================[.rst:
FindFoo
-------

Finds the Foo library.

Imported Targets
^^^^^^^^^^^^^^^^

This module provides the following imported targets, if found:

``Foo::Foo``
  The Foo library

Result Variables
^^^^^^^^^^^^^^^^

This will define the following variables:

``Foo_FOUND``
  True if the system has the Foo library.
``Foo_VERSION``
  The version of the Foo library which was found.
``Foo_INCLUDE_DIRS``
  Include directories needed to use Foo.
``Foo_LIBRARIES``
  Libraries needed to link to Foo.

Cache Variables
^^^^^^^^^^^^^^^

The following cache variables may also be set:

``Foo_INCLUDE_DIR``
  The directory containing ``foo.h``.
``Foo_LIBRARY``
  The path to the Foo library.

#]=======================================================================]

The module documentation consists of:

  • An underlined heading specifying the module name.

  • A simple description of what the module finds. More description may be required for some packages. If there are caveats or other details users of the module should be aware of, specify them here.

  • A section listing imported targets provided by the module, if any.

  • A section listing result variables provided by the module.

  • Optionally a section listing cache variables used by the module, if any.

If the package provides any macros or functions, they should be listed in an additional section, but can be documented by additional .rst: comment blocks immediately above where those macros or functions are defined.

The find module implementation may begin below the documentation block. Now the actual libraries and so on have to be found. The code here will obviously vary from module to module (dealing with that, after all, is the point of find modules), but there tends to be a common pattern for libraries.

First, we try to use pkg-config to find the library. Note that we cannot rely on this, as it may not be available, but it provides a good starting point.

find_package(PkgConfig)
pkg_check_modules(PC_Foo QUIET Foo)

This should define some variables starting PC_Foo_ that contain the information from the Foo.pc file.

Now we need to find the libraries and include files; we use the information from pkg-config to provide hints to CMake about where to look.

find_path(Foo_INCLUDE_DIR
  NAMES foo.h
  PATHS ${PC_Foo_INCLUDE_DIRS}
  PATH_SUFFIXES Foo
)
find_library(Foo_LIBRARY
  NAMES foo
  PATHS ${PC_Foo_LIBRARY_DIRS}
)

If you have a good way of getting the version (from a header file, for example), you can use that information to set Foo_VERSION (although note that find modules have traditionally used Foo_VERSION_STRING, so you may want to set both). Otherwise, attempt to use the information from pkg-config

set(Foo_VERSION ${PC_Foo_VERSION})

Now we can use FindPackageHandleStandardArgs to do most of the rest of the work for us

include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(Foo
  FOUND_VAR Foo_FOUND
  REQUIRED_VARS
    Foo_LIBRARY
    Foo_INCLUDE_DIR
  VERSION_VAR Foo_VERSION
)

This will check that the REQUIRED_VARS contain values (that do not end in -NOTFOUND) and set Foo_FOUND appropriately. It will also cache those values. If Foo_VERSION is set, and a required version was passed to find_package(), it will check the requested version against the one in Foo_VERSION. It will also print messages as appropriate; note that if the package was found, it will print the contents of the first required variable to indicate where it was found.

At this point, we have to provide a way for users of the find module to link to the library or libraries that were found. There are two approaches, as discussed in the Find Modules section above. The traditional variable approach looks like

if(Foo_FOUND)
  set(Foo_LIBRARIES ${Foo_LIBRARY})
  set(Foo_INCLUDE_DIRS ${Foo_INCLUDE_DIR})
  set(Foo_DEFINITIONS ${PC_Foo_CFLAGS_OTHER})
endif()

If more than one library was found, all of them should be included in these variables (see the Standard Variable Names section for more information).

When providing imported targets, these should be namespaced (hence the Foo:: prefix); CMake will recognize that values passed to target_link_libraries() that contain :: in their name are supposed to be imported targets (rather than just library names), and will produce appropriate diagnostic messages if that target does not exist (see policy CMP0028).

if(Foo_FOUND AND NOT TARGET Foo::Foo)
  add_library(Foo::Foo UNKNOWN IMPORTED)
  set_target_properties(Foo::Foo PROPERTIES
    IMPORTED_LOCATION "${Foo_LIBRARY}"
    INTERFACE_COMPILE_OPTIONS "${PC_Foo_CFLAGS_OTHER}"
    INTERFACE_INCLUDE_DIRECTORIES "${Foo_INCLUDE_DIR}"
  )
endif()

One thing to note about this is that the INTERFACE_INCLUDE_DIRECTORIES and similar properties should only contain information about the target itself, and not any of its dependencies. Instead, those dependencies should also be targets, and CMake should be told that they are dependencies of this target. CMake will then combine all the necessary information automatically.

The type of the IMPORTED target created in the add_library() command can always be specified as UNKNOWN type. This simplifies the code in cases where static or shared variants may be found, and CMake will determine the type by inspecting the files.

If the library is available with multiple configurations, the IMPORTED_CONFIGURATIONS target property should also be populated:

if(Foo_FOUND)
  if (NOT TARGET Foo::Foo)
    add_library(Foo::Foo UNKNOWN IMPORTED)
  endif()
  if (Foo_LIBRARY_RELEASE)
    set_property(TARGET Foo::Foo APPEND PROPERTY
      IMPORTED_CONFIGURATIONS RELEASE
    )
    set_target_properties(Foo::Foo PROPERTIES
      IMPORTED_LOCATION_RELEASE "${Foo_LIBRARY_RELEASE}"
    )
  endif()
  if (Foo_LIBRARY_DEBUG)
    set_property(TARGET Foo::Foo APPEND PROPERTY
      IMPORTED_CONFIGURATIONS DEBUG
    )
    set_target_properties(Foo::Foo PROPERTIES
      IMPORTED_LOCATION_DEBUG "${Foo_LIBRARY_DEBUG}"
    )
  endif()
  set_target_properties(Foo::Foo PROPERTIES
    INTERFACE_COMPILE_OPTIONS "${PC_Foo_CFLAGS_OTHER}"
    INTERFACE_INCLUDE_DIRECTORIES "${Foo_INCLUDE_DIR}"
  )
endif()

The RELEASE variant should be listed first in the property so that the variant is chosen if the user uses a configuration which is not an exact match for any listed IMPORTED_CONFIGURATIONS.

Most of the cache variables should be hidden in the ccmake interface unless the user explicitly asks to edit them.

mark_as_advanced(
  Foo_INCLUDE_DIR
  Foo_LIBRARY
)

If this module replaces an older version, you should set compatibility variables to cause the least disruption possible.

# compatibility variables
set(Foo_VERSION_STRING ${Foo_VERSION})