Provided by: cmake-data_3.16.3-1ubuntu1.20.04.1_all 
NAME
cmake-developer - CMake Developer Reference
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 Target 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})
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