Provided by: cmake-data_3.22.1-1ubuntu1.22.04.2_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 is available, 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_,
which (unless otherwise noted) must match exactly the name of the FindXxx.cmake file,
including upper/lowercase. This prefix on the variable names ensures that they do not
conflict with variables of other Find modules. The same pattern should also be followed
for any macros, functions and imported targets defined by the Find module.
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 (note that this also means this variable should
not be used as the result variable of a find_path() command - see Xxx_INCLUDE_DIR
below for that).
Xxx_LIBRARIES
The libraries to use with the module. These may be CMake targets, full absolute
paths to a library binary or the name of a library that the linker must find in its
search path. This should not be a cache entry (note that this also means this
variable should not be used as the result variable of a find_library() command -
see Xxx_LIBRARY below for that).
Xxx_DEFINITIONS
The compile definitions to use when compiling code that uses the module. 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
The full absolute path to an executable. In this case, Xxx might not be the name
of the module, it might be the name of the tool (usually converted to all
uppercase), assuming that tool has such a well-known name that it is unlikely that
another tool with the same name exists. It would be appropriate to use this as the
result variable of a find_program() command.
Xxx_YYY_EXECUTABLE
Similar to Xxx_EXECUTABLE except here the Xxx is always the module name and YYY is
the tool name (again, usually fully uppercase). Prefer this form if the tool name
is not very widely known or has the potential to clash with another tool. For
greater consistency, also prefer this form if the module provides more than one
executable.
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 the module.
Xxx_VERSION_VV
Variables of this form specify whether the Xxx module being provided is version VV
of the module. There should not be more than one variable of this form set to true
for a given module. For example, a module Barry might have evolved over many years
and gone through a number of different major versions. Version 3 of the Barry
module might set the variable Barry_VERSION_3 to true, whereas an older version of
the module might set Barry_VERSION_2 to true instead. It would be an error for
both Barry_VERSION_3 and Barry_VERSION_2 to both be set to true.
Xxx_WRAP_YY
When a variable of this form is set to false, it indicates that the relevant
wrapping command should not be used. The wrapping command depends on the module,
it may be implied by the module name or it might be specified by the YY part of the
variable.
Xxx_Yy_FOUND
For variables of this form, Yy is the name of a component for the module. It
should match exactly one of the valid component names that may be passed to the
find_package() command for the module. If a variable of this form is set to false,
it means that the Yy component of module Xxx was not found or is not available.
Variables of this form would typically be used for optional components so that the
caller can check whether an optional component is available.
Xxx_FOUND
When the find_package() command returns to the caller, this variable will be set to
true if the module was deemed to have been found successfully.
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. Use this
instead of calling message() directly to report a reason for failing to find the
module or package.
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. They are intended
for use by Find modules to specify and cache the locations of specific files or
directories. Users are typically able to set and edit these variables to control the
behavior of Find modules (like entering the path to a library manually):
Xxx_LIBRARY
The path of the library. Use this form only when the module provides a single
library. It is appropriate to use this as the result variable in a find_library()
command.
Xxx_Yy_LIBRARY
The path of library Yy provided by the module Xxx. Use this form when the module
provides more than one library or where other modules may also provide a library of
the same name. It is also appropriate to use this form as the result variable in a
find_library() command.
Xxx_INCLUDE_DIR
When the module provides only a single library, this variable can be used to
specify where to find headers for using the library (or more accurately, the path
that consumers of the library should add to their header search path). It would be
appropriate to use this as the result variable in a find_path() command.
Xxx_Yy_INCLUDE_DIR
If the module provides more than one library or where other modules may also
provide a library of the same name, this form is recommended for specifying where
to find headers for using library Yy provided by the module. Again, it would be
appropriate to use this as the result variable in a find_path() command.
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. The SelectLibraryConfigurations module can be
helpful for such cases.
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}
)
Alternatively, if the library is available with multiple configurations, you can use
SelectLibraryConfigurations to automatically set the Foo_LIBRARY variable instead:
find_library(Foo_LIBRARY_RELEASE
NAMES foo
PATHS ${PC_Foo_LIBRARY_DIRS}/Release
)
find_library(Foo_LIBRARY_DEBUG
NAMES foo
PATHS ${PC_Foo_LIBRARY_DIRS}/Debug
)
include(SelectLibraryConfigurations)
select_library_configurations(Foo)
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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