CMake/Help/manual/cmake-developer.7.rst

.. cmake-manual-description: CMake Developer Reference

cmake-developer(7)
******************

.. only:: html

   .. contents::

Introduction
============

This manual is intended for reference by developers working with
:manual:`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`:

Find Modules
============

A "find module" is a ``Find<PackageName>.cmake`` file to be loaded by the
:command:`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 :ref:`config file package <Config File Packages>`.

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
:ref:`config file packages <Config File Packages>` files as possible, by
providing :ref:`imported target <Imported targets>`.  This has the advantage
of propagating :ref:`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 :command:`find_package` documentation for details on what
variables are set for the find module.  Most of these are dealt with by
using :module:`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.


.. _`CMake Developer Standard Variable Names`:

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
  :command:`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 :command:`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
  :command:`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 :command:`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 :command:`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
  :command:`find_package` command and by
  :command:`find_package_handle_standard_args` to inform the user about the
  problem.  Use this instead of calling :command:`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 :command:`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 :command:`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
  :command:`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 :command:`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 :module:`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 :ref:`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.

.. code-block:: cmake

  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.

.. code-block:: cmake

  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``

.. code-block:: cmake

  set(Foo_VERSION ${PC_Foo_VERSION})

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

.. code-block:: cmake

  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 :command:`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

.. code-block:: cmake

  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
:command:`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 :policy:`CMP0028`).

.. code-block:: cmake

  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 :prop_tgt:`IMPORTED` target created in the
:command:`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
:prop_tgt:`IMPORTED_CONFIGURATIONS` target property should also be
populated:

.. code-block:: cmake

  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.

.. code-block:: cmake

  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.

.. code-block:: cmake

  # compatibility variables
  set(Foo_VERSION_STRING ${Foo_VERSION})