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 modifying the CMake
source tree itself.


Permitted C++ Subset
====================

CMake is required to build with ancient C++ compilers and standard library
implementations.  Some common C++ constructs may not be used in CMake in order
to build with such toolchains.

std::set const iterators
------------------------

The ``find()`` member function of a ``const`` ``std::set`` instance may not be
used in a comparison with the iterator returned by ``end()``:

.. code-block:: c++

  const std::set<std::string>& someSet = getSet();
  if (someSet.find("needle") == someSet.end()) // Wrong
    {
    // ...
    }

The return value of ``find()`` must be assigned to an intermediate
``const_iterator`` for comparison:

.. code-block:: c++

  const std::set<std::string>& someSet;
  const std::set<std::string>::const_iterator i = someSet.find("needle");
  if (i != propSet.end()) // Ok
    {
    // ...
    }

std::auto_ptr
-------------

Some implementations have a ``std::auto_ptr`` which can not be used as a
return value from a function. ``std::auto_ptr`` may not be used. Use
``cmsys::auto_ptr`` instead.

std::vector::insert and std::set
--------------------------------

Use of ``std::vector::insert`` with an iterator whose ``element_type`` requires
conversion is not allowed:

.. code-block:: c++

  std::set<const char*> theSet;
  std::vector<std::string> theVector;
  theVector.insert(theVector.end(), theSet.begin(), theSet.end()); // Wrong

A loop must be used instead:

.. code-block:: c++

  std::set<const char*> theSet;
  std::vector<std::string> theVector;
  for(std::set<const char*>::iterator li = theSet.begin();
      li != theSet.end(); ++li)
    {
    theVector.push_back(*li);
    }

std::set::insert
----------------

Use of ``std::set::insert`` is not allowed with any source container:

.. code-block:: c++

  std::set<cmTarget*> theSet;
  theSet.insert(targets.begin(), targets.end()); // Wrong

A loop must be used instead:

.. code-block:: c++

  ConstIterator it = targets.begin();
  const ConstIterator end = targets.end();
  for ( ; it != end; ++it)
    {
    theSet.insert(*it);
    }

.. SunCC 5.9

Template Parameter Defaults
---------------------------

On ancient compilers, C++ template must use template parameters in function
arguments.  If no parameter of that type is needed, the common workaround is
to add a defaulted pointer to the type to the templated function. However,
this does not work with other ancient compilers:

.. code-block:: c++

  template<typename PropertyType>
  PropertyType getTypedProperty(cmTarget* tgt, const char* prop,
                                PropertyType* = 0) // Wrong
    {

    }

.. code-block:: c++

  template<typename PropertyType>
  PropertyType getTypedProperty(cmTarget* tgt, const char* prop,
                                PropertyType*) // Ok
    {

    }

and invoke it with the value ``0`` explicitly in all cases.

size_t
------

Various implementations have differing implementation of ``size_t``.  When
assigning the result of ``.size()`` on a container for example, the result
should be assigned to ``size_t`` not to ``std::size_t``, ``unsigned int`` or
similar types.

Templates
---------

Some template code is permitted, but with some limitations. Member templates
may not be used, and template friends may not be used.

Adding Compile Features
=======================

CMake reports an error if a compiler whose features are known does not report
support for a particular requested feature.  A compiler is considered to have
known features if it reports support for at least one feature.

When adding a new compile feature to CMake, it is therefore necessary to list
support for the feature for all CompilerIds which already have one or more
feature supported, if the new feature is available for any version of the
compiler.

When adding the first supported feature to a particular CompilerId, it is
necessary to list support for all features known to cmake (See
:variable:`CMAKE_C_COMPILE_FEATURES` and
:variable:`CMAKE_CXX_COMPILE_FEATURES` as appropriate), where available for
the compiler.

It is sensible to record the features for the most recent version of a
particular CompilerId first, and then work backwards.  It is sensible to
try to create a continuous range of versions of feature releases of the
compiler.  Gaps in the range indicate incorrect features recorded for
intermediate releases.

Generally, features are made available for a particular version if the
compiler vendor documents availability of the feature with that
version.  Note that sometimes partially implemented features appear to
be functional in previous releases (such as ``cxx_constexpr`` in GNU 4.6,
though availability is documented in GNU 4.7), and sometimes compiler vendors
document availability of features, though supporting infrastructure is
not available (such as ``__has_feature(cxx_generic_lambdas)`` indicating
non-availability in Clang 3.4, though it is documented as available, and
fixed in Clang 3.5).  Similar cases for other compilers and versions
need to be investigated when extending CMake to support them.

When a vendor releases a new version of a known compiler which supports
a previously unsupported feature, and there are already known features for
that compiler, the feature should be listed as supported in CMake for
that version of the compiler as soon as reasonably possible.

Standard-specific/compiler-specific variables such
``CMAKE_CXX98_COMPILE_FEATURES`` are deliberately not documented.  They
only exist for the compiler-specific implementation of adding the ``-std``
compile flag for compilers which need that.

Help
====

The ``Help`` directory contains CMake help manual source files.
They are written using the `reStructuredText`_ markup syntax and
processed by `Sphinx`_ to generate the CMake help manuals.

.. _`reStructuredText`: http://docutils.sourceforge.net/docs/ref/rst/introduction.html
.. _`Sphinx`: http://sphinx-doc.org

Markup Constructs
-----------------

In addition to using Sphinx to generate the CMake help manuals, we
also use a C++-implemented document processor to print documents for
the ``--help-*`` command-line help options.  It supports a subset of
reStructuredText markup.  When authoring or modifying documents,
please verify that the command-line help looks good in addition to the
Sphinx-generated html and man pages.

The command-line help processor supports the following constructs
defined by reStructuredText, Sphinx, and a CMake extension to Sphinx.

..
 Note: This list must be kept consistent with the cmRST implementation.

CMake Domain directives
 Directives defined in the `CMake Domain`_ for defining CMake
 documentation objects are printed in command-line help output as
 if the lines were normal paragraph text with interpretation.

CMake Domain interpreted text roles
 Interpreted text roles defined in the `CMake Domain`_ for
 cross-referencing CMake documentation objects are replaced by their
 link text in command-line help output.  Other roles are printed
 literally and not processed.

``code-block`` directive
 Add a literal code block without interpretation.  The command-line
 help processor prints the block content without the leading directive
 line and with common indentation replaced by one space.

``include`` directive
 Include another document source file.  The command-line help
 processor prints the included document inline with the referencing
 document.

literal block after ``::``
 A paragraph ending in ``::`` followed by a blank line treats
 the following indented block as literal text without interpretation.
 The command-line help processor prints the ``::`` literally and
 prints the block content with common indentation replaced by one
 space.

``note`` directive
 Call out a side note.  The command-line help processor prints the
 block content as if the lines were normal paragraph text with
 interpretation.

``parsed-literal`` directive
 Add a literal block with markup interpretation.  The command-line
 help processor prints the block content without the leading
 directive line and with common indentation replaced by one space.

``productionlist`` directive
 Render context-free grammar productions.  The command-line help
 processor prints the block content as if the lines were normal
 paragraph text with interpretation.

``replace`` directive
 Define a ``|substitution|`` replacement.
 The command-line help processor requires a substitution replacement
 to be defined before it is referenced.

``|substitution|`` reference
 Reference a substitution replacement previously defined by
 the ``replace`` directive.  The command-line help processor
 performs the substitution and replaces all newlines in the
 replacement text with spaces.

``toctree`` directive
 Include other document sources in the Table-of-Contents
 document tree.  The command-line help processor prints
 the referenced documents inline as part of the referencing
 document.

Inline markup constructs not listed above are printed literally in the
command-line help output.  We prefer to use inline markup constructs that
look correct in source form, so avoid use of \\-escapes in favor of inline
literals when possible.

Explicit markup blocks not matching directives listed above are removed from
command-line help output.  Do not use them, except for plain ``..`` comments
that are removed by Sphinx too.

Note that nested indentation of blocks is not recognized by the
command-line help processor.  Therefore:

* Explicit markup blocks are recognized only when not indented
  inside other blocks.

* Literal blocks after paragraphs ending in ``::`` but not
  at the top indentation level may consume all indented lines
  following them.

Try to avoid these cases in practice.

CMake Domain
------------

CMake adds a `Sphinx Domain`_ called ``cmake``, also called the
"CMake Domain".  It defines several "object" types for CMake
documentation:

``command``
 A CMake language command.

``generator``
 A CMake native build system generator.
 See the :manual:`cmake(1)` command-line tool's ``-G`` option.

``manual``
 A CMake manual page, like this :manual:`cmake-developer(7)` manual.

``module``
 A CMake module.
 See the :manual:`cmake-modules(7)` manual
 and the :command:`include` command.

``policy``
 A CMake policy.
 See the :manual:`cmake-policies(7)` manual
 and the :command:`cmake_policy` command.

``prop_cache, prop_dir, prop_gbl, prop_sf, prop_inst, prop_test, prop_tgt``
 A CMake cache, directory, global, source file, installed file, test,
 or target property, respectively.  See the :manual:`cmake-properties(7)`
 manual and the :command:`set_property` command.

``variable``
 A CMake language variable.
 See the :manual:`cmake-variables(7)` manual
 and the :command:`set` command.

Documentation objects in the CMake Domain come from two sources.
First, the CMake extension to Sphinx transforms every document named
with the form ``Help/<type>/<file-name>.rst`` to a domain object with
type ``<type>``.  The object name is extracted from the document title,
which is expected to be of the form::

 <object-name>
 -------------

and to appear at or near the top of the ``.rst`` file before any other
lines starting in a letter, digit, or ``<``.  If no such title appears
literally in the ``.rst`` file, the object name is the ``<file-name>``.
If a title does appear, it is expected that ``<file-name>`` is equal
to ``<object-name>`` with any ``<`` and ``>`` characters removed.

Second, the CMake Domain provides directives to define objects inside
other documents:

.. code-block:: rst

 .. command:: <command-name>

  This indented block documents <command-name>.

 .. variable:: <variable-name>

  This indented block documents <variable-name>.

Object types for which no directive is available must be defined using
the first approach above.

.. _`Sphinx Domain`: http://sphinx-doc.org/domains.html

Cross-References
----------------

Sphinx uses reStructuredText interpreted text roles to provide
cross-reference syntax.  The `CMake Domain`_ provides for each
domain object type a role of the same name to cross-reference it.
CMake Domain roles are inline markup of the forms::

 :type:`name`
 :type:`text <name>`

where ``type`` is the domain object type and ``name`` is the
domain object name.  In the first form the link text will be
``name`` (or ``name()`` if the type is ``command``) and in
the second form the link text will be the explicit ``text``.
For example, the code:

.. code-block:: rst

 * The :command:`list` command.
 * The :command:`list(APPEND)` sub-command.
 * The :command:`list() command <list>`.
 * The :command:`list(APPEND) sub-command <list>`.
 * The :variable:`CMAKE_VERSION` variable.
 * The :prop_tgt:`OUTPUT_NAME_<CONFIG>` target property.

produces:

* The :command:`list` command.
* The :command:`list(APPEND)` sub-command.
* The :command:`list() command <list>`.
* The :command:`list(APPEND) sub-command <list>`.
* The :variable:`CMAKE_VERSION` variable.
* The :prop_tgt:`OUTPUT_NAME_<CONFIG>` target property.

Note that CMake Domain roles differ from Sphinx and reStructuredText
convention in that the form ``a<b>``, without a space preceding ``<``,
is interpreted as a name instead of link text with an explicit target.
This is necessary because we use ``<placeholders>`` frequently in
object names like ``OUTPUT_NAME_<CONFIG>``.  The form ``a <b>``,
with a space preceding ``<``, is still interpreted as a link text
with an explicit target.

Style
-----

Style: Section Headers
^^^^^^^^^^^^^^^^^^^^^^

When marking section titles, make the section decoration line as long as
the title text.  Use only a line below the title, not above. For
example:

.. code-block:: rst

  Title Text
  ----------

Capitalize the first letter of each non-minor word in the title.

The section header underline character hierarchy is

* ``#``: Manual group (part) in the master document
* ``*``: Manual (chapter) title
* ``=``: Section within a manual
* ``-``: Subsection or `CMake Domain`_ object document title
* ``^``: Subsubsection or `CMake Domain`_ object document section
* ``"``: Paragraph or `CMake Domain`_ object document subsection

Style: Whitespace
^^^^^^^^^^^^^^^^^

Use two spaces for indentation.  Use two spaces between sentences in
prose.

Style: Line Length
^^^^^^^^^^^^^^^^^^

Prefer to restrict the width of lines to 75-80 columns.  This is not a
hard restriction, but writing new paragraphs wrapped at 75 columns
allows space for adding minor content without significant re-wrapping of
content.

Style: Prose
^^^^^^^^^^^^

Use American English spellings in prose.

Style: Starting Literal Blocks
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Prefer to mark the start of literal blocks with ``::`` at the end of
the preceding paragraph. In cases where the following block gets
a ``code-block`` marker, put a single ``:`` at the end of the preceding
paragraph.

Style: CMake Command Signatures
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Command signatures should be marked up as plain literal blocks, not as
cmake ``code-blocks``.

Signatures are separated from preceding content by a section header.
That is, use:

.. code-block:: rst

  ... preceding paragraph.

  Normal Libraries
  ^^^^^^^^^^^^^^^^

  ::

    add_library(<lib> ...)

  This signature is used for ...

Signatures of commands should wrap optional parts with square brackets,
and should mark list of optional arguments with an ellipsis (``...``).
Elements of the signature which are specified by the user should be
specified with angle brackets, and may be referred to in prose using
``inline-literal`` syntax.

Style: Boolean Constants
^^^^^^^^^^^^^^^^^^^^^^^^

Use "``OFF``" and "``ON``" for boolean values which can be modified by
the user, such as :prop_tgt:`POSITION_INDEPENDENT_CODE`. Such properties
may be "enabled" and "disabled". Use "``True``" and "``False``" for
inherent values which can't be modified after being set, such as the
:prop_tgt:`IMPORTED` property of a build target.

Style: Inline Literals
^^^^^^^^^^^^^^^^^^^^^^

Mark up references to keywords in signatures, file names, and other
technical terms with ``inline-literal`` syntax, for example:

.. code-block:: rst

  If ``WIN32`` is used with :command:`add_executable`, the
  :prop_tgt:`WIN32_EXECUTABLE` target property is enabled. That command
  creates the file ``<name>.exe`` on Windows.

Style: Cross-References
^^^^^^^^^^^^^^^^^^^^^^^

Mark up linkable references as links, including repeats.
An alternative, which is used by wikipedia
(`<http://en.wikipedia.org/wiki/WP:REPEATLINK>`_),
is to link to a reference only once per article. That style is not used
in CMake documentation.

Style: Referencing CMake Concepts
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If referring to a concept which corresponds to a property, and that
concept is described in a high-level manual, prefer to link to the
manual section instead of the property. For example:

.. code-block:: rst

  This command creates an :ref:`Imported Target <Imported Targets>`.

instead of:

.. code-block:: rst

  This command creates an :prop_tgt:`IMPORTED` target.

The latter should be used only when referring specifically to the
property.

References to manual sections are not automatically created by creating
a section, but code such as:

.. code-block:: rst

  .. _`Imported Targets`:

creates a suitable anchor.  Use an anchor name which matches the name
of the corresponding section.  Refer to the anchor using a
cross-reference with specified text.

Imported Targets need the ``IMPORTED`` term marked up with care in
particular because the term may refer to a command keyword
(``IMPORTED``), a target property (:prop_tgt:`IMPORTED`), or a
concept (:ref:`Imported Targets`).

Where a property, command or variable is related conceptually to others,
by for example, being related to the buildsystem description, generator
expressions or Qt, each relevant property, command or variable should
link to the primary manual, which provides high-level information.  Only
particular information relating to the command should be in the
documentation of the command.

Style: Referencing CMake Domain Objects
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When referring to `CMake Domain`_ objects such as properties, variables,
commands etc, prefer to link to the target object and follow that with
the type of object it is.  For example:

.. code-block:: rst

  Set the :prop_tgt:`AUTOMOC` target property to ``ON``.

Instead of

.. code-block:: rst

  Set the target property :prop_tgt:`AUTOMOC` to ``ON``.

The ``policy`` directive is an exception, and the type us usually
referred to before the link:

.. code-block:: rst

  If policy :prop_tgt:`CMP0022` is set to ``NEW`` the behavior is ...

However, markup self-references with ``inline-literal`` syntax.
For example, within the :command:`add_executable` command
documentation, use

.. code-block:: rst

  ``add_executable``

not

.. code-block:: rst

  :command:`add_executable`

which is used elsewhere.

Modules
=======

The ``Modules`` directory contains CMake-language ``.cmake`` module files.

Module Documentation
--------------------

To document CMake module ``Modules/<module-name>.cmake``, modify
``Help/manual/cmake-modules.7.rst`` to reference the module in the
``toctree`` directive, in sorted order, as::

 /module/<module-name>

Then add the module document file ``Help/module/<module-name>.rst``
containing just the line::

 .. cmake-module:: ../../Modules/<module-name>.cmake

The ``cmake-module`` directive will scan the module file to extract
reStructuredText markup from comment blocks that start in ``.rst:``.
Add to the top of ``Modules/<module-name>.cmake`` a
:ref:`Line Comment` block of the form:

.. code-block:: cmake

 #.rst:
 # <module-name>
 # -------------
 #
 # <reStructuredText documentation of module>

or a :ref:`Bracket Comment` of the form:

.. code-block:: cmake

 #[[.rst:
 <module-name>
 -------------

 <reStructuredText documentation of module>
 #]]

Any number of ``=`` may be used in the opening and closing brackets
as long as they match.  Content on the line containing the closing
bracket is excluded if and only if the line starts in ``#``.

Additional such ``.rst:`` comments may appear anywhere in the module file.
All such comments must start with ``#`` in the first column.

For example, a ``Modules/Findxxx.cmake`` module may contain:

.. code-block:: cmake

 #.rst:
 # FindXxx
 # -------
 #
 # This is a cool module.
 # This module does really cool stuff.
 # It can do even more than you think.
 #
 # It even needs two paragraphs to tell you about it.
 # And it defines the following variables:
 #
 # * VAR_COOL: this is great isn't it?
 # * VAR_REALLY_COOL: cool right?

 <code>

 #[========================================[.rst:
 .. command:: xxx_do_something

  This command does something for Xxx::

   xxx_do_something(some arguments)
 #]========================================]
 macro(xxx_do_something)
   <code>
 endmacro()

After the top documentation block, leave a *BLANK* line, and then add a
copyright and licence notice block like this one (change only the year
range and name)

.. code-block:: cmake

  #=============================================================================
  # Copyright 2009-2011 Your Name
  #
  # Distributed under the OSI-approved BSD License (the "License");
  # see accompanying file Copyright.txt for details.
  #
  # This software is distributed WITHOUT ANY WARRANTY; without even the
  # implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  # See the License for more information.
  #=============================================================================
  # (To distribute this file outside of CMake, substitute the full
  #  License text for the above reference.)

Test the documentation formatting by running
``cmake --help-module <module-name>``, and also by enabling the
``SPHINX_HTML`` and ``SPHINX_MAN`` options to build the documentation.
Edit the comments until generated documentation looks satisfactory.  To
have a .cmake file in this directory NOT show up in the modules
documentation, simply leave out the ``Help/module/<module-name>.rst``
file and the ``Help/manual/cmake-modules.7.rst`` toctree entry.


Find Modules
------------

A "find module" is a ``Modules/Find<package>.cmake`` file to be loaded
by the :command:`find_package` command when invoked for ``<package>``.

The primary task of a find module is to determine whether a package
exists on the system, set the ``<package>_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.

Like all modules, find modules should be properly documented.  To add a
module to the CMake documentation, follow the steps in the `Module
Documentation`_ section above.



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 sytem 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
  :command:`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 :command:`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 first thing that is needed is documentation.  CMake's documentation
system requires you to start the file with a documentation marker and
the name of the module.  You should follow this with a simple statement
of what the module does.

.. code-block:: cmake

  #.rst:
  # FindFoo
  # -------
  #
  # Finds the Foo library
  #

More description may be required for some packages.  If there are
caveats or other details users of the module should be aware of, you can
add further paragraphs below this.  Then you need to document what
variables and imported targets are set by the module, such as

.. code-block:: cmake

  # 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
  #
  # and the following imported targets::
  #
  #   Foo::Foo   - The Foo library

If the package provides any macros, they should be listed here, but can
be documented where they are defined.  See the `Module
Documentation`_ section above for more details.

After the documentation, leave a blank line, and then add a copyright and
licence notice block

.. code-block:: cmake

  #=============================================================================
  # Copyright 2009-2011 Your Name
  #
  # Distributed under the OSI-approved BSD License (the "License");
  # see accompanying file Copyright.txt for details.
  #
  # This software is distributed WITHOUT ANY WARRANTY; without even the
  # implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  # See the License for more information.
  #=============================================================================
  # (To distribute this file outside of CMake, substitute the full
  #  License text for the above reference.)

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}
    # if you need to put #include <Foo/foo.h> in your code, add:
    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 that 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})