CMake Documentation Guide

The following is a guide to the CMake documentation source for developers. See documentation on CMake Development_ for more information.

.. _CMake Development: README.rst

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: https://docutils.sourceforge.net/docs/ref/rst/introduction.html .. _Sphinx: https://sphinx-doc.org

To generate the HTML and/or man documentation locally from within the CMake repository to build/html and build/man directories:

.. code-block:: console

$ cmake -S Utilities/Sphinx -B build -DSPHINX_HTML=ON -DSPHINX_MAN=ON $ cmake --build build

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.

versionadded, versionchanged directives Specify that something was added or changed by a named CMake version. The command-line help processor prints the block content as if the lines were normal paragraph text with interpretation.

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.

cpack_gen A CPack package generator. See the cpack(1)_ command-line tool's -G option.

envvar An environment variable. See the cmake-env-variables(7)_ manual and the set()_ command.

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

genex A CMake generator expression. See the cmake-generator-expressions(7)_ manual.

manual A CMake manual page, like the cmake(1)_ manual.

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

policy A CMake policy. See the cmake-policies(7)_ manual and the 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 cmake-properties(7)_ manual and the set_property()_ command.

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

Documentation objects in the CMake Domain come from two sources:

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, or in the case of a $<genex-name> or $<genex-name:...>, the genex-name.
CMake Domain directives_ may be used in documents to explicitly define some object types:
- command directive_
- envvar directive_
- genex directive_
- variable directive_
Object types for which no directive is available must be defined using the document transform above.

CMake Domain Directives

The CMake Domain provides the following directives.

command directive ^^^^^^^^^^^^^^^^^^^^^

Document a "command" object:

.. code-block:: rst

.. command:: <command-name>

This indented block documents <command-name>.

The directive requires a single argument, the command name.

envvar directive ^^^^^^^^^^^^^^^^^^^^

Document an "envvar" object:

.. code-block:: rst

.. envvar:: <envvar-name>

This indented block documents <envvar-name>.

The directive requires a single argument, the environment variable name.

genex directive ^^^^^^^^^^^^^^^^^^^

Document a "genex" object:

.. code-block:: rst

.. genex:: <genex-name>

This indented block documents <genex-name>.

The directive requires a single argument, the generator expression name.

The optional :target: option allows a custom target name to be specified. Because this will affect the ability to reference the "genex" object using the :genex: role, this option should be used very sparingly.

signature directive ^^^^^^^^^^^^^^^^^^^^^^^

Document CMake Command Signatures <Style: CMake Command Signatures_>_ within a Help/command/<command-name>.rst document.

.. code-block:: rst

.. signature:: <command-name>(<signature>)

This indented block documents one or more signatures of a CMake command.

The signature directive requires one argument, the signature summary:

One or more signatures must immediately follow the ::. The first signature may optionally be placed on the same line. A blank line following the signature directive will result in a documentation generation error: 1 argument(s) required, 0 supplied.
Signatures may be split across multiple lines, but the final ) of each signature must be the last character on its line.
Blank lines between signatures are not allowed. (Content after a blank line is treated as part of the description.)
Whitespace in signatures is not preserved. To document a complex signature, abbreviate it in the signature directive argument and specify the full signature in a code-block in the description.

The signature directive generates a hyperlink target for each signature:

Default target names are automatically extracted from leading "keyword" arguments in the signatures, where a keyword is any sequence of non-space starting with a letter. For example, the signature string(REGEX REPLACE <match-regex> ...) generates the target REGEX REPLACE, similar to .. _`REGEX REPLACE`:.
Custom target names may be specified using a :target: option. For example:

.. code-block:: rst

.. signature:: cmake_path(GET <path-var> ROOT_NAME <out-var>) cmake_path(GET <path-var> ROOT_PATH <out-var>) :target: GET ROOT_NAME GET ROOT_PATH

Provide a custom target name for each signature, one per line. The first target may optionally be placed on the same line as :target:.
If a target name is already in use earlier in the document, no hyperlink target will be generated.
The targets may be referenced from within the same document using REF`_`` or TEXT <REF_>_`` syntax. Like reStructuredText section headers, the targets do not work with Sphinx ``:ref:`` syntax, however they can be globally referenced using e.g. ``:command:string(APPEND)```.

Although whitespace in the signature is not preserved, by default, line breaks are suppressed inside of square- or angle-brackets. This behavior can be controlled using the :break: option; note, however, that there is no way to force a line break. The default value is 'smart'. Allowable values are:

all Allow line breaks at any whitespace.

smart (default) Allow line breaks at whitespace, except between matched square- or angle-brackets. For example, if a signature contains the text <input>... [OUTPUT_VARIABLE <out-var>], a line break would be allowed after <input>... but not between OUTPUT_VARIABLE and <out-var>.

verbatim Allow line breaks only where the source document contains a newline.

The directive treats its content as the documentation of the signature(s). Indent the signature documentation accordingly.

variable directive ^^^^^^^^^^^^^^^^^^^^^^

Document a "variable" object:

.. code-block:: rst

.. variable:: <variable-name>

This indented block documents <variable-name>.

The directive requires a single argument, the variable name.

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 list()_ command.
The list(APPEND)_ sub-command.
The list() command_.
The list(APPEND) sub-command_.
The CMAKE_VERSION_ variable.
The 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.

Additionally, the cref role may be used to create references to local targets that have literal styling. This is especially useful for referencing a subcommand in the command's documentation.

.. _list(): https://cmake.org/cmake/help/latest/command/list.html .. _list(APPEND): https://cmake.org/cmake/help/latest/command/list.html .. _list(APPEND) sub-command: https://cmake.org/cmake/help/latest/command/list.html .. _list() command: https://cmake.org/cmake/help/latest/command/list.html .. _CMAKE_VERSION: https://cmake.org/cmake/help/latest/variable/CMAKE_VERSION.html .. _OUTPUT_NAME_<CONFIG>: https://cmake.org/cmake/help/latest/prop_tgt/OUTPUT_NAME_CONFIG.html

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
~: CMake Domain_ object document subsubsection

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 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A Help/command/<command-name>.rst document defines one command object in the CMake Domain, but some commands have multiple signatures. Use the CMake Domain's signature directive to document each signature. Separate signatures from preceding content by a section header. For example:

.. code-block:: rst

... preceding paragraph.

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

.. signature:: add_library(<lib> ...)

This signature is used for ...

Use the following conventions in command signature documentation:

Use an angle-bracket <placeholder> for arguments to be specified by the caller. Refer to them in prose using inline literal <Style: Inline Literals_>_ syntax.
Wrap optional parts with square brackets.
Mark repeatable parts with a trailing ellipsis (...).

The signature directive may be used multiple times for different signatures of the same command.

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

Use "OFF" and "ON" for boolean values which can be modified by the user, such as 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 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 (<https://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, a target property, or a concept.

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 :policy:CMP0022 is set to NEW the behavior is ...

However, markup self-references with inline-literal syntax. For example, within the 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:. At the top of Modules/<module-name>.cmake, begin with the following license notice:

.. code-block:: cmake

Distributed under the OSI-approved BSD 3-Clause License. See accompanying

file LICENSE.rst or https://cmake.org/licensing for details.

After this notice, add a BLANK line. Then, add documentation using a 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 FindXxx.cmake module may contain:

.. code-block:: cmake

Distributed under the OSI-approved BSD 3-Clause License. See accompanying

file LICENSE.rst or https://cmake.org/licensing for details.

#[=======================================================================[.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()

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.

.. _Bracket Comment: https://cmake.org/cmake/help/latest/manual/cmake-language.7.html#bracket-comment

Module Functions and Macros

Modules may provide CMake functions and macros defined by the function()_ and macro()_ commands. To avoid conflicts across modules, name the functions and macros using the prefix <ModuleName>_ followed by the rest of the name, where <ModuleName> is the exact-case spelling of the module name. We have no convention for the portion of names after the <ModuleName>_ prefix.

For historical reasons, some modules that come with CMake do not follow this prefix convention. When adding new functions to these modules, discussion during review can decide whether to follow their existing convention or to use the module name prefix.

Documentation of public functions and macros should be provided in the module, typically in the main module documentation_ at the top. For example, a MyModule module may document a function like this::

#[=======================================================================[.rst: MyModule

This is my module. It provides some functions.

.. command:: MyModule_Some_Function

This is some function:

.. code-block:: cmake

  MyModule_Some_Function(...)

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

Documentation may alternatively be placed just before each definition. For example, a MyModule module may document another function like this::

#[=======================================================================[.rst: .. command:: MyModule_Other_Function

This is another function:

.. code-block:: cmake

  MyModule_Other_Function(...)

#]=======================================================================] function(MyModule_Other_Function ...) # ... endfunction()

.. _function(): https://cmake.org/cmake/help/latest/command/function.html .. _macro(): https://cmake.org/cmake/help/latest/command/macro.html