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CMake Documentation Guide

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

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.

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.

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.
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. 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, or in the case of a $<genex-name> or $<genex-name:...>, the genex-name.

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

.. command:: <command-name>

 This indented block documents <command-name>.

.. envvar:: <envvar-name>

 This indented block documents <envvar-name>.

.. genex:: <genex-name>

 This indented block documents <genex-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.

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:

* 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:

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:

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:

... 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 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:

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:

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

instead of:

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:

.. _`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:

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

Instead of

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:

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

``add_executable``

not

: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:

# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
# file Copyright.txt or https://cmake.org/licensing for details.

After this notice, add a BLANK line. Then, add documentation using a Bracket Comment of the form:

#[=======================================================================[.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:

# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
# file Copyright.txt 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.