1 ===========================
2 Writing kernel-doc comments
3 ===========================
5 The Linux kernel source files may contain structured documentation
6 comments in the kernel-doc format to describe the functions, types
7 and design of the code. It is easier to keep documentation up-to-date
8 when it is embedded in source files.
10 .. note:: The kernel-doc format is deceptively similar to javadoc,
11 gtk-doc or Doxygen, yet distinctively different, for historical
12 reasons. The kernel source contains tens of thousands of kernel-doc
13 comments. Please stick to the style described here.
15 The kernel-doc structure is extracted from the comments, and proper
16 `Sphinx C Domain`_ function and type descriptions with anchors are
17 generated from them. The descriptions are filtered for special kernel-doc
18 highlights and cross-references. See below for details.
20 .. _Sphinx C Domain: http://www.sphinx-doc.org/en/stable/domains.html
22 Every function that is exported to loadable modules using
23 ``EXPORT_SYMBOL`` or ``EXPORT_SYMBOL_GPL`` should have a kernel-doc
24 comment. Functions and data structures in header files which are intended
25 to be used by modules should also have kernel-doc comments.
27 It is good practice to also provide kernel-doc formatted documentation
28 for functions externally visible to other kernel files (not marked
29 ``static``). We also recommend providing kernel-doc formatted
30 documentation for private (file ``static``) routines, for consistency of
31 kernel source code layout. This is lower priority and at the discretion
32 of the maintainer of that kernel source file.
34 How to format kernel-doc comments
35 ---------------------------------
37 The opening comment mark ``/**`` is used for kernel-doc comments. The
38 ``kernel-doc`` tool will extract comments marked this way. The rest of
39 the comment is formatted like a normal multi-line comment with a column
40 of asterisks on the left side, closing with ``*/`` on a line by itself.
42 The function and type kernel-doc comments should be placed just before
43 the function or type being described in order to maximise the chance
44 that somebody changing the code will also change the documentation. The
45 overview kernel-doc comments may be placed anywhere at the top indentation
48 Running the ``kernel-doc`` tool with increased verbosity and without actual
49 output generation may be used to verify proper formatting of the
50 documentation comments. For example::
52 scripts/kernel-doc -v -none drivers/foo/bar.c
54 The documentation format is verified by the kernel build when it is
55 requested to perform extra gcc checks::
59 Function documentation
60 ----------------------
62 The general format of a function and function-like macro kernel-doc comment is::
65 * function_name() - Brief description of function.
66 * @arg1: Describe the first argument.
67 * @arg2: Describe the second argument.
68 * One can provide multiple line descriptions
71 * A longer description, with more discussion of the function function_name()
72 * that might be useful to those using or modifying it. Begins with an
73 * empty comment line, and may include additional embedded empty
76 * The longer description may have multiple paragraphs.
78 * Context: Describes whether the function can sleep, what locks it takes,
79 * releases, or expects to be held. It can extend over multiple
81 * Return: Describe the return value of function_name.
83 * The return value description can also have multiple paragraphs, and should
84 * be placed at the end of the comment block.
87 The brief description following the function name may span multiple lines, and
88 ends with an argument description, a blank comment line, or the end of the
94 Each function argument should be described in order, immediately following
95 the short function description. Do not leave a blank line between the
96 function description and the arguments, nor between the arguments.
98 Each ``@argument:`` description may span multiple lines.
102 If the ``@argument`` description has multiple lines, the continuation
103 of the description should start at the same column as the previous line::
105 * @argument: some long description
106 * that continues on next lines
111 * some long description
112 * that continues on next lines
114 If a function has a variable number of arguments, its description should
115 be written in kernel-doc notation as::
122 The context in which a function can be called should be described in a
123 section named ``Context``. This should include whether the function
124 sleeps or can be called from interrupt context, as well as what locks
125 it takes, releases and expects to be held by its caller.
129 * Context: Any context.
130 * Context: Any context. Takes and releases the RCU lock.
131 * Context: Any context. Expects <lock> to be held by caller.
132 * Context: Process context. May sleep if @gfp flags permit.
133 * Context: Process context. Takes and releases <mutex>.
134 * Context: Softirq or process context. Takes and releases <lock>, BH-safe.
135 * Context: Interrupt context.
140 The return value, if any, should be described in a dedicated section
145 #) The multi-line descriptive text you provide does *not* recognize
146 line breaks, so if you try to format some text nicely, as in::
150 * -EINVAL - invalid argument
151 * -ENOMEM - out of memory
153 this will all run together and produce::
155 Return: 0 - OK -EINVAL - invalid argument -ENOMEM - out of memory
157 So, in order to produce the desired line breaks, you need to use a
161 * * 0 - OK to runtime suspend the device
162 * * -EBUSY - Device should not be runtime suspended
164 #) If the descriptive text you provide has lines that begin with
165 some phrase followed by a colon, each of those phrases will be taken
166 as a new section heading, which probably won't produce the desired
169 Structure, union, and enumeration documentation
170 -----------------------------------------------
172 The general format of a struct, union, and enum kernel-doc comment is::
175 * struct struct_name - Brief description.
176 * @member1: Description of member1.
177 * @member2: Description of member2.
178 * One can provide multiple line descriptions
181 * Description of the structure.
184 You can replace the ``struct`` in the above example with ``union`` or
185 ``enum`` to describe unions or enums. ``member`` is used to mean struct
186 and union member names as well as enumerations in an enum.
188 The brief description following the structure name may span multiple
189 lines, and ends with a member description, a blank comment line, or the
190 end of the comment block.
195 Members of structs, unions and enums should be documented the same way
196 as function parameters; they immediately succeed the short description
197 and may be multi-line.
199 Inside a struct or union description, you can use the ``private:`` and
200 ``public:`` comment tags. Structure fields that are inside a ``private:``
201 area are not listed in the generated output documentation.
203 The ``private:`` and ``public:`` tags must begin immediately following a
204 ``/*`` comment marker. They may optionally include comments between the
205 ``:`` and the ending ``*/`` marker.
210 * struct my_struct - short description
220 /* private: internal use only */
222 /* public: the next one is public */
226 Nested structs/unions
227 ~~~~~~~~~~~~~~~~~~~~~
229 It is possible to document nested structs and unions, like::
232 * struct nested_foobar - a struct with nested unions and structs
233 * @memb1: first member of anonymous union/anonymous struct
234 * @memb2: second member of anonymous union/anonymous struct
235 * @memb3: third member of anonymous union/anonymous struct
236 * @memb4: fourth member of anonymous union/anonymous struct
237 * @bar: non-anonymous union
238 * @bar.st1: struct st1 inside @bar
239 * @bar.st2: struct st2 inside @bar
240 * @bar.st1.memb1: first member of struct st1 on union bar
241 * @bar.st1.memb2: second member of struct st1 on union bar
242 * @bar.st2.memb1: first member of struct st2 on union bar
243 * @bar.st2.memb2: second member of struct st2 on union bar
245 struct nested_foobar {
246 /* Anonymous union/struct*/
271 #) When documenting nested structs or unions, if the struct/union ``foo``
272 is named, the member ``bar`` inside it should be documented as
274 #) When the nested struct/union is anonymous, the member ``bar`` in it
275 should be documented as ``@bar:``
277 In-line member documentation comments
278 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
280 The structure members may also be documented in-line within the definition.
281 There are two styles, single-line comments where both the opening ``/**`` and
282 closing ``*/`` are on the same line, and multi-line comments where they are each
283 on a line of their own, like all other kernel-doc comments::
286 * struct foo - Brief description.
287 * @foo: The Foo member.
292 * @bar: The Bar member.
296 * @baz: The Baz member.
298 * Here, the member description may contain several paragraphs.
302 /** @foobar: Single line description. */
305 /** @bar2: Description for struct @bar2 inside @foo */
308 * @bar2.barbar: Description for @barbar inside @foo.bar2
314 Typedef documentation
315 ---------------------
317 The general format of a typedef kernel-doc comment is::
320 * typedef type_name - Brief description.
322 * Description of the type.
325 Typedefs with function prototypes can also be documented::
328 * typedef type_name - Brief description.
329 * @arg1: description of arg1
330 * @arg2: description of arg2
332 * Description of the type.
334 * Context: Locking context.
335 * Return: Meaning of the return value.
337 typedef void (*type_name)(struct v4l2_ctrl *arg1, void *arg2);
339 Highlights and cross-references
340 -------------------------------
342 The following special patterns are recognized in the kernel-doc comment
343 descriptive text and converted to proper reStructuredText markup and `Sphinx C
346 .. attention:: The below are **only** recognized within kernel-doc comments,
347 **not** within normal reStructuredText documents.
353 Name of a function parameter. (No cross-referencing, just formatting.)
356 Name of a constant. (No cross-referencing, just formatting.)
359 A literal block that should be handled as-is. The output will use a
362 Useful if you need to use special characters that would otherwise have some
363 meaning either by kernel-doc script or by reStructuredText.
365 This is particularly useful if you need to use things like ``%ph`` inside
366 a function description.
369 Name of an environment variable. (No cross-referencing, just formatting.)
380 ``&struct_name->member`` or ``&struct_name.member``
381 Structure or union member reference. The cross-reference will be to the struct
382 or union definition, not the member directly.
385 A generic type reference. Prefer using the full reference described above
386 instead. This is mostly for legacy comments.
388 Cross-referencing from reStructuredText
389 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
391 No additional syntax is needed to cross-reference the functions and types
392 defined in the kernel-doc comments from reStructuredText documents.
393 Just end function names with ``()`` and write ``struct``, ``union``, ``enum``
394 or ``typedef`` before types.
403 However, if you want custom text in the cross-reference link, that can be done
404 through the following syntax::
406 See :c:func:`my custom link text for function foo <foo>`.
407 See :c:type:`my custom link text for struct bar <bar>`.
409 For further details, please refer to the `Sphinx C Domain`_ documentation.
411 Overview documentation comments
412 -------------------------------
414 To facilitate having source code and comments close together, you can include
415 kernel-doc documentation blocks that are free-form comments instead of being
416 kernel-doc for functions, structures, unions, enums, or typedefs. This could be
417 used for something like a theory of operation for a driver or library code, for
420 This is done by using a ``DOC:`` section keyword with a section title.
422 The general format of an overview or high-level documentation comment is::
425 * DOC: Theory of Operation
427 * The whizbang foobar is a dilly of a gizmo. It can do whatever you
428 * want it to do, at any time. It reads your mind. Here's how it works.
432 * The only drawback to this gizmo is that is can sometimes damage
433 * hardware, software, or its subject(s).
436 The title following ``DOC:`` acts as a heading within the source file, but also
437 as an identifier for extracting the documentation comment. Thus, the title must
438 be unique within the file.
440 =============================
441 Including kernel-doc comments
442 =============================
444 The documentation comments may be included in any of the reStructuredText
445 documents using a dedicated kernel-doc Sphinx directive extension.
447 The kernel-doc directive is of the format::
449 .. kernel-doc:: source
452 The *source* is the path to a source file, relative to the kernel source
453 tree. The following directive options are supported:
455 export: *[source-pattern ...]*
456 Include documentation for all functions in *source* that have been exported
457 using ``EXPORT_SYMBOL`` or ``EXPORT_SYMBOL_GPL`` either in *source* or in any
458 of the files specified by *source-pattern*.
460 The *source-pattern* is useful when the kernel-doc comments have been placed
461 in header files, while ``EXPORT_SYMBOL`` and ``EXPORT_SYMBOL_GPL`` are next to
462 the function definitions.
466 .. kernel-doc:: lib/bitmap.c
469 .. kernel-doc:: include/net/mac80211.h
470 :export: net/mac80211/*.c
472 internal: *[source-pattern ...]*
473 Include documentation for all functions and types in *source* that have
474 **not** been exported using ``EXPORT_SYMBOL`` or ``EXPORT_SYMBOL_GPL`` either
475 in *source* or in any of the files specified by *source-pattern*.
479 .. kernel-doc:: drivers/gpu/drm/i915/intel_audio.c
482 identifiers: *[ function/type ...]*
483 Include documentation for each *function* and *type* in *source*.
484 If no *function* is specified, the documentation for all functions
485 and types in the *source* will be included.
489 .. kernel-doc:: lib/bitmap.c
490 :identifiers: bitmap_parselist bitmap_parselist_user
492 .. kernel-doc:: lib/idr.c
495 no-identifiers: *[ function/type ...]*
496 Exclude documentation for each *function* and *type* in *source*.
500 .. kernel-doc:: lib/bitmap.c
501 :no-identifiers: bitmap_parselist
503 functions: *[ function/type ...]*
504 This is an alias of the 'identifiers' directive and deprecated.
507 Include documentation for the ``DOC:`` paragraph identified by *title* in
508 *source*. Spaces are allowed in *title*; do not quote the *title*. The *title*
509 is only used as an identifier for the paragraph, and is not included in the
510 output. Please make sure to have an appropriate heading in the enclosing
511 reStructuredText document.
515 .. kernel-doc:: drivers/gpu/drm/i915/intel_audio.c
516 :doc: High Definition Audio over HDMI and Display Port
518 Without options, the kernel-doc directive includes all documentation comments
519 from the source file.
521 The kernel-doc extension is included in the kernel source tree, at
522 ``Documentation/sphinx/kerneldoc.py``. Internally, it uses the
523 ``scripts/kernel-doc`` script to extract the documentation comments from the
528 How to use kernel-doc to generate man pages
529 -------------------------------------------
531 If you just want to use kernel-doc to generate man pages you can do this
532 from the kernel git tree::
534 $ scripts/kernel-doc -man \
535 $(git grep -l '/\*\*' -- :^Documentation :^tools) \
536 | scripts/split-man.pl /tmp/man
538 Some older versions of git do not support some of the variants of syntax for
539 path exclusion. One of the following commands may work for those versions::
541 $ scripts/kernel-doc -man \
542 $(git grep -l '/\*\*' -- . ':!Documentation' ':!tools') \
543 | scripts/split-man.pl /tmp/man
545 $ scripts/kernel-doc -man \
546 $(git grep -l '/\*\*' -- . ":(exclude)Documentation" ":(exclude)tools") \
547 | scripts/split-man.pl /tmp/man