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1	NOTE: this document is outdated and will eventually be removed. See
2	Documentation/kernel-documentation.rst for current information.
3
4	kernel-doc nano-HOWTO
5	=====================
6
7	How to format kernel-doc comments
8	---------------------------------
9
10	In order to provide embedded, 'C' friendly, easy to maintain,
11	but consistent and extractable documentation of the functions and
12	data structures in the Linux kernel, the Linux kernel has adopted
13	a consistent style for documenting functions and their parameters,
14	and structures and their members.
15
16	The format for this documentation is called the kernel-doc format.
17	It is documented in this Documentation/kernel-doc-nano-HOWTO.txt file.
18
19	This style embeds the documentation within the source files, using
20	a few simple conventions. The scripts/kernel-doc perl script, some
21	SGML templates in Documentation/DocBook, and other tools understand
22	these conventions, and are used to extract this embedded documentation
23	into various documents.
24
25	In order to provide good documentation of kernel functions and data
26	structures, please use the following conventions to format your
27	kernel-doc comments in Linux kernel source.
28
29	We definitely need kernel-doc formatted documentation for functions
30	that are exported to loadable modules using EXPORT_SYMBOL.
31
32	We also look to provide kernel-doc formatted documentation for
33	functions externally visible to other kernel files (not marked
34	"static").
35
36	We also recommend providing kernel-doc formatted documentation
37	for private (file "static") routines, for consistency of kernel
38	source code layout. But this is lower priority and at the
39	discretion of the MAINTAINER of that kernel source file.
40
41	Data structures visible in kernel include files should also be
42	documented using kernel-doc formatted comments.
43
44	The opening comment mark "/**" is reserved for kernel-doc comments.
45	Only comments so marked will be considered by the kernel-doc scripts,
46	and any comment so marked must be in kernel-doc format. Do not use
47	"/**" to be begin a comment block unless the comment block contains
48	kernel-doc formatted comments. The closing comment marker for
49	kernel-doc comments can be either "*/" or "**/", but "*/" is
50	preferred in the Linux kernel tree.
51
52	Kernel-doc comments should be placed just before the function
53	or data structure being described.
54
55	Example kernel-doc function comment:
56
57	/**
58	* foobar() - short function description of foobar
59	* @arg1: Describe the first argument to foobar.
60	* @arg2: Describe the second argument to foobar.
61	* One can provide multiple line descriptions
62	* for arguments.
63	*
64	* A longer description, with more discussion of the function foobar()
65	* that might be useful to those using or modifying it. Begins with
66	* empty comment line, and may include additional embedded empty
67	* comment lines.
68	*
69	* The longer description can have multiple paragraphs.
70	*
71	* Return: Describe the return value of foobar.
72	*/
73
74	The short description following the subject can span multiple lines
75	and ends with an @argument description, an empty line or the end of
76	the comment block.
77
78	The @argument descriptions must begin on the very next line following
79	this opening short function description line, with no intervening
80	empty comment lines.
81
82	If a function parameter is "..." (varargs), it should be listed in
83	kernel-doc notation as:
84	* @...: description
85
86	The return value, if any, should be described in a dedicated section
87	named "Return".
88
89	Example kernel-doc data structure comment.
90
91	/**
92	* struct blah - the basic blah structure
93	* @mem1: describe the first member of struct blah
94	* @mem2: describe the second member of struct blah,
95	* perhaps with more lines and words.
96	*
97	* Longer description of this structure.
98	*/
99
100	The kernel-doc function comments describe each parameter to the
101	function, in order, with the @name lines.
102
103	The kernel-doc data structure comments describe each structure member
104	in the data structure, with the @name lines.
105
106	The longer description formatting is "reflowed", losing your line
107	breaks. So presenting carefully formatted lists within these
108	descriptions won't work so well; derived documentation will lose
109	the formatting.
110
111	See the section below "How to add extractable documentation to your
112	source files" for more details and notes on how to format kernel-doc
113	comments.
114
115	Components of the kernel-doc system
116	-----------------------------------
117
118	Many places in the source tree have extractable documentation in the
119	form of block comments above functions. The components of this system
120	are:
121
122	- scripts/kernel-doc
123
124	This is a perl script that hunts for the block comments and can mark
125	them up directly into DocBook, man, text, and HTML. (No, not
126	texinfo.)
127
128	- Documentation/DocBook/*.tmpl
129
130	These are SGML template files, which are normal SGML files with
131	special place-holders for where the extracted documentation should
132	go.
133
134	- scripts/docproc.c
135
136	This is a program for converting SGML template files into SGML
137	files. When a file is referenced it is searched for symbols
138	exported (EXPORT_SYMBOL), to be able to distinguish between internal
139	and external functions.
140	It invokes kernel-doc, giving it the list of functions that
141	are to be documented.
142	Additionally it is used to scan the SGML template files to locate
143	all the files referenced herein. This is used to generate dependency
144	information as used by make.
145
146	- Makefile
147
148	The targets 'xmldocs', 'psdocs', 'pdfdocs', and 'htmldocs' are used
149	to build XML DocBook files, PostScript files, PDF files, and html files
150	in Documentation/DocBook. The older target 'sgmldocs' is equivalent
151	to 'xmldocs'.
152
153	- Documentation/DocBook/Makefile
154
155	This is where C files are associated with SGML templates.
156
157
158	How to extract the documentation
159	--------------------------------
160
161	If you just want to read the ready-made books on the various
162	subsystems (see Documentation/DocBook/*.tmpl), just type 'make
163	psdocs', or 'make pdfdocs', or 'make htmldocs', depending on your
164	preference. If you would rather read a different format, you can type
165	'make xmldocs' and then use DocBook tools to convert
166	Documentation/DocBook/*.xml to a format of your choice (for example,
167	'db2html ...' if 'make htmldocs' was not defined).
168
169	If you want to see man pages instead, you can do this:
170
171	$ cd linux
172	$ scripts/kernel-doc -man $(find -name '*.c') | split-man.pl /tmp/man
173	$ scripts/kernel-doc -man $(find -name '*.h') | split-man.pl /tmp/man
174
175	Here is split-man.pl:
176
177	-->
178	#!/usr/bin/perl
179
180	if ($#ARGV < 0) {
181	die "where do I put the results?\n";
182	}
183
184	mkdir $ARGV[0],0777;
185	$state = 0;
186	while (<STDIN>) {
187	if (/^\.TH \"[^\"]*\" 9 \"([^\"]*)\"/) {
188	if ($state == 1) { close OUT }
189	$state = 1;
190	$fn = "$ARGV[0]/$1.9";
191	print STDERR "Creating $fn\n";
192	open OUT, ">$fn" or die "can't open $fn: $!\n";
193	print OUT $_;
194	} elsif ($state != 0) {
195	print OUT $_;
196	}
197	}
198
199	close OUT;
200	<--
201
202	If you just want to view the documentation for one function in one
203	file, you can do this:
204
205	$ scripts/kernel-doc -man -function fn file | nroff -man | less
206
207	or this:
208
209	$ scripts/kernel-doc -text -function fn file
210
211
212	How to add extractable documentation to your source files
213	---------------------------------------------------------
214
215	The format of the block comment is like this:
216
217	/**
218	* function_name(:)? (- short description)?
219	(* @parameterx(space)*: (description of parameter x)?)*
220	(* a blank line)?
221	* (Description:)? (Description of function)?
222	* (section header: (section description)? )*
223	(*)?*/
224
225	All "description" text can span multiple lines, although the
226	function_name & its short description are traditionally on a single line.
227	Description text may also contain blank lines (i.e., lines that contain
228	only a "*").
229
230	"section header:" names must be unique per function (or struct,
231	union, typedef, enum).
232
233	Use the section header "Return" for sections describing the return value
234	of a function.
235
236	Avoid putting a spurious blank line after the function name, or else the
237	description will be repeated!
238
239	All descriptive text is further processed, scanning for the following special
240	patterns, which are highlighted appropriately.
241
242	'funcname()' - function
243	'$ENVVAR' - environment variable
244	'&struct_name' - name of a structure (up to two words including 'struct')
245	'@parameter' - name of a parameter
246	'%CONST' - name of a constant.
247
248	NOTE 1: The multi-line descriptive text you provide does *not* recognize
249	line breaks, so if you try to format some text nicely, as in:
250
251	Return:
252	0 - cool
253	1 - invalid arg
254	2 - out of memory
255
256	this will all run together and produce:
257
258	Return: 0 - cool 1 - invalid arg 2 - out of memory
259
260	NOTE 2: If the descriptive text you provide has lines that begin with
261	some phrase followed by a colon, each of those phrases will be taken as
262	a new section heading, which means you should similarly try to avoid text
263	like:
264
265	Return:
266	0: cool
267	1: invalid arg
268	2: out of memory
269
270	every line of which would start a new section. Again, probably not
271	what you were after.
272
273	Take a look around the source tree for examples.
274
275
276	kernel-doc for structs, unions, enums, and typedefs
277	---------------------------------------------------
278
279	Beside functions you can also write documentation for structs, unions,
280	enums and typedefs. Instead of the function name you must write the name
281	of the declaration; the struct/union/enum/typedef must always precede
282	the name. Nesting of declarations is not supported.
283	Use the argument mechanism to document members or constants.
284
285	Inside a struct description, you can use the "private:" and "public:"
286	comment tags. Structure fields that are inside a "private:" area
287	are not listed in the generated output documentation. The "private:"
288	and "public:" tags must begin immediately following a "/*" comment
289	marker. They may optionally include comments between the ":" and the
290	ending "*/" marker.
291
292	Example:
293
294	/**
295	* struct my_struct - short description
296	* @a: first member
297	* @b: second member
298	*
299	* Longer description
300	*/
301	struct my_struct {
302	int a;
303	int b;
304	/* private: internal use only */
305	int c;
306	};
307
308
309	Including documentation blocks in source files
310	----------------------------------------------
311
312	To facilitate having source code and comments close together, you can
313	include kernel-doc documentation blocks that are free-form comments
314	instead of being kernel-doc for functions, structures, unions,
315	enums, or typedefs. This could be used for something like a
316	theory of operation for a driver or library code, for example.
317
318	This is done by using a DOC: section keyword with a section title. E.g.:
319
320	/**
321	* DOC: Theory of Operation
322	*
323	* The whizbang foobar is a dilly of a gizmo. It can do whatever you
324	* want it to do, at any time. It reads your mind. Here's how it works.
325	*
326	* foo bar splat
327	*
328	* The only drawback to this gizmo is that is can sometimes damage
329	* hardware, software, or its subject(s).
330	*/
331
332	DOC: sections are used in SGML templates files as indicated below.
333
334
335	How to make new SGML template files
336	-----------------------------------
337
338	SGML template files (*.tmpl) are like normal SGML files, except that
339	they can contain escape sequences where extracted documentation should
340	be inserted.
341
342	!E<filename> is replaced by the documentation, in <filename>, for
343	functions that are exported using EXPORT_SYMBOL: the function list is
344	collected from files listed in Documentation/DocBook/Makefile.
345
346	!I<filename> is replaced by the documentation for functions that are
347	_not_ exported using EXPORT_SYMBOL.
348
349	!D<filename> is used to name additional files to search for functions
350	exported using EXPORT_SYMBOL.
351
352	!F<filename> <function [functions...]> is replaced by the
353	documentation, in <filename>, for the functions listed.
354
355	!P<filename> <section title> is replaced by the contents of the DOC:
356	section titled <section title> from <filename>.
357	Spaces are allowed in <section title>; do not quote the <section title>.
358
359	!C<filename> is replaced by nothing, but makes the tools check that
360	all DOC: sections and documented functions, symbols, etc. are used.
361	This makes sense to use when you use !F/!P only and want to verify
362	that all documentation is included.
363
364	Tim.
365	*/ <twaugh@redhat.com>
366