Root/Documentation/kernel-doc-nano-HOWTO.txt

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

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