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

  1 NOTE: this document is outdated and will eventually be removed.  See
  2 Documentation/doc-guide/ 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>

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