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