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 'sgmldocs', 'psdocs', 'pdfdocs', and 'htmldocs' are used
 146  to build DocBook files, PostScript files, PDF files, and html files
 147  in Documentation/DocBook.
 148
 149- Documentation/DocBook/Makefile
 150
 151  This is where C files are associated with SGML templates.
 152
 153
 154How to extract the documentation
 155--------------------------------
 156
 157If you just want to read the ready-made books on the various
 158subsystems (see Documentation/DocBook/*.tmpl), just type 'make
 159psdocs', or 'make pdfdocs', or 'make htmldocs', depending on your
 160preference.  If you would rather read a different format, you can type
 161'make sgmldocs' and then use DocBook tools to convert
 162Documentation/DocBook/*.sgml to a format of your choice (for example,
 163'db2html ...' if 'make htmldocs' was not defined).
 164
 165If you want to see man pages instead, you can do this:
 166
 167$ cd linux
 168$ scripts/kernel-doc -man $(find -name '*.c') | split-man.pl /tmp/man
 169$ scripts/kernel-doc -man $(find -name '*.h') | split-man.pl /tmp/man
 170
 171Here is split-man.pl:
 172
 173-->
 174#!/usr/bin/perl
 175
 176if ($#ARGV < 0) {
 177   die "where do I put the results?\n";
 178}
 179
 180mkdir $ARGV[0],0777;
 181$state = 0;
 182while (<STDIN>) {
 183    if (/^\.TH \"[^\"]*\" 9 \"([^\"]*)\"/) {
 184        if ($state == 1) { close OUT }
 185        $state = 1;
 186        $fn = "$ARGV[0]/$1.9";
 187        print STDERR "Creating $fn\n";
 188        open OUT, ">$fn" or die "can't open $fn: $!\n";
 189        print OUT $_;
 190    } elsif ($state != 0) {
 191        print OUT $_;
 192    }
 193}
 194
 195close OUT;
 196<--
 197
 198If you just want to view the documentation for one function in one
 199file, you can do this:
 200
 201$ scripts/kernel-doc -man -function fn file | nroff -man | less
 202
 203or this:
 204
 205$ scripts/kernel-doc -text -function fn file
 206
 207
 208How to add extractable documentation to your source files
 209---------------------------------------------------------
 210
 211The format of the block comment is like this:
 212
 213/**
 214 * function_name(:)? (- short description)?
 215(* @parameterx(space)*: (description of parameter x)?)*
 216(* a blank line)?
 217 * (Description:)? (Description of function)?
 218 * (section header: (section description)? )*
 219(*)?*/
 220
 221All "description" text can span multiple lines, although the
 222function_name & its short description are traditionally on a single line.
 223Description text may also contain blank lines (i.e., lines that contain
 224only a "*").
 225
 226"section header:" names must be unique per function (or struct,
 227union, typedef, enum).
 228
 229Use the section header "Return" for sections describing the return value
 230of a function.
 231
 232Avoid putting a spurious blank line after the function name, or else the
 233description will be repeated!
 234
 235All descriptive text is further processed, scanning for the following special
 236patterns, which are highlighted appropriately.
 237
 238'funcname()' - function
 239'$ENVVAR' - environment variable
 240'&struct_name' - name of a structure (up to two words including 'struct')
 241'@parameter' - name of a parameter
 242'%CONST' - name of a constant.
 243
 244NOTE 1:  The multi-line descriptive text you provide does *not* recognize
 245line breaks, so if you try to format some text nicely, as in:
 246
 247  Return:
 248    0 - cool
 249    1 - invalid arg
 250    2 - out of memory
 251
 252this will all run together and produce:
 253
 254  Return: 0 - cool 1 - invalid arg 2 - out of memory
 255
 256NOTE 2:  If the descriptive text you provide has lines that begin with
 257some phrase followed by a colon, each of those phrases will be taken as
 258a new section heading, which means you should similarly try to avoid text
 259like:
 260
 261  Return:
 262    0: cool
 263    1: invalid arg
 264    2: out of memory
 265
 266every line of which would start a new section.  Again, probably not
 267what you were after.
 268
 269Take a look around the source tree for examples.
 270
 271
 272kernel-doc for structs, unions, enums, and typedefs
 273---------------------------------------------------
 274
 275Beside functions you can also write documentation for structs, unions,
 276enums and typedefs. Instead of the function name you must write the name
 277of the declaration;  the struct/union/enum/typedef must always precede
 278the name. Nesting of declarations is not supported.
 279Use the argument mechanism to document members or constants.
 280
 281Inside a struct description, you can use the "private:" and "public:"
 282comment tags.  Structure fields that are inside a "private:" area
 283are not listed in the generated output documentation.  The "private:"
 284and "public:" tags must begin immediately following a "/*" comment
 285marker.  They may optionally include comments between the ":" and the
 286ending "*/" marker.
 287
 288Example:
 289
 290/**
 291 * struct my_struct - short description
 292 * @a: first member
 293 * @b: second member
 294 *
 295 * Longer description
 296 */
 297struct my_struct {
 298    int a;
 299    int b;
 300/* private: internal use only */
 301    int c;
 302};
 303
 304
 305Including documentation blocks in source files
 306----------------------------------------------
 307
 308To facilitate having source code and comments close together, you can
 309include kernel-doc documentation blocks that are free-form comments
 310instead of being kernel-doc for functions, structures, unions,
 311enums, or typedefs.  This could be used for something like a
 312theory of operation for a driver or library code, for example.
 313
 314This is done by using a DOC: section keyword with a section title.  E.g.:
 315
 316/**
 317 * DOC: Theory of Operation
 318 *
 319 * The whizbang foobar is a dilly of a gizmo.  It can do whatever you
 320 * want it to do, at any time.  It reads your mind.  Here's how it works.
 321 *
 322 * foo bar splat
 323 *
 324 * The only drawback to this gizmo is that is can sometimes damage
 325 * hardware, software, or its subject(s).
 326 */
 327
 328DOC: sections are used in SGML templates files as indicated below.
 329
 330
 331How to make new SGML template files
 332-----------------------------------
 333
 334SGML template files (*.tmpl) are like normal SGML files, except that
 335they can contain escape sequences where extracted documentation should
 336be inserted.
 337
 338!E<filename> is replaced by the documentation, in <filename>, for
 339functions that are exported using EXPORT_SYMBOL: the function list is
 340collected from files listed in Documentation/DocBook/Makefile.
 341
 342!I<filename> is replaced by the documentation for functions that are
 343_not_ exported using EXPORT_SYMBOL.
 344
 345!D<filename> is used to name additional files to search for functions
 346exported using EXPORT_SYMBOL.
 347
 348!F<filename> <function [functions...]> is replaced by the
 349documentation, in <filename>, for the functions listed.
 350
 351!P<filename> <section title> is replaced by the contents of the DOC:
 352section titled <section title> from <filename>.
 353Spaces are allowed in <section title>; do not quote the <section title>.
 354
 355!C<filename> is replaced by nothing, but makes the tools check that
 356all DOC: sections and documented functions, symbols, etc. are used.
 357This makes sense to use when you use !F/!P only and want to verify
 358that all documentation is included.
 359
 360Tim.
 361*/ <twaugh@redhat.com>
 362
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