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