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