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