1NOTE: this document is outdated and will eventually be removed. See 2Documentation/kernel-documentation.rst for current information. 3 4kernel-doc nano-HOWTO 5===================== 6 7How to format kernel-doc comments 8--------------------------------- 9 10In order to provide embedded, 'C' friendly, easy to maintain, 11but consistent and extractable documentation of the functions and 12data structures in the Linux kernel, the Linux kernel has adopted 13a consistent style for documenting functions and their parameters, 14and structures and their members. 15 16The format for this documentation is called the kernel-doc format. 17It is documented in this Documentation/kernel-doc-nano-HOWTO.txt file. 18 19This style embeds the documentation within the source files, using 20a few simple conventions. The scripts/kernel-doc perl script, some 21SGML templates in Documentation/DocBook, and other tools understand 22these conventions, and are used to extract this embedded documentation 23into various documents. 24 25In order to provide good documentation of kernel functions and data 26structures, please use the following conventions to format your 27kernel-doc comments in Linux kernel source. 28 29We definitely need kernel-doc formatted documentation for functions 30that are exported to loadable modules using EXPORT_SYMBOL. 31 32We also look to provide kernel-doc formatted documentation for 33functions externally visible to other kernel files (not marked 34"static"). 35 36We also recommend providing kernel-doc formatted documentation 37for private (file "static") routines, for consistency of kernel 38source code layout. But this is lower priority and at the 39discretion of the MAINTAINER of that kernel source file. 40 41Data structures visible in kernel include files should also be 42documented using kernel-doc formatted comments. 43 44The opening comment mark "/**" is reserved for kernel-doc comments. 45Only comments so marked will be considered by the kernel-doc scripts, 46and any comment so marked must be in kernel-doc format. Do not use 47"/**" to be begin a comment block unless the comment block contains 48kernel-doc formatted comments. The closing comment marker for 49kernel-doc comments can be either "*/" or "**/", but "*/" is 50preferred in the Linux kernel tree. 51 52Kernel-doc comments should be placed just before the function 53or data structure being described. 54 55Example kernel-doc function comment: 56 57/** 58 * foobar() - short function description of foobar 59 * @arg1: Describe the first argument to foobar. 60 * @arg2: Describe the second argument to foobar. 61 * One can provide multiple line descriptions 62 * for arguments. 63 * 64 * A longer description, with more discussion of the function foobar() 65 * that might be useful to those using or modifying it. Begins with 66 * empty comment line, and may include additional embedded empty 67 * comment lines. 68 * 69 * The longer description can have multiple paragraphs. 70 * 71 * Return: Describe the return value of foobar. 72 */ 73 74The short description following the subject can span multiple lines 75and ends with an @argument description, an empty line or the end of 76the comment block. 77 78The @argument descriptions must begin on the very next line following 79this opening short function description line, with no intervening 80empty comment lines. 81 82If a function parameter is "..." (varargs), it should be listed in 83kernel-doc notation as: 84 * @...: description 85 86The return value, if any, should be described in a dedicated section 87named "Return". 88 89Example kernel-doc data structure comment. 90 91/** 92 * struct blah - the basic blah structure 93 * @mem1: describe the first member of struct blah 94 * @mem2: describe the second member of struct blah, 95 * perhaps with more lines and words. 96 * 97 * Longer description of this structure. 98 */ 99 100The kernel-doc function comments describe each parameter to the 101function, in order, with the @name lines. 102 103The kernel-doc data structure comments describe each structure member 104in the data structure, with the @name lines. 105 106The longer description formatting is "reflowed", losing your line 107breaks. So presenting carefully formatted lists within these 108descriptions won't work so well; derived documentation will lose 109the formatting. 110 111See the section below "How to add extractable documentation to your 112source files" for more details and notes on how to format kernel-doc 113comments. 114 115Components of the kernel-doc system 116----------------------------------- 117 118Many places in the source tree have extractable documentation in the 119form of block comments above functions. The components of this system 120are: 121 122- scripts/kernel-doc 123 124 This is a perl script that hunts for the block comments and can mark 125 them up directly into DocBook, man, text, and HTML. (No, not 126 texinfo.) 127 128- Documentation/DocBook/*.tmpl 129 130 These are SGML template files, which are normal SGML files with 131 special place-holders for where the extracted documentation should 132 go. 133 134- scripts/docproc.c 135 136 This is a program for converting SGML template files into SGML 137 files. When a file is referenced it is searched for symbols 138 exported (EXPORT_SYMBOL), to be able to distinguish between internal 139 and external functions. 140 It invokes kernel-doc, giving it the list of functions that 141 are to be documented. 142 Additionally it is used to scan the SGML template files to locate 143 all the files referenced herein. This is used to generate dependency 144 information as used by make. 145 146- Makefile 147 148 The targets 'xmldocs', 'psdocs', 'pdfdocs', and 'htmldocs' are used 149 to build XML DocBook files, PostScript files, PDF files, and html files 150 in Documentation/DocBook. The older target 'sgmldocs' is equivalent 151 to 'xmldocs'. 152 153- Documentation/DocBook/Makefile 154 155 This is where C files are associated with SGML templates. 156 157 158How to extract the documentation 159-------------------------------- 160 161If you just want to read the ready-made books on the various 162subsystems (see Documentation/DocBook/*.tmpl), just type 'make 163psdocs', or 'make pdfdocs', or 'make htmldocs', depending on your 164preference. If you would rather read a different format, you can type 165'make xmldocs' and then use DocBook tools to convert 166Documentation/DocBook/*.xml to a format of your choice (for example, 167'db2html ...' if 'make htmldocs' was not defined). 168 169If you want to see man pages instead, you can do this: 170 171$ cd linux 172$ scripts/kernel-doc -man $(find -name '*.c') | split-man.pl /tmp/man 173$ scripts/kernel-doc -man $(find -name '*.h') | split-man.pl /tmp/man 174 175Here is split-man.pl: 176 177--> 178#!/usr/bin/perl 179 180if ($#ARGV < 0) { 181 die "where do I put the results?\n"; 182} 183 184mkdir $ARGV[0],0777; 185$state = 0; 186while (<STDIN>) { 187 if (/^\.TH \"[^\"]*\" 9 \"([^\"]*)\"/) { 188 if ($state == 1) { close OUT } 189 $state = 1; 190 $fn = "$ARGV[0]/$1.9"; 191 print STDERR "Creating $fn\n"; 192 open OUT, ">$fn" or die "can't open $fn: $!\n"; 193 print OUT $_; 194 } elsif ($state != 0) { 195 print OUT $_; 196 } 197} 198 199close OUT; 200<-- 201 202If you just want to view the documentation for one function in one 203file, you can do this: 204 205$ scripts/kernel-doc -man -function fn file | nroff -man | less 206 207or this: 208 209$ scripts/kernel-doc -text -function fn file 210 211 212How to add extractable documentation to your source files 213--------------------------------------------------------- 214 215The format of the block comment is like this: 216 217/** 218 * function_name(:)? (- short description)? 219(* @parameterx(space)*: (description of parameter x)?)* 220(* a blank line)? 221 * (Description:)? (Description of function)? 222 * (section header: (section description)? )* 223(*)?*/ 224 225All "description" text can span multiple lines, although the 226function_name & its short description are traditionally on a single line. 227Description text may also contain blank lines (i.e., lines that contain 228only a "*"). 229 230"section header:" names must be unique per function (or struct, 231union, typedef, enum). 232 233Use the section header "Return" for sections describing the return value 234of a function. 235 236Avoid putting a spurious blank line after the function name, or else the 237description will be repeated! 238 239All descriptive text is further processed, scanning for the following special 240patterns, which are highlighted appropriately. 241 242'funcname()' - function 243'$ENVVAR' - environment variable 244'&struct_name' - name of a structure (up to two words including 'struct') 245'@parameter' - name of a parameter 246'%CONST' - name of a constant. 247 248NOTE 1: The multi-line descriptive text you provide does *not* recognize 249line breaks, so if you try to format some text nicely, as in: 250 251 Return: 252 0 - cool 253 1 - invalid arg 254 2 - out of memory 255 256this will all run together and produce: 257 258 Return: 0 - cool 1 - invalid arg 2 - out of memory 259 260NOTE 2: If the descriptive text you provide has lines that begin with 261some phrase followed by a colon, each of those phrases will be taken as 262a new section heading, which means you should similarly try to avoid text 263like: 264 265 Return: 266 0: cool 267 1: invalid arg 268 2: out of memory 269 270every line of which would start a new section. Again, probably not 271what you were after. 272 273Take a look around the source tree for examples. 274 275 276kernel-doc for structs, unions, enums, and typedefs 277--------------------------------------------------- 278 279Beside functions you can also write documentation for structs, unions, 280enums and typedefs. Instead of the function name you must write the name 281of the declaration; the struct/union/enum/typedef must always precede 282the name. Nesting of declarations is not supported. 283Use the argument mechanism to document members or constants. 284 285Inside a struct description, you can use the "private:" and "public:" 286comment tags. Structure fields that are inside a "private:" area 287are not listed in the generated output documentation. The "private:" 288and "public:" tags must begin immediately following a "/*" comment 289marker. They may optionally include comments between the ":" and the 290ending "*/" marker. 291 292Example: 293 294/** 295 * struct my_struct - short description 296 * @a: first member 297 * @b: second member 298 * 299 * Longer description 300 */ 301struct my_struct { 302 int a; 303 int b; 304/* private: internal use only */ 305 int c; 306}; 307 308 309Including documentation blocks in source files 310---------------------------------------------- 311 312To facilitate having source code and comments close together, you can 313include kernel-doc documentation blocks that are free-form comments 314instead of being kernel-doc for functions, structures, unions, 315enums, or typedefs. This could be used for something like a 316theory of operation for a driver or library code, for example. 317 318This is done by using a DOC: section keyword with a section title. E.g.: 319 320/** 321 * DOC: Theory of Operation 322 * 323 * The whizbang foobar is a dilly of a gizmo. It can do whatever you 324 * want it to do, at any time. It reads your mind. Here's how it works. 325 * 326 * foo bar splat 327 * 328 * The only drawback to this gizmo is that is can sometimes damage 329 * hardware, software, or its subject(s). 330 */ 331 332DOC: sections are used in SGML templates files as indicated below. 333 334 335How to make new SGML template files 336----------------------------------- 337 338SGML template files (*.tmpl) are like normal SGML files, except that 339they can contain escape sequences where extracted documentation should 340be inserted. 341 342!E<filename> is replaced by the documentation, in <filename>, for 343functions that are exported using EXPORT_SYMBOL: the function list is 344collected from files listed in Documentation/DocBook/Makefile. 345 346!I<filename> is replaced by the documentation for functions that are 347_not_ exported using EXPORT_SYMBOL. 348 349!D<filename> is used to name additional files to search for functions 350exported using EXPORT_SYMBOL. 351 352!F<filename> <function [functions...]> is replaced by the 353documentation, in <filename>, for the functions listed. 354 355!P<filename> <section title> is replaced by the contents of the DOC: 356section titled <section title> from <filename>. 357Spaces are allowed in <section title>; do not quote the <section title>. 358 359!C<filename> is replaced by nothing, but makes the tools check that 360all DOC: sections and documented functions, symbols, etc. are used. 361This makes sense to use when you use !F/!P only and want to verify 362that all documentation is included. 363 364Tim. 365*/ <twaugh@redhat.com> 366