1 \input texinfo @c -*- texinfo -*-
3 @settitle Developer Documentation
5 @center @titlefont{Developer Documentation}
12 @chapter Developers Guide
16 @item libavcodec is the library containing the codecs (both encoding and
17 decoding). Look at @file{libavcodec/apiexample.c} to see how to use it.
19 @item libavformat is the library containing the file format handling (mux and
20 demux code for several formats). Look at @file{avplay.c} to use it in a
21 player. See @file{libavformat/output-example.c} to use it to generate
22 audio or video streams.
26 @section Integrating libav in your program
28 Shared libraries should be used whenever is possible in order to reduce
29 the effort distributors have to pour to support programs and to ensure
30 only the public API is used.
32 You can use Libav in your commercial program, but you must abide to the
33 license, LGPL or GPL depending on the specific features used, please refer
34 to @uref{http://libav.org/legal.html, our legal page} for a quick checklist and to
35 the following links for the exact text of each license:
36 @uref{http://git.libav.org/?p=libav.git;a=blob;f=COPYING.GPLv2, GPL version 2},
37 @uref{http://git.libav.org/?p=libav.git;a=blob;f=COPYING.GPLv3, GPL version 3},
38 @uref{http://git.libav.org/?p=libav.git;a=blob;f=COPYING.LGPLv2.1, LGPL version 2.1},
39 @uref{http://git.libav.org/?p=libav.git;a=blob;f=COPYING.LGPLv3, LGPL version 3}.
40 Any modification to the source code can be suggested for inclusion.
41 The best way to proceed is to send your patches to the
42 @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel}
48 @subsection Code formatting conventions
49 The code is written in K&R C style. That means the following:
52 The control statements are formatted by putting space between the statement
53 and parenthesis in the following way:
55 for (i = 0; i < filter->input_count; i++) @{
58 The case statement is always located at the same level as the switch itself:
60 switch (link->init_state) @{
63 case AVLINK_STARTINIT:
64 av_log(filter, AV_LOG_INFO, "circular filter chain detected");
68 Braces in function declarations are written on the new line:
70 const char *avfilter_configuration(void)
72 return LIBAV_CONFIGURATION;
76 Do not check for NULL values by comparison, @samp{if (p)} and
77 @samp{if (!p)} are correct; @samp{if (p == NULL)} and @samp{if (p != NULL)}
80 In case of a single-statement if, no curly braces are required:
86 Do not put spaces immediately inside parentheses. @samp{if (ret)} is
87 a valid style; @samp{if ( ret )} is not.
90 There are the following guidelines regarding the indentation in files:
95 The TAB character is forbidden outside of Makefiles as is any
96 form of trailing whitespace. Commits containing either will be
97 rejected by the git repository.
99 You should try to limit your code lines to 80 characters; however, do so if
100 and only if this improves readability.
102 The presentation is one inspired by 'indent -i4 -kr -nut'.
104 The main priority in Libav is simplicity and small code size in order to
105 minimize the bug count.
108 Use the JavaDoc/Doxygen format (see examples below) so that code documentation
109 can be generated automatically. All nontrivial functions should have a comment
110 above them explaining what the function does, even if it is just one sentence.
111 All structures and their member variables should be documented, too.
113 Avoid Qt-style and similar Doxygen syntax with @code{!} in it, i.e. replace
114 @code{//!} with @code{///} and similar. Also @@ syntax should be employed
115 for markup commands, i.e. use @code{@@param} and not @code{\param}.
129 typedef struct Foobar@{
130 int var1; /**< var1 description */
131 int var2; ///< var2 description
132 /** var3 description */
140 * @@param my_parameter description of my_parameter
141 * @@return return value description
143 int myfunc(int my_parameter)
147 @subsection C language features
149 Libav is programmed in the ISO C90 language with a few additional
150 features from ISO C99, namely:
153 the @samp{inline} keyword;
157 designated struct initializers (@samp{struct s x = @{ .i = 17 @};})
159 compound literals (@samp{x = (struct s) @{ 17, 23 @};})
162 These features are supported by all compilers we care about, so we will not
163 accept patches to remove their use unless they absolutely do not impair
164 clarity and performance.
166 All code must compile with recent versions of GCC and a number of other
167 currently supported compilers. To ensure compatibility, please do not use
168 additional C99 features or GCC extensions. Especially watch out for:
171 mixing statements and declarations;
173 @samp{long long} (use @samp{int64_t} instead);
175 @samp{__attribute__} not protected by @samp{#ifdef __GNUC__} or similar;
177 GCC statement expressions (@samp{(x = (@{ int y = 4; y; @})}).
180 @subsection Naming conventions
181 All names should be composed with underscores (_), not CamelCase. For example,
182 @samp{avfilter_get_video_buffer} is an acceptable function name and
183 @samp{AVFilterGetVideo} is not. The only exception are structure
184 names; they should always be CamelCase.
186 There are the following conventions for naming variables and functions:
189 For local variables no prefix is required.
191 For variables and functions declared as @code{static} no prefix is required.
193 For variables and functions used internally by a library an @code{ff_}
194 prefix should be used, e.g. @samp{ff_w64_demuxer}.
196 For variables and functions used internally across multiple libraries, use
197 @code{avpriv_}. For example, @samp{avpriv_aac_parse_header}.
199 For externally visible symbols, each library has its own prefix. Check
200 the existing code and choose names accordingly.
203 Furthermore, name space reserved for the system should not be invaded.
204 Identifiers ending in @code{_t} are reserved by
205 @url{http://pubs.opengroup.org/onlinepubs/007904975/functions/xsh_chap02_02.html#tag_02_02_02, POSIX}.
206 Also avoid names starting with @code{__} or @code{_} followed by an uppercase
207 letter as they are reserved by the C standard. Names starting with @code{_}
208 are reserved at the file level and may not be used for externally visible
209 symbols. If in doubt, just avoid names starting with @code{_} altogether.
211 @subsection Miscellaneous conventions
214 fprintf and printf are forbidden in libavformat and libavcodec,
215 please use av_log() instead.
217 Casts should be used only when necessary. Unneeded parentheses
218 should also be avoided if they don't make the code easier to understand.
221 @subsection Editor configuration
222 In order to configure Vim to follow Libav formatting conventions, paste
223 the following snippet into your @file{.vimrc}:
225 " Indentation rules for Libav: 4 spaces, no tabs.
231 " Allow tabs in Makefiles.
232 autocmd FileType make,automake set noexpandtab shiftwidth=8 softtabstop=8
233 " Trailing whitespace and tabs are forbidden, so highlight them.
234 highlight ForbiddenWhitespace ctermbg=red guibg=red
235 match ForbiddenWhitespace /\s\+$\|\t/
236 " Do not highlight spaces at the end of line while typing on that line.
237 autocmd InsertEnter * match ForbiddenWhitespace /\t\|\s\+\%#\@@<!$/
240 For Emacs, add these roughly equivalent lines to your @file{.emacs.d/init.el}:
245 (indent-tabs-mode . nil)
246 (show-trailing-whitespace . t)
248 (statement-cont . (c-lineup-assignments +)))
251 (setq c-default-style "libav")
254 @section Development Policy
258 Contributions should be licensed under the
259 @uref{http://www.gnu.org/licenses/lgpl-2.1.html, LGPL 2.1},
260 including an "or any later version" clause, or, if you prefer
261 a gift-style license, the
262 @uref{http://www.isc.org/software/license/, ISC} or
263 @uref{http://mit-license.org/, MIT} license.
264 @uref{http://www.gnu.org/licenses/gpl-2.0.html, GPL 2} including
265 an "or any later version" clause is also acceptable, but LGPL is
268 All the patches MUST be reviewed in the mailing list before they are
271 The Libav coding style should remain consistent. Changes to
272 conform will be suggested during the review or implemented on commit.
274 Patches should be generated using @code{git format-patch} or directly sent
275 using @code{git send-email}.
276 Please make sure you give the proper credit by setting the correct author
279 The commit message should have a short first line in the form of
280 a @samp{topic: short description} as a header, separated by a newline
281 from the body consisting of an explanation of why the change is necessary.
282 If the commit fixes a known bug on the bug tracker, the commit message
283 should include its bug ID. Referring to the issue on the bug tracker does
284 not exempt you from writing an excerpt of the bug in the commit message.
285 If the patch is a bug fix which should be backported to stable releases,
286 i.e. a non-API/ABI-breaking bug fix, add @code{CC: libav-stable@@libav.org}
287 to the bottom of your commit message, and make sure to CC your patch to
288 this address, too. Some git setups will do this automatically.
290 Work in progress patches should be sent to the mailing list with the [WIP]
293 Branches in public personal repos are advised as way to
294 work on issues collaboratively.
296 You do not have to over-test things. If it works for you and you think it
297 should work for others, send it to the mailing list for review.
298 If you have doubt about portability please state it in the submission so
299 people with specific hardware could test it.
301 Do not commit unrelated changes together, split them into self-contained
302 pieces. Also do not forget that if part B depends on part A, but A does not
303 depend on B, then A can and should be committed first and separate from B.
304 Keeping changes well split into self-contained parts makes reviewing and
305 understanding them on the commit log mailing list easier. This also helps
306 in case of debugging later on.
308 Patches that change behavior of the programs (renaming options etc) or
309 public API or ABI should be discussed in depth and possible few days should
310 pass between discussion and commit.
311 Changes to the build system (Makefiles, configure script) which alter
312 the expected behavior should be considered in the same regard.
314 When applying patches that have been discussed (at length) on the mailing
315 list, reference the thread in the log message.
318 @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel} and
319 @uref{https://lists.libav.org/mailman/listinfo/libav-commits, libav-commits}
321 Bugs and possible improvements or general questions regarding commits
322 are discussed on libav-devel. We expect you to react if problems with
323 your code are uncovered.
325 Update the documentation if you change behavior or add features. If you are
326 unsure how best to do this, send an [RFC] patch to libav-devel.
328 All discussions and decisions should be reported on the public developer
329 mailing list, so that there is a reference to them.
330 Other media (e.g. IRC) should be used for coordination and immediate
333 Never write to unallocated memory, never write over the end of arrays,
334 always check values read from some untrusted source before using them
335 as array index or other risky things. Always use valgrind to double-check.
337 Remember to check if you need to bump versions for the specific libav
338 parts (libavutil, libavcodec, libavformat) you are changing. You need
339 to change the version integer.
340 Incrementing the first component means no backward compatibility to
341 previous versions (e.g. removal of a function from the public API).
342 Incrementing the second component means backward compatible change
343 (e.g. addition of a function to the public API or extension of an
344 existing data structure).
345 Incrementing the third component means a noteworthy binary compatible
346 change (e.g. encoder bug fix that matters for the decoder).
348 Compiler warnings indicate potential bugs or code with bad style.
349 If it is a bug, the bug has to be fixed. If it is not, the code should
350 be changed to not generate a warning unless that causes a slowdown
351 or obfuscates the code.
352 If a type of warning leads to too many false positives, that warning
353 should be disabled, not the code changed.
355 If you add a new file, give it a proper license header. Do not copy and
356 paste it from a random place, use an existing file as template.
359 We think our rules are not too hard. If you have comments, contact us.
361 @section Submitting patches
363 First, read the @ref{Coding Rules} above if you did not yet, in particular
364 the rules regarding patch submission.
366 As stated already, please do not submit a patch which contains several
368 Split it into separate, self-contained pieces. This does not mean splitting
369 file by file. Instead, make the patch as small as possible while still
370 keeping it as a logical unit that contains an individual change, even
371 if it spans multiple files. This makes reviewing your patches much easier
372 for us and greatly increases your chances of getting your patch applied.
374 Use the patcheck tool of Libav to check your patch.
375 The tool is located in the tools directory.
377 Run the @ref{Regression Tests} before submitting a patch in order to verify
378 it does not cause unexpected problems.
380 It also helps quite a bit if you tell us what the patch does (for example
381 'replaces lrint by lrintf'), and why (for example '*BSD isn't C99 compliant
382 and has no lrint()'). This kind of explanation should be the body of the
385 Also please if you send several patches, send each patch as a separate mail,
386 do not attach several unrelated patches to the same mail.
388 Patches should be posted to the
389 @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel}
390 mailing list. Use @code{git send-email} when possible since it will properly
391 send patches without requiring extra care. If you cannot, then send patches
392 as base64-encoded attachments, so your patch is not trashed during
395 Your patch will be reviewed on the mailing list. You will likely be asked
396 to make some changes and are expected to send in an improved version that
397 incorporates the requests from the review. This process may go through
398 several iterations. Once your patch is deemed good enough, it will be
399 committed to the official Libav tree.
401 Give us a few days to react. But if some time passes without reaction,
402 send a reminder by email. Your patch should eventually be dealt with.
405 @section New codecs or formats checklist
409 Did you use av_cold for codec initialization and close functions?
411 Did you add a long_name under NULL_IF_CONFIG_SMALL to the AVCodec or
412 AVInputFormat/AVOutputFormat struct?
414 Did you bump the minor version number (and reset the micro version
415 number) in @file{libavcodec/version.h} or @file{libavformat/version.h}?
417 Did you register it in @file{allcodecs.c} or @file{allformats.c}?
419 Did you add the AVCodecID to @file{avcodec.h}?
420 When adding new codec IDs, also add an entry to the codec descriptor
421 list in @file{libavcodec/codec_desc.c}.
423 If it has a FourCC, did you add it to @file{libavformat/riff.c},
424 even if it is only a decoder?
426 Did you add a rule to compile the appropriate files in the Makefile?
427 Remember to do this even if you are just adding a format to a file that
428 is already being compiled by some other rule, like a raw demuxer.
430 Did you add an entry to the table of supported formats or codecs in
431 @file{doc/general.texi}?
433 Did you add an entry in the Changelog?
435 If it depends on a parser or a library, did you add that dependency in
438 Did you @code{git add} the appropriate files before committing?
440 Did you make sure it compiles standalone, i.e. with
441 @code{configure --disable-everything --enable-decoder=foo}
442 (or @code{--enable-demuxer} or whatever your component is)?
446 @section patch submission checklist
450 Does @code{make check} pass with the patch applied?
452 Is the patch against latest Libav git master branch?
454 Are you subscribed to the
455 @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel}
456 mailing list? (Only list subscribers are allowed to post.)
458 Have you checked that the changes are minimal, so that the same cannot be
459 achieved with a smaller patch and/or simpler final code?
461 If the change is to speed critical code, did you benchmark it?
463 If you did any benchmarks, did you provide them in the mail?
465 Have you checked that the patch does not introduce buffer overflows or
466 other security issues?
468 Did you test your decoder or demuxer against damaged data? If no, see
469 tools/trasher, the noise bitstream filter, and
470 @uref{http://caca.zoy.org/wiki/zzuf, zzuf}. Your decoder or demuxer
471 should not crash, end in a (near) infinite loop, or allocate ridiculous
472 amounts of memory when fed damaged data.
474 Does the patch not mix functional and cosmetic changes?
476 Did you add tabs or trailing whitespace to the code? Both are forbidden.
478 Is the patch attached to the email you send?
480 Is the mime type of the patch correct? It should be text/x-diff or
481 text/x-patch or at least text/plain and not application/octet-stream.
483 If the patch fixes a bug, did you provide a verbose analysis of the bug?
485 If the patch fixes a bug, did you provide enough information, including
486 a sample, so the bug can be reproduced and the fix can be verified?
487 Note please do not attach samples >100k to mails but rather provide a
488 URL, you can upload to ftp://upload.libav.org
490 Did you provide a verbose summary about what the patch does change?
492 Did you provide a verbose explanation why it changes things like it does?
494 Did you provide a verbose summary of the user visible advantages and
495 disadvantages if the patch is applied?
497 Did you provide an example so we can verify the new feature added by the
500 If you added a new file, did you insert a license header? It should be
501 taken from Libav, not randomly copied and pasted from somewhere else.
503 You should maintain alphabetical order in alphabetically ordered lists as
504 long as doing so does not break API/ABI compatibility.
506 Lines with similar content should be aligned vertically when doing so
507 improves readability.
509 Make sure you check the return values of function and return appropriate
510 error codes. Especially memory allocation functions like @code{malloc()}
511 are notoriously left unchecked, which is a serious problem.
514 @section Patch review process
516 All patches posted to the
517 @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel}
518 mailing list will be reviewed, unless they contain a
519 clear note that the patch is not for the git master branch.
520 Reviews and comments will be posted as replies to the patch on the
521 mailing list. The patch submitter then has to take care of every comment,
522 that can be by resubmitting a changed patch or by discussion. Resubmitted
523 patches will themselves be reviewed like any other patch. If at some point
524 a patch passes review with no comments then it is approved, that can for
525 simple and small patches happen immediately while large patches will generally
526 have to be changed and reviewed many times before they are approved.
527 After a patch is approved it will be committed to the repository.
529 We will review all submitted patches, but sometimes we are quite busy so
530 especially for large patches this can take several weeks.
532 When resubmitting patches, if their size grew or during the review different
533 issues arisen please split the patch so each issue has a specific patch.
535 @anchor{Regression Tests}
536 @section Regression Tests
538 Before submitting a patch (or committing to the repository), you should at
539 least make sure that it does not break anything.
541 If the code changed has already a test present in FATE you should run it,
542 otherwise it is advised to add it.
544 Improvements to codec or demuxer might change the FATE results. Make sure
545 to commit the update reference with the change and to explain in the comment
546 why the expected result changed.
548 Please refer to @url{fate.html}.