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{doc/examples/decoding_encoding.c} to see how to use
20 @item libavformat is the library containing the file format handling (mux and
21 demux code for several formats). Look at @file{ffplay.c} to use it in a
22 player. See @file{doc/examples/muxing.c} to use it to generate audio or video
27 @section Integrating libavcodec or libavformat in your program
29 You can integrate all the source code of the libraries to link them
30 statically to avoid any version problem. All you need is to provide a
31 'config.mak' and a 'config.h' in the parent directory. See the defines
32 generated by ./configure to understand what is needed.
34 You can use libavcodec or libavformat in your commercial program, but
35 @emph{any patch you make must be published}. The best way to proceed is
36 to send your patches to the FFmpeg mailing list.
40 There are 3 ways by which code gets into ffmpeg.
42 @item Submitting Patches to the main developer mailing list
43 see @ref{Submitting patches} for details.
44 @item Directly committing changes to the main tree.
45 @item Committing changes to a git clone, for example on github.com or
46 gitorious.org. And asking us to merge these changes.
49 Whichever way, changes should be reviewed by the maintainer of the code
50 before they are committed. And they should follow the @ref{Coding Rules}.
51 The developer making the commit and the author are responsible for their changes
52 and should try to fix issues their commit causes.
57 @subsection Code formatting conventions
59 There are the following guidelines regarding the indentation in files:
64 The TAB character is forbidden outside of Makefiles as is any
65 form of trailing whitespace. Commits containing either will be
66 rejected by the git repository.
68 You should try to limit your code lines to 80 characters; however, do so if
69 and only if this improves readability.
71 The presentation is one inspired by 'indent -i4 -kr -nut'.
73 The main priority in FFmpeg is simplicity and small code size in order to
74 minimize the bug count.
77 Use the JavaDoc/Doxygen format (see examples below) so that code documentation
78 can be generated automatically. All nontrivial functions should have a comment
79 above them explaining what the function does, even if it is just one sentence.
80 All structures and their member variables should be documented, too.
82 Avoid Qt-style and similar Doxygen syntax with @code{!} in it, i.e. replace
83 @code{//!} with @code{///} and similar. Also @@ syntax should be employed
84 for markup commands, i.e. use @code{@@param} and not @code{\param}.
98 typedef struct Foobar@{
99 int var1; /**< var1 description */
100 int var2; ///< var2 description
101 /** var3 description */
109 * @@param my_parameter description of my_parameter
110 * @@return return value description
112 int myfunc(int my_parameter)
116 @subsection C language features
118 FFmpeg is programmed in the ISO C90 language with a few additional
119 features from ISO C99, namely:
122 the @samp{inline} keyword;
126 designated struct initializers (@samp{struct s x = @{ .i = 17 @};})
128 compound literals (@samp{x = (struct s) @{ 17, 23 @};})
131 These features are supported by all compilers we care about, so we will not
132 accept patches to remove their use unless they absolutely do not impair
133 clarity and performance.
135 All code must compile with recent versions of GCC and a number of other
136 currently supported compilers. To ensure compatibility, please do not use
137 additional C99 features or GCC extensions. Especially watch out for:
140 mixing statements and declarations;
142 @samp{long long} (use @samp{int64_t} instead);
144 @samp{__attribute__} not protected by @samp{#ifdef __GNUC__} or similar;
146 GCC statement expressions (@samp{(x = (@{ int y = 4; y; @})}).
149 @subsection Naming conventions
150 All names are using underscores (_), not CamelCase. For example, @samp{avfilter_get_video_buffer} is
151 a valid function name and @samp{AVFilterGetVideo} is not. The exception from this are type names, like
152 for example structs and enums; they should always be in the CamelCase
155 There are following conventions for naming variables and functions:
158 For local variables no prefix is required.
160 For variables and functions declared as @code{static} no prefixes are required.
162 For variables and functions used internally by the library, @code{ff_} prefix
164 For example, @samp{ff_w64_demuxer}.
166 For variables and functions used internally across multiple libraries, use
167 @code{avpriv_}. For example, @samp{avpriv_aac_parse_header}.
169 For exported names, each library has its own prefixes. Just check the existing
170 code and name accordingly.
173 @subsection Miscellaneous conventions
176 fprintf and printf are forbidden in libavformat and libavcodec,
177 please use av_log() instead.
179 Casts should be used only when necessary. Unneeded parentheses
180 should also be avoided if they don't make the code easier to understand.
183 @subsection Editor configuration
184 In order to configure Vim to follow FFmpeg formatting conventions, paste
185 the following snippet into your @file{.vimrc}:
187 " indentation rules for FFmpeg: 4 spaces, no tabs
193 " Allow tabs in Makefiles.
194 autocmd FileType make set noexpandtab shiftwidth=8 softtabstop=8
195 " Trailing whitespace and tabs are forbidden, so highlight them.
196 highlight ForbiddenWhitespace ctermbg=red guibg=red
197 match ForbiddenWhitespace /\s\+$\|\t/
198 " Do not highlight spaces at the end of line while typing on that line.
199 autocmd InsertEnter * match ForbiddenWhitespace /\t\|\s\+\%#\@@<!$/
202 For Emacs, add these roughly equivalent lines to your @file{.emacs.d/init.el}:
204 (c-add-style "ffmpeg"
207 (indent-tabs-mode . nil)
208 (show-trailing-whitespace . t)
210 (statement-cont . (c-lineup-assignments +)))
213 (setq c-default-style "ffmpeg")
216 @section Development Policy
220 Contributions should be licensed under the LGPL 2.1, including an
221 "or any later version" clause, or the MIT license. GPL 2 including
222 an "or any later version" clause is also acceptable, but LGPL is
225 You must not commit code which breaks FFmpeg! (Meaning unfinished but
226 enabled code which breaks compilation or compiles but does not work or
227 breaks the regression tests)
228 You can commit unfinished stuff (for testing etc), but it must be disabled
229 (#ifdef etc) by default so it does not interfere with other developers'
232 The commit message should have a short first line in the form of
233 a @samp{topic: short description} as a header, separated by a newline
234 from the body consisting of an explanation of why the change is necessary.
235 If the commit fixes a known bug on the bug tracker, the commit message
236 should include its bug ID. Referring to the issue on the bug tracker does
237 not exempt you from writing an excerpt of the bug in the commit message.
239 You do not have to over-test things. If it works for you, and you think it
240 should work for others, then commit. If your code has problems
241 (portability, triggers compiler bugs, unusual environment etc) they will be
242 reported and eventually fixed.
244 Do not commit unrelated changes together, split them into self-contained
245 pieces. Also do not forget that if part B depends on part A, but A does not
246 depend on B, then A can and should be committed first and separate from B.
247 Keeping changes well split into self-contained parts makes reviewing and
248 understanding them on the commit log mailing list easier. This also helps
249 in case of debugging later on.
250 Also if you have doubts about splitting or not splitting, do not hesitate to
251 ask/discuss it on the developer mailing list.
253 Do not change behavior of the programs (renaming options etc) or public
254 API or ABI without first discussing it on the ffmpeg-devel mailing list.
255 Do not remove functionality from the code. Just improve!
257 Note: Redundant code can be removed.
259 Do not commit changes to the build system (Makefiles, configure script)
260 which change behavior, defaults etc, without asking first. The same
261 applies to compiler warning fixes, trivial looking fixes and to code
262 maintained by other developers. We usually have a reason for doing things
263 the way we do. Send your changes as patches to the ffmpeg-devel mailing
264 list, and if the code maintainers say OK, you may commit. This does not
265 apply to files you wrote and/or maintain.
267 We refuse source indentation and other cosmetic changes if they are mixed
268 with functional changes, such commits will be rejected and removed. Every
269 developer has his own indentation style, you should not change it. Of course
270 if you (re)write something, you can use your own style, even though we would
271 prefer if the indentation throughout FFmpeg was consistent (Many projects
272 force a given indentation style - we do not.). If you really need to make
273 indentation changes (try to avoid this), separate them strictly from real
276 NOTE: If you had to put if()@{ .. @} over a large (> 5 lines) chunk of code,
277 then either do NOT change the indentation of the inner part within (do not
278 move it to the right)! or do so in a separate commit
280 Always fill out the commit log message. Describe in a few lines what you
281 changed and why. You can refer to mailing list postings if you fix a
282 particular bug. Comments such as "fixed!" or "Changed it." are unacceptable.
284 area changed: Short 1 line description
286 details describing what and why and giving references.
288 Make sure the author of the commit is set correctly. (see git commit --author)
289 If you apply a patch, send an
290 answer to ffmpeg-devel (or wherever you got the patch from) saying that
291 you applied the patch.
293 When applying patches that have been discussed (at length) on the mailing
294 list, reference the thread in the log message.
296 Do NOT commit to code actively maintained by others without permission.
297 Send a patch to ffmpeg-devel instead. If no one answers within a reasonable
298 timeframe (12h for build failures and security fixes, 3 days small changes,
299 1 week for big patches) then commit your patch if you think it is OK.
300 Also note, the maintainer can simply ask for more time to review!
302 Subscribe to the ffmpeg-cvslog mailing list. The diffs of all commits
303 are sent there and reviewed by all the other developers. Bugs and possible
304 improvements or general questions regarding commits are discussed there. We
305 expect you to react if problems with your code are uncovered.
307 Update the documentation if you change behavior or add features. If you are
308 unsure how best to do this, send a patch to ffmpeg-devel, the documentation
309 maintainer(s) will review and commit your stuff.
311 Try to keep important discussions and requests (also) on the public
312 developer mailing list, so that all developers can benefit from them.
314 Never write to unallocated memory, never write over the end of arrays,
315 always check values read from some untrusted source before using them
316 as array index or other risky things.
318 Remember to check if you need to bump versions for the specific libav*
319 parts (libavutil, libavcodec, libavformat) you are changing. You need
320 to change the version integer.
321 Incrementing the first component means no backward compatibility to
322 previous versions (e.g. removal of a function from the public API).
323 Incrementing the second component means backward compatible change
324 (e.g. addition of a function to the public API or extension of an
325 existing data structure).
326 Incrementing the third component means a noteworthy binary compatible
327 change (e.g. encoder bug fix that matters for the decoder). The third
328 component always starts at 100 to distinguish FFmpeg from Libav.
330 Compiler warnings indicate potential bugs or code with bad style. If a type of
331 warning always points to correct and clean code, that warning should
332 be disabled, not the code changed.
333 Thus the remaining warnings can either be bugs or correct code.
334 If it is a bug, the bug has to be fixed. If it is not, the code should
335 be changed to not generate a warning unless that causes a slowdown
336 or obfuscates the code.
338 If you add a new file, give it a proper license header. Do not copy and
339 paste it from a random place, use an existing file as template.
342 We think our rules are not too hard. If you have comments, contact us.
344 Note, these rules are mostly borrowed from the MPlayer project.
346 @anchor{Submitting patches}
347 @section Submitting patches
349 First, read the @ref{Coding Rules} above if you did not yet, in particular
350 the rules regarding patch submission.
352 When you submit your patch, please use @code{git format-patch} or
353 @code{git send-email}. We cannot read other diffs :-)
355 Also please do not submit a patch which contains several unrelated changes.
356 Split it into separate, self-contained pieces. This does not mean splitting
357 file by file. Instead, make the patch as small as possible while still
358 keeping it as a logical unit that contains an individual change, even
359 if it spans multiple files. This makes reviewing your patches much easier
360 for us and greatly increases your chances of getting your patch applied.
362 Use the patcheck tool of FFmpeg to check your patch.
363 The tool is located in the tools directory.
365 Run the @ref{Regression tests} before submitting a patch in order to verify
366 it does not cause unexpected problems.
368 It also helps quite a bit if you tell us what the patch does (for example
369 'replaces lrint by lrintf'), and why (for example '*BSD isn't C99 compliant
372 Also please if you send several patches, send each patch as a separate mail,
373 do not attach several unrelated patches to the same mail.
375 Patches should be posted to the
376 @uref{http://lists.ffmpeg.org/mailman/listinfo/ffmpeg-devel, ffmpeg-devel}
377 mailing list. Use @code{git send-email} when possible since it will properly
378 send patches without requiring extra care. If you cannot, then send patches
379 as base64-encoded attachments, so your patch is not trashed during
382 Your patch will be reviewed on the mailing list. You will likely be asked
383 to make some changes and are expected to send in an improved version that
384 incorporates the requests from the review. This process may go through
385 several iterations. Once your patch is deemed good enough, some developer
386 will pick it up and commit it to the official FFmpeg tree.
388 Give us a few days to react. But if some time passes without reaction,
389 send a reminder by email. Your patch should eventually be dealt with.
392 @section New codecs or formats checklist
396 Did you use av_cold for codec initialization and close functions?
398 Did you add a long_name under NULL_IF_CONFIG_SMALL to the AVCodec or
399 AVInputFormat/AVOutputFormat struct?
401 Did you bump the minor version number (and reset the micro version
402 number) in @file{libavcodec/version.h} or @file{libavformat/version.h}?
404 Did you register it in @file{allcodecs.c} or @file{allformats.c}?
406 Did you add the AVCodecID to @file{avcodec.h}?
407 When adding new codec IDs, also add an entry to the codec descriptor
408 list in @file{libavcodec/codec_desc.c}.
410 If it has a FourCC, did you add it to @file{libavformat/riff.c},
411 even if it is only a decoder?
413 Did you add a rule to compile the appropriate files in the Makefile?
414 Remember to do this even if you're just adding a format to a file that is
415 already being compiled by some other rule, like a raw demuxer.
417 Did you add an entry to the table of supported formats or codecs in
418 @file{doc/general.texi}?
420 Did you add an entry in the Changelog?
422 If it depends on a parser or a library, did you add that dependency in
425 Did you @code{git add} the appropriate files before committing?
427 Did you make sure it compiles standalone, i.e. with
428 @code{configure --disable-everything --enable-decoder=foo}
429 (or @code{--enable-demuxer} or whatever your component is)?
433 @section patch submission checklist
437 Does @code{make fate} pass with the patch applied?
439 Was the patch generated with git format-patch or send-email?
441 Did you sign off your patch? (git commit -s)
442 See @url{http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob_plain;f=Documentation/SubmittingPatches} for the meaning
445 Did you provide a clear git commit log message?
447 Is the patch against latest FFmpeg git master branch?
449 Are you subscribed to ffmpeg-devel?
450 (the list is subscribers only due to spam)
452 Have you checked that the changes are minimal, so that the same cannot be
453 achieved with a smaller patch and/or simpler final code?
455 If the change is to speed critical code, did you benchmark it?
457 If you did any benchmarks, did you provide them in the mail?
459 Have you checked that the patch does not introduce buffer overflows or
460 other security issues?
462 Did you test your decoder or demuxer against damaged data? If no, see
463 tools/trasher, the noise bitstream filter, and
464 @uref{http://caca.zoy.org/wiki/zzuf, zzuf}. Your decoder or demuxer
465 should not crash, end in a (near) infinite loop, or allocate ridiculous
466 amounts of memory when fed damaged data.
468 Does the patch not mix functional and cosmetic changes?
470 Did you add tabs or trailing whitespace to the code? Both are forbidden.
472 Is the patch attached to the email you send?
474 Is the mime type of the patch correct? It should be text/x-diff or
475 text/x-patch or at least text/plain and not application/octet-stream.
477 If the patch fixes a bug, did you provide a verbose analysis of the bug?
479 If the patch fixes a bug, did you provide enough information, including
480 a sample, so the bug can be reproduced and the fix can be verified?
481 Note please do not attach samples >100k to mails but rather provide a
482 URL, you can upload to ftp://upload.ffmpeg.org
484 Did you provide a verbose summary about what the patch does change?
486 Did you provide a verbose explanation why it changes things like it does?
488 Did you provide a verbose summary of the user visible advantages and
489 disadvantages if the patch is applied?
491 Did you provide an example so we can verify the new feature added by the
494 If you added a new file, did you insert a license header? It should be
495 taken from FFmpeg, not randomly copied and pasted from somewhere else.
497 You should maintain alphabetical order in alphabetically ordered lists as
498 long as doing so does not break API/ABI compatibility.
500 Lines with similar content should be aligned vertically when doing so
501 improves readability.
503 Consider to add a regression test for your code.
505 If you added YASM code please check that things still work with --disable-yasm
507 Make sure you check the return values of function and return appropriate
508 error codes. Especially memory allocation functions like @code{av_malloc()}
509 are notoriously left unchecked, which is a serious problem.
512 @section Patch review process
514 All patches posted to ffmpeg-devel will be reviewed, unless they contain a
515 clear note that the patch is not for the git master branch.
516 Reviews and comments will be posted as replies to the patch on the
517 mailing list. The patch submitter then has to take care of every comment,
518 that can be by resubmitting a changed patch or by discussion. Resubmitted
519 patches will themselves be reviewed like any other patch. If at some point
520 a patch passes review with no comments then it is approved, that can for
521 simple and small patches happen immediately while large patches will generally
522 have to be changed and reviewed many times before they are approved.
523 After a patch is approved it will be committed to the repository.
525 We will review all submitted patches, but sometimes we are quite busy so
526 especially for large patches this can take several weeks.
528 If you feel that the review process is too slow and you are willing to try to
529 take over maintainership of the area of code you change then just clone
530 git master and maintain the area of code there. We will merge each area from
531 where its best maintained.
533 When resubmitting patches, please do not make any significant changes
534 not related to the comments received during review. Such patches will
535 be rejected. Instead, submit significant changes or new features as
538 @anchor{Regression tests}
539 @section Regression tests
541 Before submitting a patch (or committing to the repository), you should at least
542 test that you did not break anything.
544 Running 'make fate' accomplishes this, please see @url{fate.html} for details.
546 [Of course, some patches may change the results of the regression tests. In
547 this case, the reference results of the regression tests shall be modified
550 @subsection Adding files to the fate-suite dataset
552 When there is no muxer or encoder available to generate test media for a
553 specific test then the media has to be inlcuded in the fate-suite.
554 First please make sure that the sample file is as small as possible to test the
555 respective decoder or demuxer sufficiently. Large files increase network
556 bandwidth and disk space requirements.
557 Once you have a working fate test and fate sample, provide in the commit
558 message or introductionary message for the patch series that you post to
559 the ffmpeg-devel mailing list, a direct link to download the sample media.