\input texinfo @c -*- texinfo -*- @settitle Developer Documentation @titlepage @center @titlefont{Developer Documentation} @end titlepage @top @contents @chapter Developers Guide @section API @itemize @bullet @item libavcodec is the library containing the codecs (both encoding and decoding). Look at @file{libavcodec/apiexample.c} to see how to use it. @item libavformat is the library containing the file format handling (mux and demux code for several formats). Look at @file{avplay.c} to use it in a player. See @file{libavformat/output-example.c} to use it to generate audio or video streams. @end itemize @section Integrating libav in your program Shared libraries should be used whenever is possible in order to reduce the effort distributors have to pour to support programs and to ensure only the public api is used. You can use Libav in your commercial program, but you must abide to the license, LGPL or GPL depending on the specific features used, please refer to @uref{http://libav.org/legal.html, our legal page} for a quick checklist and to the following links for the exact text of each license: @uref{http://git.libav.org/?p=libav.git;a=blob;f=COPYING.GPLv2, GPL version 2}, @uref{http://git.libav.org/?p=libav.git;a=blob;f=COPYING.GPLv3, GPL version 3}, @uref{http://git.libav.org/?p=libav.git;a=blob;f=COPYING.LGPLv2.1, LGPL version 2.1}, @uref{http://git.libav.org/?p=libav.git;a=blob;f=COPYING.LGPLv3, LGPL version 3}. Any modification to the source code can be suggested for inclusion. The best way to proceed is to send your patches to the @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel} mailing list. @anchor{Coding Rules} @section Coding Rules @subsection Code formatting conventions The code is written in K&R C style. That means the following: @itemize @bullet @item The control statements are formatted by putting space between the statement and parenthesis in the following way: @example for (i = 0; i < filter->input_count; i++) @{ @end example @item The case statement is always located at the same level as the switch itself: @example switch (link->init_state) @{ case AVLINK_INIT: continue; case AVLINK_STARTINIT: av_log(filter, AV_LOG_INFO, "circular filter chain detected"); return 0; @end example @item Braces in function declarations are written on the new line: @example const char *avfilter_configuration(void) @{ return LIBAV_CONFIGURATION; @} @end example @item Do not check for NULL values by comparison, @samp{if (p)} and @samp{if (!p)} are correct; @samp{if (p == NULL)} and @samp{if (p != NULL)} are not. @item In case of a single-statement if, no curly braces are required: @example if (!pic || !picref) goto fail; @end example @item Do not put spaces immediately inside parentheses. @samp{if (ret)} is a valid style; @samp{if ( ret )} is not. @end itemize There are the following guidelines regarding the indentation in files: @itemize @bullet @item Indent size is 4. @item The TAB character is forbidden outside of Makefiles as is any form of trailing whitespace. Commits containing either will be rejected by the git repository. @item You should try to limit your code lines to 80 characters; however, do so if and only if this improves readability. @end itemize The presentation is one inspired by 'indent -i4 -kr -nut'. The main priority in Libav is simplicity and small code size in order to minimize the bug count. @subsection Comments Use the JavaDoc/Doxygen format (see examples below) so that code documentation can be generated automatically. All nontrivial functions should have a comment above them explaining what the function does, even if it is just one sentence. All structures and their member variables should be documented, too. Avoid Qt-style and similar Doxygen syntax with @code{!} in it, i.e. replace @code{//!} with @code{///} and similar. Also @@ syntax should be employed for markup commands, i.e. use @code{@@param} and not @code{\param}. @example /** * @@file * MPEG codec. * @@author ... */ /** * Summary sentence. * more text ... * ... */ typedef struct Foobar@{ int var1; /**< var1 description */ int var2; ///< var2 description /** var3 description */ int var3; @} Foobar; /** * Summary sentence. * more text ... * ... * @@param my_parameter description of my_parameter * @@return return value description */ int myfunc(int my_parameter) ... @end example @subsection C language features Libav is programmed in the ISO C90 language with a few additional features from ISO C99, namely: @itemize @bullet @item the @samp{inline} keyword; @item @samp{//} comments; @item designated struct initializers (@samp{struct s x = @{ .i = 17 @};}) @item compound literals (@samp{x = (struct s) @{ 17, 23 @};}) @end itemize These features are supported by all compilers we care about, so we will not accept patches to remove their use unless they absolutely do not impair clarity and performance. All code must compile with recent versions of GCC and a number of other currently supported compilers. To ensure compatibility, please do not use additional C99 features or GCC extensions. Especially watch out for: @itemize @bullet @item mixing statements and declarations; @item @samp{long long} (use @samp{int64_t} instead); @item @samp{__attribute__} not protected by @samp{#ifdef __GNUC__} or similar; @item GCC statement expressions (@samp{(x = (@{ int y = 4; y; @})}). @end itemize @subsection Naming conventions All names are using underscores (_), not CamelCase. For example, @samp{avfilter_get_video_buffer} is a valid function name and @samp{AVFilterGetVideo} is not. The only exception from this are structure names; they should always be in the CamelCase There are following conventions for naming variables and functions: @itemize @bullet @item For local variables no prefix is required. @item For variables and functions declared as @code{static} no prefixes are required. @item For variables and functions used internally by the library, @code{ff_} prefix should be used. For example, @samp{ff_w64_demuxer}. @item For variables and functions used internally across multiple libraries, use @code{avpriv_}. For example, @samp{avpriv_aac_parse_header}. @item For exported names, each library has its own prefixes. Just check the existing code and name accordingly. @end itemize @subsection Miscellanous conventions @itemize @bullet @item fprintf and printf are forbidden in libavformat and libavcodec, please use av_log() instead. @item Casts should be used only when necessary. Unneeded parentheses should also be avoided if they don't make the code easier to understand. @end itemize @subsection Editor configuration In order to configure Vim to follow Libav formatting conventions, paste the following snippet into your @file{.vimrc}: @example " indentation rules for libav: 4 spaces, no tabs set expandtab set shiftwidth=4 set softtabstop=4 set cindent set cinoptions=(0 " allow tabs in Makefiles autocmd FileType make set noexpandtab shiftwidth=8 softtabstop=8 " Trailing whitespace and tabs are forbidden, so highlight them. highlight ForbiddenWhitespace ctermbg=red guibg=red match ForbiddenWhitespace /\s\+$\|\t/ " Do not highlight spaces at the end of line while typing on that line. autocmd InsertEnter * match ForbiddenWhitespace /\t\|\s\+\%#\@@<!$/ @end example For Emacs, add these roughly equivalent lines to your @file{.emacs.d/init.el}: @example (c-add-style "libav" '("k&r" (c-basic-offset . 4) (indent-tabs-mode nil) (show-trailing-whitespace t) (c-offsets-alist (statement-cont . (c-lineup-assignments +))) ) ) (setq c-default-style "libav") @end example @section Development Policy @enumerate @item Contributions should be licensed under the LGPL 2.1, including an "or any later version" clause, or the MIT license. GPL 2 including an "or any later version" clause is also acceptable, but LGPL is preferred. @item All the patches MUST be reviewed in the mailing list before they are committed. @item The Libav coding style should remain consistent. Changes to conform will be suggested during the review or implemented on commit. @item Patches should be generated using @code{git format-patch} or directly sent using @code{git send-email}. Please make sure you give the proper credit by setting the correct author in the commit. @item The commit message should have a short first line in the form of @samp{topic: short description} as header, separated by a newline from the body consting in few lines explaining the reason of the patch. Referring to the issue on the bug tracker does not exempt to report an excerpt of the bug. @item Work in progress patches should be sent to the mailing list with the [WIP] or the [RFC] tag. @item Branches in public personal repos are advised as way to work on issues collaboratively. @item You do not have to over-test things. If it works for you and you think it should work for others, send it to the mailing list for review. If you have doubt about portability please state it in the submission so people with specific hardware could test it. @item Do not commit unrelated changes together, split them into self-contained pieces. Also do not forget that if part B depends on part A, but A does not depend on B, then A can and should be committed first and separate from B. Keeping changes well split into self-contained parts makes reviewing and understanding them on the commit log mailing list easier. This also helps in case of debugging later on. @item Patches that change behavior of the programs (renaming options etc) or public API or ABI should be discussed in depth and possible few days should pass between discussion and commit. Changes to the build system (Makefiles, configure script) which alter the expected behavior should be considered in the same regard. @item When applying patches that have been discussed (at length) on the mailing list, reference the thread in the log message. @item Subscribe to the @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel} and @uref{https://lists.libav.org/mailman/listinfo/libav-commits, libav-commits} mailing lists. Bugs and possible improvements or general questions regarding commits are discussed on libav-devel. We expect you to react if problems with your code are uncovered. @item Update the documentation if you change behavior or add features. If you are unsure how best to do this, send an [RFC] patch to libav-devel. @item All discussions and decisions should be reported on the public developer mailing list, so that there is a reference to them. Other media (e.g. IRC) should be used for coordination and immediate collaboration. @item Never write to unallocated memory, never write over the end of arrays, always check values read from some untrusted source before using them as array index or other risky things. Always use valgrind to doublecheck. @item Remember to check if you need to bump versions for the specific libav parts (libavutil, libavcodec, libavformat) you are changing. You need to change the version integer. Incrementing the first component means no backward compatibility to previous versions (e.g. removal of a function from the public API). Incrementing the second component means backward compatible change (e.g. addition of a function to the public API or extension of an existing data structure). Incrementing the third component means a noteworthy binary compatible change (e.g. encoder bug fix that matters for the decoder). @item Compiler warnings indicate potential bugs or code with bad style. If it is a bug, the bug has to be fixed. If it is not, the code should be changed to not generate a warning unless that causes a slowdown or obfuscates the code. If a type of warning leads to too many false positives, that warning should be disabled, not the code changed. @item If you add a new file, give it a proper license header. Do not copy and paste it from a random place, use an existing file as template. @end enumerate We think our rules are not too hard. If you have comments, contact us. Note, some rules were borrowed from the MPlayer project. @section Submitting patches First, read the @ref{Coding Rules} above if you did not yet, in particular the rules regarding patch submission. As stated already, please do not submit a patch which contains several unrelated changes. Split it into separate, self-contained pieces. This does not mean splitting file by file. Instead, make the patch as small as possible while still keeping it as a logical unit that contains an individual change, even if it spans multiple files. This makes reviewing your patches much easier for us and greatly increases your chances of getting your patch applied. Use the patcheck tool of Libav to check your patch. The tool is located in the tools directory. Run the @ref{Regression Tests} before submitting a patch in order to verify it does not cause unexpected problems. Patches should be posted as base64 encoded attachments (or any other encoding which ensures that the patch will not be trashed during transmission) to the @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel} mailing list. It also helps quite a bit if you tell us what the patch does (for example 'replaces lrint by lrintf'), and why (for example '*BSD isn't C99 compliant and has no lrint()'). This kind of explanation should be the body of the commit message. Also please if you send several patches, send each patch as a separate mail, do not attach several unrelated patches to the same mail. Use @code{git send-email} when possible since it will properly send patches without requiring extra care. Your patch will be reviewed on the mailing list. You will likely be asked to make some changes and are expected to send in an improved version that incorporates the requests from the review. This process may go through several iterations. Once your patch is deemed good enough, it will be committed to the official Libav tree. Give us a few days to react. But if some time passes without reaction, send a reminder by email. Your patch should eventually be dealt with. @section New codecs or formats checklist @enumerate @item Did you use av_cold for codec initialization and close functions? @item Did you add a long_name under NULL_IF_CONFIG_SMALL to the AVCodec or AVInputFormat/AVOutputFormat struct? @item Did you bump the minor version number (and reset the micro version number) in @file{libavcodec/version.h} or @file{libavformat/version.h}? @item Did you register it in @file{allcodecs.c} or @file{allformats.c}? @item Did you add the CodecID to @file{avcodec.h}? @item If it has a fourcc, did you add it to @file{libavformat/riff.c}, even if it is only a decoder? @item Did you add a rule to compile the appropriate files in the Makefile? Remember to do this even if you are just adding a format to a file that is already being compiled by some other rule, like a raw demuxer. @item Did you add an entry to the table of supported formats or codecs in @file{doc/general.texi}? @item Did you add an entry in the Changelog? @item If it depends on a parser or a library, did you add that dependency in configure? @item Did you @code{git add} the appropriate files before committing? @item Did you make sure it compiles standalone, i.e. with @code{configure --disable-everything --enable-decoder=foo} (or @code{--enable-demuxer} or whatever your component is)? @end enumerate @section patch submission checklist @enumerate @item Does @code{make check} pass with the patch applied? @item Is the patch against latest Libav git master branch? @item Are you subscribed to the @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel} mailing list? (Only list subscribers are allowed to post.) @item Have you checked that the changes are minimal, so that the same cannot be achieved with a smaller patch and/or simpler final code? @item If the change is to speed critical code, did you benchmark it? @item If you did any benchmarks, did you provide them in the mail? @item Have you checked that the patch does not introduce buffer overflows or other security issues? @item Did you test your decoder or demuxer against damaged data? If no, see tools/trasher and the noise bitstream filter. Your decoder or demuxer should not crash or end in a (near) infinite loop when fed damaged data. @item Does the patch not mix functional and cosmetic changes? @item Did you add tabs or trailing whitespace to the code? Both are forbidden. @item Is the patch attached to the email you send? @item Is the mime type of the patch correct? It should be text/x-diff or text/x-patch or at least text/plain and not application/octet-stream. @item If the patch fixes a bug, did you provide a verbose analysis of the bug? @item If the patch fixes a bug, did you provide enough information, including a sample, so the bug can be reproduced and the fix can be verified? Note please do not attach samples >100k to mails but rather provide a URL, you can upload to ftp://upload.libav.org @item Did you provide a verbose summary about what the patch does change? @item Did you provide a verbose explanation why it changes things like it does? @item Did you provide a verbose summary of the user visible advantages and disadvantages if the patch is applied? @item Did you provide an example so we can verify the new feature added by the patch easily? @item If you added a new file, did you insert a license header? It should be taken from Libav, not randomly copied and pasted from somewhere else. @item You should maintain alphabetical order in alphabetically ordered lists as long as doing so does not break API/ABI compatibility. @item Lines with similar content should be aligned vertically when doing so improves readability. @item Make sure you check the return values of function and return appropriate error codes. Especially memory allocation functions like @code{malloc()} are notoriously left unchecked, which is a serious problem. @end enumerate @section Patch review process All patches posted to the @uref{https://lists.libav.org/mailman/listinfo/libav-devel, libav-devel} mailing list will be reviewed, unless they contain a clear note that the patch is not for the git master branch. Reviews and comments will be posted as replies to the patch on the mailing list. The patch submitter then has to take care of every comment, that can be by resubmitting a changed patch or by discussion. Resubmitted patches will themselves be reviewed like any other patch. If at some point a patch passes review with no comments then it is approved, that can for simple and small patches happen immediately while large patches will generally have to be changed and reviewed many times before they are approved. After a patch is approved it will be committed to the repository. We will review all submitted patches, but sometimes we are quite busy so especially for large patches this can take several weeks. When resubmitting patches, if their size grew or during the review different issues arisen please split the patch so each issue has a specific patch. @anchor{Regression Tests} @section Regression Tests Before submitting a patch (or committing to the repository), you should at least make sure that it does not break anything. If the code changed has already a test present in FATE you should run it, otherwise it is advised to add it. Improvements to codec or demuxer might change the FATE results. Make sure to commit the update reference with the change and to explain in the comment why the expected result changed. Please refer to @url{fate.html}. @bye