\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 anddecoding). Look at @file{libavcodec/apiexample.c} to see how to use it. @item libavformat is the library containing the file format handling (mux anddemux code for several formats). Look at @file{avplay.c} to use it in aplayer. See @file{libavformat/output-example.c} to use it to generateaudio or video streams. @end itemize @section Integrating libav in your program Shared libraries should be used whenever is possible in order to reducethe effort distributors have to pour to support programs and to ensureonly the public api is used. You can use Libav in your commercial program, but you must abide to thelicense, LGPL or GPL depending on the specific features used, please referto @uref{http://libav.org/legal.html, our legal page} for a quick checklist and tothe 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 conventionsThe code is written in K&R C style. That means the following:@itemize @bullet@itemThe control statements are formatted by putting space between the statementand parenthesis in the following way:@examplefor (i = 0; i < filter->input_count; i++) @{@end example@itemThe case statement is always located at the same level as the switch itself:@exampleswitch (link->init_state) @{case AVLINK_INIT:    continue;case AVLINK_STARTINIT:    av_log(filter, AV_LOG_INFO, "circular filter chain detected");    return 0;@end example@itemBraces in function declarations are written on the new line:@exampleconst char *avfilter_configuration(void)@{    return LIBAV_CONFIGURATION;@}@end example@itemDo 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.@itemIn case of a single-statement if, no curly braces are required:@exampleif (!pic || !picref)    goto fail;@end example@itemDo not put spaces immediately inside parentheses. @samp{if (ret)} isa valid style; @samp{if ( ret )} is not.@end itemize There are the following guidelines regarding the indentation in files:@itemize @bullet@itemIndent size is 4.@itemThe TAB character is forbidden outside of Makefiles as is anyform of trailing whitespace. Commits containing either will berejected by the git repository.@itemYou should try to limit your code lines to 80 characters; however, do so ifand only if this improves readability.@end itemizeThe presentation is one inspired by 'indent -i4 -kr -nut'. The main priority in Libav is simplicity and small code size in order tominimize the bug count. @subsection CommentsUse the JavaDoc/Doxygen  format (see examples below) so that code documentationcan be generated automatically. All nontrivial functions should have a commentabove 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 employedfor 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 additionalfeatures from ISO C99, namely:@itemize @bullet@itemthe @samp{inline} keyword;@item@samp{//} comments;@itemdesignated struct initializers (@samp{struct s x = @{ .i = 17 @};})@itemcompound literals (@samp{x = (struct s) @{ 17, 23 @};})@end itemize These features are supported by all compilers we care about, so we will notaccept patches to remove their use unless they absolutely do not impairclarity and performance. All code must compile with recent versions of GCC and a number of othercurrently supported compilers. To ensure compatibility, please do not useadditional C99 features or GCC extensions. Especially watch out for:@itemize @bullet@itemmixing statements and declarations;@item@samp{long long} (use @samp{int64_t} instead);@item@samp{__attribute__} not protected by @samp{#ifdef __GNUC__} or similar;@itemGCC statement expressions (@samp{(x = (@{ int y = 4; y; @})}).@end itemize @subsection Naming conventionsAll 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 structurenames; they should always be in the CamelCase There are following conventions for naming variables and functions:@itemize @bullet@itemFor local variables no prefix is required.@itemFor variables and functions declared as @code{static} no prefixes are required.@itemFor variables and functions used internally by the library, @code{ff_} prefixshould be used.For example, @samp{ff_w64_demuxer}.@itemFor variables and functions used internally across multiple libraries, use@code{avpriv_}. For example, @samp{avpriv_aac_parse_header}.@itemFor exported names, each library has its own prefixes. Just check the existingcode and name accordingly.@end itemize @subsection Miscellanous conventions@itemize @bullet@itemfprintf and printf are forbidden in libavformat and libavcodec,please use av_log() instead.@itemCasts should be used only when necessary. Unneeded parenthesesshould also be avoided if they don't make the code easier to understand.@end itemize @subsection Editor configurationIn order to configure Vim to follow Libav formatting conventions, pastethe following snippet into your @file{.vimrc}:@example" indentation rules for libav: 4 spaces, no tabsset expandtabset shiftwidth=4set softtabstop=4set cindentset cinoptions=(0" allow tabs in Makefilesautocmd FileType make set noexpandtab shiftwidth=8 softtabstop=8" Trailing whitespace and tabs are forbidden, so highlight them.highlight ForbiddenWhitespace ctermbg=red guibg=redmatch 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 particularthe rules regarding patch submission. As stated already, please do not submit a patch which contains severalunrelated changes.Split it into separate, self-contained pieces. This does not mean splittingfile by file. Instead, make the patch as small as possible while stillkeeping it as a logical unit that contains an individual change, evenif it spans multiple files. This makes reviewing your patches much easierfor 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 verifyit does not cause unexpected problems. Patches should be posted as base64 encoded attachments (or any otherencoding which ensures that the patch will not be trashed duringtransmission) 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 compliantand has no lrint()'). This kind of explanation should be the body of thecommit 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 patcheswithout requiring extra care. Your patch will be reviewed on the mailing list. You will likely be askedto make some changes and are expected to send in an improved version thatincorporates the requests from the review. This process may go throughseveral iterations. Once your patch is deemed good enough, it will becommitted 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.@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 aclear note that the patch is not for the git master branch.Reviews and comments will be posted as replies to the patch on themailing list. The patch submitter then has to take care of every comment,

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