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<a name="SEC_Top"></a>
<h1 class="settitle">Developer Documentation</h1>

<a name="SEC_Contents"></a>
<h1>Table of Contents</h1>
<div class="contents">

<ul class="toc">
  <li><a name="toc-Developers-Guide" href="#Developers-Guide">1. Developers Guide</a>
  <ul class="toc">
    <li><a name="toc-API" href="#API">1.1 API</a></li>
    <li><a name="toc-Integrating-libav-in-your-program" href="#Integrating-libav-in-your-program">1.2 Integrating libav in your program</a></li>
    <li><a name="toc-Coding-Rules-1" href="#Coding-Rules-1">1.3 Coding Rules</a>
    <ul class="toc">
      <li><a name="toc-Code-formatting-conventions" href="#Code-formatting-conventions">1.3.1 Code formatting conventions</a></li>
      <li><a name="toc-Comments" href="#Comments">1.3.2 Comments</a></li>
      <li><a name="toc-C-language-features" href="#C-language-features">1.3.3 C language features</a></li>
      <li><a name="toc-Naming-conventions" href="#Naming-conventions">1.3.4 Naming conventions</a></li>
      <li><a name="toc-Miscellanous-conventions" href="#Miscellanous-conventions">1.3.5 Miscellanous conventions</a></li>
      <li><a name="toc-Editor-configuration" href="#Editor-configuration">1.3.6 Editor configuration</a></li>
    </ul></li>
    <li><a name="toc-Development-Policy" href="#Development-Policy">1.4 Development Policy</a></li>
    <li><a name="toc-Submitting-patches" href="#Submitting-patches">1.5 Submitting patches</a></li>
    <li><a name="toc-New-codecs-or-formats-checklist" href="#New-codecs-or-formats-checklist">1.6 New codecs or formats checklist</a></li>
    <li><a name="toc-patch-submission-checklist" href="#patch-submission-checklist">1.7 patch submission checklist</a></li>
    <li><a name="toc-Patch-review-process" href="#Patch-review-process">1.8 Patch review process</a></li>
    <li><a name="toc-Regression-Tests-1" href="#Regression-Tests-1">1.9 Regression Tests</a></li>
  </ul>
</li>
</ul>
</div>

<a name="Developers-Guide"></a>
<h1 class="chapter"><a href="developer.html#toc-Developers-Guide">1. Developers Guide</a></h1>

<a name="API"></a>
<h2 class="section"><a href="developer.html#toc-API">1.1 API</a></h2>
<ul>
<li> libavcodec is the library containing the codecs (both encoding and
decoding). Look at &lsquo;<tt>libavcodec/apiexample.c</tt>&rsquo; to see how to use it.

</li><li> libavformat is the library containing the file format handling (mux and
demux code for several formats). Look at &lsquo;<tt>avplay.c</tt>&rsquo; to use it in a
player. See &lsquo;<tt>libavformat/output-example.c</tt>&rsquo; to use it to generate
audio or video streams.

</li></ul>

<a name="Integrating-libav-in-your-program"></a>
<h2 class="section"><a href="developer.html#toc-Integrating-libav-in-your-program">1.2 Integrating libav in your program</a></h2>

<p>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.
</p>
<p>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 <a href="http://libav.org/legal.html">our legal page</a> for a quick checklist and to
the following links for the exact text of each license:
<a href="http://git.libav.org/?p=libav.git;a=blob;f=COPYING.GPLv2">GPL version 2</a>,
<a href="http://git.libav.org/?p=libav.git;a=blob;f=COPYING.GPLv3">GPL version 3</a>,
<a href="http://git.libav.org/?p=libav.git;a=blob;f=COPYING.LGPLv2.1">LGPL version 2.1</a>,
<a href="http://git.libav.org/?p=libav.git;a=blob;f=COPYING.LGPLv3">LGPL version 3</a>.
Any modification to the source code can be suggested for inclusion.
The best way to proceed is to send your patches to the
<a href="https://lists.libav.org/mailman/listinfo/libav-devel">libav-devel</a>
mailing list.
</p>
<p><a name="Coding-Rules"></a>
</p><a name="Coding-Rules-1"></a>
<h2 class="section"><a href="developer.html#toc-Coding-Rules-1">1.3 Coding Rules</a></h2>

<a name="Code-formatting-conventions"></a>
<h3 class="subsection"><a href="developer.html#toc-Code-formatting-conventions">1.3.1 Code formatting conventions</a></h3>
<p>The code is written in K&amp;R C style. That means the following:
</p><ul>
<li>
The control statements are formatted by putting space between the statement
and parenthesis in the following way:
<table><tr><td>&nbsp;</td><td><pre class="example">for (i = 0; i &lt; filter-&gt;input_count; i++) {
</pre></td></tr></table>
</li><li>
The case statement is always located at the same level as the switch itself:
<table><tr><td>&nbsp;</td><td><pre class="example">switch (link-&gt;init_state) {
case AVLINK_INIT:
    continue;
case AVLINK_STARTINIT:
    av_log(filter, AV_LOG_INFO, &quot;circular filter chain detected&quot;);
    return 0;
</pre></td></tr></table>
</li><li>
Braces in function declarations are written on the new line:
<table><tr><td>&nbsp;</td><td><pre class="example">const char *avfilter_configuration(void)
{
    return LIBAV_CONFIGURATION;
}
</pre></td></tr></table>
</li><li>
In case of a single-statement if, no curly braces are required:
<table><tr><td>&nbsp;</td><td><pre class="example">if (!pic || !picref)
    goto fail;
</pre></td></tr></table>
</li><li>
Do not put spaces immediately inside parentheses. &lsquo;<samp>if (ret)</samp>&rsquo; is
a valid style; &lsquo;<samp>if ( ret )</samp>&rsquo; is not.
</li></ul>

<p>There are the following guidelines regarding the indentation in files:
</p><ul>
<li>
Indent size is 4.
</li><li>
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.
</li><li>
You should try to limit your code lines to 80 characters; however, do so if
and only if this improves readability.
</li></ul>
<p>The presentation is one inspired by &rsquo;indent -i4 -kr -nut&rsquo;.
</p>
<p>The main priority in Libav is simplicity and small code size in order to
minimize the bug count.
</p>
<a name="Comments"></a>
<h3 class="subsection"><a href="developer.html#toc-Comments">1.3.2 Comments</a></h3>
<p>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.
</p>
<p>Avoid Qt-style and similar Doxygen syntax with <code>!</code> in it, i.e. replace
<code>//!</code> with <code>///</code> and similar.  Also @ syntax should be employed
for markup commands, i.e. use <code>@param</code> and not <code>\param</code>.
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">/**
 * @file
 * MPEG codec.
 * @author ...
 */

/**
 * Summary sentence.
 * more text ...
 * ...
 */
typedef struct Foobar{
    int var1; /**&lt; var1 description */
    int var2; ///&lt; 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)
...
</pre></td></tr></table>

<a name="C-language-features"></a>
<h3 class="subsection"><a href="developer.html#toc-C-language-features">1.3.3 C language features</a></h3>

<p>Libav is programmed in the ISO C90 language with a few additional
features from ISO C99, namely:
</p><ul>
<li>
the &lsquo;<samp>inline</samp>&rsquo; keyword;
</li><li>
&lsquo;<samp>//</samp>&rsquo; comments;
</li><li>
designated struct initializers (&lsquo;<samp>struct s x = { .i = 17 };</samp>&rsquo;)
</li><li>
compound literals (&lsquo;<samp>x = (struct s) { 17, 23 };</samp>&rsquo;)
</li></ul>

<p>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.
</p>
<p>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:
</p><ul>
<li>
mixing statements and declarations;
</li><li>
&lsquo;<samp>long long</samp>&rsquo; (use &lsquo;<samp>int64_t</samp>&rsquo; instead);
</li><li>
&lsquo;<samp>__attribute__</samp>&rsquo; not protected by &lsquo;<samp>#ifdef __GNUC__</samp>&rsquo; or similar;
</li><li>
GCC statement expressions (&lsquo;<samp>(x = ({ int y = 4; y; })</samp>&rsquo;).
</li></ul>

<a name="Naming-conventions"></a>
<h3 class="subsection"><a href="developer.html#toc-Naming-conventions">1.3.4 Naming conventions</a></h3>
<p>All names are using underscores (_), not CamelCase. For example,
&lsquo;<samp>avfilter_get_video_buffer</samp>&rsquo; is a valid function name and
&lsquo;<samp>AVFilterGetVideo</samp>&rsquo; is not. The only exception from this are structure
names; they should always be in the CamelCase
</p>
<p>There are following conventions for naming variables and functions:
</p><ul>
<li>
For local variables no prefix is required.
</li><li>
For variables and functions declared as <code>static</code> no prefixes are required.
</li><li>
For variables and functions used internally by the library, <code>ff_</code> prefix
should be used.
For example, &lsquo;<samp>ff_w64_demuxer</samp>&rsquo;.
</li><li>
For variables and functions used internally across multiple libraries, use
<code>avpriv_</code>. For example, &lsquo;<samp>avpriv_aac_parse_header</samp>&rsquo;.
</li><li>
For exported names, each library has its own prefixes. Just check the existing
code and name accordingly.
</li></ul>

<a name="Miscellanous-conventions"></a>
<h3 class="subsection"><a href="developer.html#toc-Miscellanous-conventions">1.3.5 Miscellanous conventions</a></h3>
<ul>
<li>
fprintf and printf are forbidden in libavformat and libavcodec,
please use av_log() instead.
</li><li>
Casts should be used only when necessary. Unneeded parentheses
should also be avoided if they don&rsquo;t make the code easier to understand.
</li></ul>

<a name="Editor-configuration"></a>
<h3 class="subsection"><a href="developer.html#toc-Editor-configuration">1.3.6 Editor configuration</a></h3>
<p>In order to configure Vim to follow Libav formatting conventions, paste
the following snippet into your &lsquo;<tt>.vimrc</tt>&rsquo;:
</p><table><tr><td>&nbsp;</td><td><pre class="example">&quot; indentation rules for libav: 4 spaces, no tabs
set expandtab
set shiftwidth=4
set softtabstop=4
&quot; allow tabs in Makefiles
autocmd FileType make set noexpandtab shiftwidth=8 softtabstop=8
&quot; Trailing whitespace and tabs are forbidden, so highlight them.
highlight ForbiddenWhitespace ctermbg=red guibg=red
match ForbiddenWhitespace /\s\+$\|\t/
&quot; Do not highlight spaces at the end of line while typing on that line.
autocmd InsertEnter * match ForbiddenWhitespace /\t\|\s\+\%#\@&lt;!$/
</pre></td></tr></table>

<p>For Emacs, add these roughly equivalent lines to your &lsquo;<tt>.emacs.d/init.el</tt>&rsquo;:
</p><table><tr><td>&nbsp;</td><td><pre class="example">(setq c-default-style &quot;k&amp;r&quot;)
(setq-default c-basic-offset 4)
(setq-default indent-tabs-mode nil)
(setq-default show-trailing-whitespace t)
</pre></td></tr></table>

<a name="Development-Policy"></a>
<h2 class="section"><a href="developer.html#toc-Development-Policy">1.4 Development Policy</a></h2>

<ol>
<li>
   Contributions should be licensed under the LGPL 2.1, including an
   &quot;or any later version&quot; clause, or the MIT license.  GPL 2 including
   an &quot;or any later version&quot; clause is also acceptable, but LGPL is
   preferred.
</li><li>
   All the patches MUST be reviewed in the mailing list before they are
   committed.
</li><li>
   The Libav coding style should remain consistent. Changes to
   conform will be suggested during the review or implemented on commit.
</li><li>
   Patches should be generated using <code>git format-patch</code> or directly sent
   using <code>git send-email</code>.
   Please make sure you give the proper credit by setting the correct author
   in the commit.
</li><li>
   The commit message should have a short first line in the form of
   &lsquo;<samp>topic: short description</samp>&rsquo; 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.
</li><li>
   Work in progress patches should be sent to the mailing list with the [WIP]
   or the [RFC] tag.
</li><li>
   Branches in public personal repos are advised as way to
   work on issues collaboratively.
</li><li>
   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.
</li><li>
   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.
</li><li>
   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.
</li><li>
   When applying patches that have been discussed (at length) on the mailing
   list, reference the thread in the log message.
</li><li>
    Subscribe to the
    <a href="https://lists.libav.org/mailman/listinfo/libav-devel">libav-devel</a> and
    <a href="https://lists.libav.org/mailman/listinfo/libav-commits">libav-commits</a>
    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.
</li><li>
    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.
</li><li>
    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.
</li><li>
    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.
</li><li>
    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).
</li><li>
    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.
</li><li>
    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.
</li></ol>

<p>We think our rules are not too hard. If you have comments, contact us.
</p>
<p>Note, some rules were borrowed from the MPlayer project.
</p>
<a name="Submitting-patches"></a>
<h2 class="section"><a href="developer.html#toc-Submitting-patches">1.5 Submitting patches</a></h2>

<p>First, read the <a href="#Coding-Rules">Coding Rules</a> above if you did not yet, in particular
the rules regarding patch submission.
</p>
<p>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.
</p>
<p>Use the patcheck tool of Libav to check your patch.
The tool is located in the tools directory.
</p>
<p>Run the <a href="#Regression-Tests">Regression Tests</a> before submitting a patch in order to verify
it does not cause unexpected problems.
</p>
<p>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
<a href="https://lists.libav.org/mailman/listinfo/libav-devel">libav-devel</a>
mailing list.
</p>
<p>It also helps quite a bit if you tell us what the patch does (for example
&rsquo;replaces lrint by lrintf&rsquo;), and why (for example &rsquo;*BSD isn&rsquo;t C99 compliant
and has no lrint()&rsquo;). This kind of explanation should be the body of the
commit message.
</p>
<p>Also please if you send several patches, send each patch as a separate mail,
do not attach several unrelated patches to the same mail.
</p>
<p>Use <code>git send-email</code> when possible since it will properly send patches
without requiring extra care.
</p>
<p>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.
</p>
<p>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.
</p>

<a name="New-codecs-or-formats-checklist"></a>
<h2 class="section"><a href="developer.html#toc-New-codecs-or-formats-checklist">1.6 New codecs or formats checklist</a></h2>

<ol>
<li>
    Did you use av_cold for codec initialization and close functions?
</li><li>
    Did you add a long_name under NULL_IF_CONFIG_SMALL to the AVCodec or
    AVInputFormat/AVOutputFormat struct?
</li><li>
    Did you bump the minor version number (and reset the micro version
    number) in &lsquo;<tt>libavcodec/version.h</tt>&rsquo; or &lsquo;<tt>libavformat/version.h</tt>&rsquo;?
</li><li>
    Did you register it in &lsquo;<tt>allcodecs.c</tt>&rsquo; or &lsquo;<tt>allformats.c</tt>&rsquo;?
</li><li>
    Did you add the CodecID to &lsquo;<tt>avcodec.h</tt>&rsquo;?
</li><li>
    If it has a fourcc, did you add it to &lsquo;<tt>libavformat/riff.c</tt>&rsquo;,
    even if it is only a decoder?
</li><li>
    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.
</li><li>
    Did you add an entry to the table of supported formats or codecs in
    &lsquo;<tt>doc/general.texi</tt>&rsquo;?
</li><li>
    Did you add an entry in the Changelog?
</li><li>
    If it depends on a parser or a library, did you add that dependency in
    configure?
</li><li>
    Did you <code>git add</code> the appropriate files before committing?
</li><li>
    Did you make sure it compiles standalone, i.e. with
    <code>configure --disable-everything --enable-decoder=foo</code>
    (or <code>--enable-demuxer</code> or whatever your component is)?
</li></ol>


<a name="patch-submission-checklist"></a>
<h2 class="section"><a href="developer.html#toc-patch-submission-checklist">1.7 patch submission checklist</a></h2>

<ol>
<li>
    Does <code>make fate</code> pass with the patch applied?
</li><li>
    Does <code>make checkheaders</code> pass with the patch applied?
</li><li>
    Is the patch against latest Libav git master branch?
</li><li>
    Are you subscribed to the
    <a href="https://lists.libav.org/mailman/listinfo/libav-devel">libav-devel</a>
    mailing list? (Only list subscribers are allowed to post.)
</li><li>
    Have you checked that the changes are minimal, so that the same cannot be
    achieved with a smaller patch and/or simpler final code?
</li><li>
    If the change is to speed critical code, did you benchmark it?
</li><li>
    If you did any benchmarks, did you provide them in the mail?
</li><li>
    Have you checked that the patch does not introduce buffer overflows or
    other security issues?
</li><li>
    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.
</li><li>
    Does the patch not mix functional and cosmetic changes?
</li><li>
    Did you add tabs or trailing whitespace to the code? Both are forbidden.
</li><li>
    Is the patch attached to the email you send?
</li><li>
    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.
</li><li>
    If the patch fixes a bug, did you provide a verbose analysis of the bug?
</li><li>
    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 &gt;100k to mails but rather provide a
    URL, you can upload to ftp://upload.libav.org
</li><li>
    Did you provide a verbose summary about what the patch does change?
</li><li>
    Did you provide a verbose explanation why it changes things like it does?
</li><li>
    Did you provide a verbose summary of the user visible advantages and
    disadvantages if the patch is applied?
</li><li>
    Did you provide an example so we can verify the new feature added by the
    patch easily?
</li><li>
    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.
</li><li>
    You should maintain alphabetical order in alphabetically ordered lists as
    long as doing so does not break API/ABI compatibility.
</li><li>
    Lines with similar content should be aligned vertically when doing so
    improves readability.
</li></ol>

<a name="Patch-review-process"></a>
<h2 class="section"><a href="developer.html#toc-Patch-review-process">1.8 Patch review process</a></h2>

<p>All patches posted to the
<a href="https://lists.libav.org/mailman/listinfo/libav-devel">libav-devel</a>
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.
</p>
<p>We will review all submitted patches, but sometimes we are quite busy so
especially for large patches this can take several weeks.
</p>
<p>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.
</p>
<p><a name="Regression-Tests"></a>
</p><a name="Regression-Tests-1"></a>
<h2 class="section"><a href="developer.html#toc-Regression-Tests-1">1.9 Regression Tests</a></h2>

<p>Before submitting a patch (or committing to the repository), you should at
least make sure that it does not break anything.
</p>
<p>If the code changed has already a test present in FATE you should run it,
otherwise it is advised to add it.
</p>
<p>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.
</p>
<p>Please refer to <a href="fate.html">fate.html</a>.
</p>
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libav-tools 4:0.8.1-0ubuntu1 / usr / share / doc / libav-tools / html / developer.html
