Developer Documentation

1. Developers Guide

1.1 API

libavcodec is the library containing the codecs (both encoding and decoding). Look at ‘libavcodec/apiexample.c’ to see how to use it.
libavformat is the library containing the file format handling (mux and demux code for several formats). Look at ‘avplay.c’ to use it in a player. See ‘libavformat/output-example.c’ to use it to generate audio or video streams.

1.2 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 our legal page for a quick checklist and to the following links for the exact text of each license: GPL version 2, GPL version 3, LGPL version 2.1, 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 libav-devel mailing list.

1.3 Coding Rules

1.3.1 Code formatting conventions

The code is written in K&R C style. That means the following:

The control statements are formatted by putting space between the statement and parenthesis in the following way:
for (i = 0; i < filter->input_count; i++) {

The case statement is always located at the same level as the switch itself:

switch (link->init_state) {
case AVLINK_INIT:
    continue;
case AVLINK_STARTINIT:
    av_log(filter, AV_LOG_INFO, "circular filter chain detected");
    return 0;

Braces in function definitions are written on the new line:

const char *avfilter_configuration(void)
{
    return LIBAV_CONFIGURATION;
}

Do not check for NULL values by comparison, ‘if (p)’ and ‘if (!p)’ are correct; ‘if (p == NULL)’ and ‘if (p != NULL)’ are not.
In case of a single-statement if, no curly braces are required:
if (!pic || !picref) goto fail;
Do not put spaces immediately inside parentheses. ‘if (ret)’ is a valid style; ‘if ( ret )’ is not.

There are the following guidelines regarding the indentation in files:

Indent size is 4.
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.
You should try to limit your code lines to 80 characters; however, do so if and only if this improves readability.

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.

1.3.2 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 ! in it, i.e. replace //! with /// and similar. Also @ syntax should be employed for markup commands, i.e. use @param and not \param.

/**
 * @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)
...

1.3.3 C language features

Libav is programmed in the ISO C90 language with a few additional features from ISO C99, namely:

the ‘inline’ keyword;
‘//’ comments;
designated struct initializers (‘struct s x = { .i = 17 };’)
compound literals (‘x = (struct s) { 17, 23 };’)

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:

mixing statements and declarations;
‘long long’ (use ‘int64_t’ instead);
‘__attribute__’ not protected by ‘#ifdef __GNUC__’ or similar;
GCC statement expressions (‘(x = ({ int y = 4; y; })’).

1.3.4 Naming conventions

All names should be composed with underscores (_), not CamelCase. For example, ‘avfilter_get_video_buffer’ is an acceptable function name and ‘AVFilterGetVideo’ is not. The only exception are structure names; they should always be CamelCase.

There are the following conventions for naming variables and functions:

For local variables no prefix is required.
For file-scope variables and functions declared as static, no prefix is required.
For variables and functions visible outside of file scope, but only used internally by a library, an ff_ prefix should be used, e.g. ‘ff_w64_demuxer’.
For variables and functions visible outside of file scope, used internally across multiple libraries, use avpriv_ as prefix, for example, ‘avpriv_aac_parse_header’.
For externally visible symbols, each library has its own prefix. Check the existing code and choose names accordingly.

Furthermore, name space reserved for the system should not be invaded. Identifiers ending in _t are reserved by POSIX. Also avoid names starting with __ or _ followed by an uppercase letter as they are reserved by the C standard. Names starting with _ are reserved at the file level and may not be used for externally visible symbols. If in doubt, just avoid names starting with _ altogether.

1.3.5 Miscellaneous conventions

fprintf and printf are forbidden in libavformat and libavcodec, please use av_log() instead.
Casts should be used only when necessary. Unneeded parentheses should also be avoided if they don’t make the code easier to understand.

1.3.6 Editor configuration

In order to configure Vim to follow Libav formatting conventions, paste the following snippet into your ‘.vimrc’:

" 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,automake set noexpandtab shiftwidth=8 softtabstop=8
" Trailing whitespace and tabs are forbidden, so highlight them.
highlight ForbiddenWhitespace ctermbg=red guibg=red
match ForbiddenWhitespace /\s\+$\|\t/
" Do not highlight spaces at the end of line while typing on that line.
autocmd InsertEnter * match ForbiddenWhitespace /\t\|\s\+\%#\@<!$/

For Emacs, add these roughly equivalent lines to your ‘.emacs.d/init.el’:

(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")

1.4 Development Policy

Contributions should be licensed under the LGPL 2.1, including an "or any later version" clause, or, if you prefer a gift-style license, the ISC or MIT license. GPL 2 including an "or any later version" clause is also acceptable, but LGPL is preferred.
All the patches MUST be reviewed in the mailing list before they are committed.
The Libav coding style should remain consistent. Changes to conform will be suggested during the review or implemented on commit.
Patches should be generated using git format-patch or directly sent using git send-email. Please make sure you give the proper credit by setting the correct author in the commit.
The commit message should have a short first line in the form of a ‘topic: short description’ as a header, separated by a newline from the body consisting of an explanation of why the change is necessary. If the commit fixes a known bug on the bug tracker, the commit message should include its bug ID. Referring to the issue on the bug tracker does not exempt you from writing an excerpt of the bug in the commit message. If the patch is a bug fix which should be backported to stable releases, i.e. a non-API/ABI-breaking bug fix, add CC: libav-stable@libav.org to the bottom of your commit message, and make sure to CC your patch to this address, too. Some git setups will do this automatically.
Work in progress patches should be sent to the mailing list with the [WIP] or the [RFC] tag.
Branches in public personal repos are advised as way to work on issues collaboratively.
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.
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.
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.
When applying patches that have been discussed (at length) on the mailing list, reference the thread in the log message.
Subscribe to the libav-devel and 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.
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.
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.
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 double-check.
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).
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.
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.

We think our rules are not too hard. If you have comments, contact us.

1.5 Submitting patches

First, read the 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 Regression Tests before submitting a patch in order to verify it does not cause unexpected problems.

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.

Patches should be posted to the libav-devel mailing list. Use git send-email when possible since it will properly send patches without requiring extra care. If you cannot, then send patches as base64-encoded attachments, so your patch is not trashed during transmission.

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.

1.6 New codecs or formats checklist

Did you use av_cold for codec initialization and close functions?
Did you add a long_name under NULL_IF_CONFIG_SMALL to the AVCodec or AVInputFormat/AVOutputFormat struct?
Did you bump the minor version number (and reset the micro version number) in ‘libavcodec/version.h’ or ‘libavformat/version.h’?
Did you register it in ‘allcodecs.c’ or ‘allformats.c’?
Did you add the AVCodecID to ‘avcodec.h’? When adding new codec IDs, also add an entry to the codec descriptor list in ‘libavcodec/codec_desc.c’.
If it has a FourCC, did you add it to ‘libavformat/riff.c’, even if it is only a decoder?
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.
Did you add an entry to the table of supported formats or codecs in ‘doc/general.texi’?
Did you add an entry in the Changelog?
If it depends on a parser or a library, did you add that dependency in configure?
Did you git add the appropriate files before committing?
Did you make sure it compiles standalone, i.e. with configure --disable-everything --enable-decoder=foo (or --enable-demuxer or whatever your component is)?

1.7 patch submission checklist

Does make check pass with the patch applied?
Is the patch against latest Libav git master branch?
Are you subscribed to the libav-devel mailing list? (Only list subscribers are allowed to post.)
Have you checked that the changes are minimal, so that the same cannot be achieved with a smaller patch and/or simpler final code?
If the change is to speed critical code, did you benchmark it?
If you did any benchmarks, did you provide them in the mail?
Have you checked that the patch does not introduce buffer overflows or other security issues?
Did you test your decoder or demuxer against damaged data? If no, see tools/trasher, the noise bitstream filter, and zzuf. Your decoder or demuxer should not crash, end in a (near) infinite loop, or allocate ridiculous amounts of memory when fed damaged data.
Does the patch not mix functional and cosmetic changes?
Did you add tabs or trailing whitespace to the code? Both are forbidden.
Is the patch attached to the email you send?
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.
If the patch fixes a bug, did you provide a verbose analysis of the bug?
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
Did you provide a verbose summary about what the patch does change?
Did you provide a verbose explanation why it changes things like it does?
Did you provide a verbose summary of the user visible advantages and disadvantages if the patch is applied?
Did you provide an example so we can verify the new feature added by the patch easily?
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.
You should maintain alphabetical order in alphabetically ordered lists as long as doing so does not break API/ABI compatibility.
Lines with similar content should be aligned vertically when doing so improves readability.
Make sure you check the return values of function and return appropriate error codes. Especially memory allocation functions like malloc() are notoriously left unchecked, which is a serious problem.

1.8 Patch review process

All patches posted to the 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.

1.9 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 fate.html.

1.9.1 Visualizing Test Coverage

The Libav build system allows visualizing the test coverage in an easy manner with the coverage tools gcov/lcov. This involves the following steps:

Configure to compile with instrumentation enabled: configure --toolchain=gcov.
Run your test case, either manually or via FATE. This can be either the full FATE regression suite, or any arbitrary invocation of any front-end tool provided by Libav, in any combination.
Run make lcov to generate coverage data in HTML format.
View lcov/index.html in your preferred HTML viewer.

You can use the command make lcov-reset to reset the coverage measurements. You will need to rerun make lcov after running a new test.

1.9.2 Using Valgrind

The configure script provides a shortcut for using valgrind to spot bugs related to memory handling. Just add the option --toolchain=valgrind-memcheck or --toolchain=valgrind-massif to your configure line, and reasonable defaults will be set for running FATE under the supervision of either the memcheck or the massif tool of the valgrind suite.

In case you need finer control over how valgrind is invoked, use the --target-exec='valgrind <your_custom_valgrind_options> option in your configure line instead.

1.10 Release process

Libav maintains a set of release branches, which are the recommended deliverable for system integrators and distributors (such as Linux distributions, etc.). At irregular times, a release manager prepares, tests and publishes tarballs on the http://libav.org website.

There are two kinds of releases:

Major releases always include the latest and greatest features and functionality.
Point releases are cut from release branches, which are named release/X, with X being the release version number.

Note that we promise to our users that shared libraries from any Libav release never break programs that have been compiled against previous versions of the same release series in any case!

However, from time to time, we do make API changes that require adaptations in applications. Such changes are only allowed in (new) major releases and require further steps such as bumping library version numbers and/or adjustments to the symbol versioning file. Please discuss such changes on the libav-devel mailing list in time to allow forward planning.

1.10.1 Criteria for Point Releases

Changes that match the following criteria are valid candidates for inclusion into a point release:

Fixes a security issue, preferably identified by a CVE number issued by http://cve.mitre.org/.
Fixes a documented bug in http://bugzilla.libav.org.
Improves the included documentation.
Retains both source code and binary compatibility with previous point releases of the same release branch.

The order for checking the rules is (1 OR 2 OR 3) AND 4.

All Libav developers are welcome to nominate commits that they push to master by mailing the libav-stable mailing list. The easiest way to do so is to include CC: libav-stable@libav.org in the commit message.

1.10.2 Release Checklist

The release process involves the following steps:

Ensure that the ‘RELEASE’ file contains the version number for the upcoming release.
File a release tracking bug in http://bugzilla.libav.org. Make sure that the bug has an alias named ReleaseX.Y for the X.Y release.
Announce the intent to do a release to the mailing list.
Reassign unresolved blocking bugs from previous release tracking bugs to the new bug.
Review patch nominations that reach the libav-stable mailing list, and push patches that fulfill the stable release criteria to the release branch.
Ensure that the FATE regression suite still passes in the release branch on at least i386 and amd64 (cf. Regression Tests).
Prepare the release tarballs in xz and gz formats, and supplementing files that contain md5 and sha1 checksums.
Publish the tarballs at http://libav.org/releases. Create and push an annotated tag in the form vX, with X containing the version number.
Build the tarballs with the Windows binaries, and publish them at http://win32.libav.org/releases.
Propose and send a patch to the libav-devel mailing list with a news entry for the website.
Publish the news entry.
Send announcement to the mailing list.

Developer Documentation

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