# Developer Documentation

# Table of Contents

# 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 declarations are written on the new line:

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

- 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 are using underscores (_), not CamelCase. For example, ‘`avfilter_get_video_buffer`’ is a valid function name and ‘`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:

- For local variables no prefix is required.
- For variables and functions declared as `static` no prefixes are required.
- For variables and functions used internally by the library, `ff_` prefix should be used. For example, ‘`ff_w64_demuxer`’.
- For variables and functions used internally across multiple libraries, use `avpriv_`. For example, ‘`avpriv_aac_parse_header`’.
- For exported names, each library has its own prefixes. Just check the existing code and name accordingly.

### 1.3.5 Miscellanous 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
    " 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`’:

    (setq c-default-style "k&r")
    (setq-default c-basic-offset 4)
    (setq-default indent-tabs-mode nil)
    (setq-default show-trailing-whitespace t)

## 1.4 Development Policy

1. 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.
2. All the patches MUST be reviewed in the mailing list before they are committed.
3. The Libav coding style should remain consistent. Changes to conform will be suggested during the review or implemented on commit.
4. 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.
5. The commit message should have a short first line in the form of ‘`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.
6. Work in progress patches should be sent to the mailing list with the [WIP] or the [RFC] tag.
7. Branches in public personal repos are advised as way to work on issues collaboratively.
8. 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.
9. 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.
10. 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.
11. When applying patches that have been discussed (at length) on the mailing list, reference the thread in the log message.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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).
17. 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.
18. 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.

Note, some rules were borrowed from the MPlayer project.

## 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.

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 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 `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.

## 1.6 New codecs or formats checklist

1. Did you use av_cold for codec initialization and close functions?
2. Did you add a long_name under NULL_IF_CONFIG_SMALL to the AVCodec or AVInputFormat/AVOutputFormat struct?
3. Did you bump the minor version number (and reset the micro version number) in ‘`libavcodec/version.h`’ or ‘`libavformat/version.h`’?
4. Did you register it in ‘`allcodecs.c`’ or ‘`allformats.c`’?
5. Did you add the CodecID to ‘`avcodec.h`’?
6. If it has a fourcc, did you add it to ‘`libavformat/riff.c`’, even if it is only a decoder?
7. 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.
8. Did you add an entry to the table of supported formats or codecs in ‘`doc/general.texi`’?
9. Did you add an entry in the Changelog?
10. If it depends on a parser or a library, did you add that dependency in configure?
11. Did you `git add` the appropriate files before committing?
12. 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

1. Does `make check` pass with the patch applied?
2. Is the patch against latest Libav git master branch?
3. Are you subscribed to the libav-devel mailing list? (Only list subscribers are allowed to post.)
4. Have you checked that the changes are minimal, so that the same cannot be achieved with a smaller patch and/or simpler final code?
5. If the change is to speed critical code, did you benchmark it?
6. If you did any benchmarks, did you provide them in the mail?
7. Have you checked that the patch does not introduce buffer overflows or other security issues?
8. 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.
9. Does the patch not mix functional and cosmetic changes?
10. Did you add tabs or trailing whitespace to the code? Both are forbidden.
11. Is the patch attached to the email you send?
12. 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.
13. If the patch fixes a bug, did you provide a verbose analysis of the bug?
14. 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
15. Did you provide a verbose summary about what the patch does change?
16. Did you provide a verbose explanation why it changes things like it does?
17. Did you provide a verbose summary of the user visible advantages and disadvantages if the patch is applied?
18. Did you provide an example so we can verify the new feature added by the patch easily?
19. 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.
20. You should maintain alphabetical order in alphabetically ordered lists as long as doing so does not break API/ABI compatibility.
21. Lines with similar content should be aligned vertically when doing so improves readability.

## 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.