AAC encoder: various fixes in M/S coding

[ffmpeg] / doc / developer.texi

\input texinfo @c -*- texinfo -*-
@documentencoding UTF-8

@settitle Developer Documentation
@titlepage
@center @titlefont{Developer Documentation}
@end titlepage

@top

@contents

@chapter Developers Guide

@section Notes for external developers

This document is mostly useful for internal FFmpeg developers.
External developers who need to use the API in their application should
refer to the API doxygen documentation in the public headers, and
check the examples in @file{doc/examples} and in the source code to
see how the public API is employed.

You can use the FFmpeg libraries in your commercial program, but you
are encouraged to @emph{publish any patch you make}. In this case the
best way to proceed is to send your patches to the ffmpeg-devel
mailing list following the guidelines illustrated in the remainder of
this document.

For more detailed legal information about the use of FFmpeg in
external programs read the @file{LICENSE} file in the source tree and
consult @url{https://ffmpeg.org/legal.html}.

@section Contributing

There are 3 ways by which code gets into FFmpeg.
@itemize @bullet
@item Submitting patches to the main developer mailing list.
      See @ref{Submitting patches} for details.
@item Directly committing changes to the main tree.
@item Committing changes to a git clone, for example on github.com or
      gitorious.org. And asking us to merge these changes.
@end itemize

Whichever way, changes should be reviewed by the maintainer of the code
before they are committed. And they should follow the @ref{Coding Rules}.
The developer making the commit and the author are responsible for their changes
and should try to fix issues their commit causes.

@anchor{Coding Rules}
@section Coding Rules

@subsection Code formatting conventions

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.

@item
K&R coding style is used.
@end itemize
The presentation is one inspired by 'indent -i4 -kr -nut'.

The main priority in FFmpeg 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

FFmpeg 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 should be composed with underscores (_), not CamelCase. For example,
@samp{avfilter_get_video_buffer} is an acceptable function name and
@samp{AVFilterGetVideo} is not. The exception from this are type names, like
for example structs and enums; they should always be in CamelCase.

There are the following conventions for naming variables and functions:

@itemize @bullet
@item
For local variables no prefix is required.

@item
For file-scope variables and functions declared as @code{static}, no prefix
is required.

@item
For variables and functions visible outside of file scope, but only used
internally by a library, an @code{ff_} prefix should be used,
e.g. @samp{ff_w64_demuxer}.

@item
For variables and functions visible outside of file scope, used internally
across multiple libraries, use @code{avpriv_} as prefix, for example,
@samp{avpriv_aac_parse_header}.

@item
Each library has its own prefix for public symbols, in addition to the
commonly used @code{av_} (@code{avformat_} for libavformat,
@code{avcodec_} for libavcodec, @code{swr_} for libswresample, etc).
Check the existing code and choose names accordingly.
Note that some symbols without these prefixes are also exported for
retro-compatibility reasons. These exceptions are declared in the
@code{lib<name>/lib<name>.v} files.
@end itemize

Furthermore, name space reserved for the system should not be invaded.
Identifiers ending in @code{_t} are reserved by
@url{http://pubs.opengroup.org/onlinepubs/007904975/functions/xsh_chap02_02.html#tag_02_02_02, POSIX}.
Also avoid names starting with @code{__} or @code{_} followed by an uppercase
letter as they are reserved by the C standard. Names starting with @code{_}
are reserved at the file level and may not be used for externally visible
symbols. If in doubt, just avoid names starting with @code{_} altogether.

@subsection Miscellaneous 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 FFmpeg formatting conventions, paste
the following snippet into your @file{.vimrc}:
@example
" indentation rules for FFmpeg: 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\+$\|\t/
" Do not highlight spaces at the end of line while typing on that line.
autocmd InsertEnter * match ForbiddenWhitespace /\t\|\s\+\%#\@@<!$/
@end example

For Emacs, add these roughly equivalent lines to your @file{.emacs.d/init.el}:
@lisp
(c-add-style "ffmpeg"
             '("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 "ffmpeg")
@end lisp

@section Development Policy

@enumerate
@item
Contributions should be licensed under the
@uref{http://www.gnu.org/licenses/lgpl-2.1.html, LGPL 2.1},
including an "or any later version" clause, or, if you prefer
a gift-style license, the
@uref{http://opensource.org/licenses/isc-license.txt, ISC} or
@uref{http://mit-license.org/, MIT} license.
@uref{http://www.gnu.org/licenses/gpl-2.0.html, GPL 2} including
an "or any later version" clause is also acceptable, but LGPL is
preferred.
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.

@item
You must not commit code which breaks FFmpeg! (Meaning unfinished but
enabled code which breaks compilation or compiles but does not work or
breaks the regression tests)
You can commit unfinished stuff (for testing etc), but it must be disabled
(#ifdef etc) by default so it does not interfere with other developers'
work.

@item
The commit message should have a short first line in the form of
a @samp{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.

@item
You do not have to over-test things. If it works for you, and you think it
should work for others, then commit. If your code has problems
(portability, triggers compiler bugs, unusual environment etc) they will be
reported and eventually fixed.

@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.
Also if you have doubts about splitting or not splitting, do not hesitate to
ask/discuss it on the developer mailing list.

@item
Do not change behavior of the programs (renaming options etc) or public
API or ABI without first discussing it on the ffmpeg-devel mailing list.
Do not remove functionality from the code. Just improve!

Note: Redundant code can be removed.

@item
Do not commit changes to the build system (Makefiles, configure script)
which change behavior, defaults etc, without asking first. The same
applies to compiler warning fixes, trivial looking fixes and to code
maintained by other developers. We usually have a reason for doing things
the way we do. Send your changes as patches to the ffmpeg-devel mailing
list, and if the code maintainers say OK, you may commit. This does not
apply to files you wrote and/or maintain.

@item
We refuse source indentation and other cosmetic changes if they are mixed
with functional changes, such commits will be rejected and removed. Every
developer has his own indentation style, you should not change it. Of course
if you (re)write something, you can use your own style, even though we would
prefer if the indentation throughout FFmpeg was consistent (Many projects
force a given indentation style - we do not.). If you really need to make
indentation changes (try to avoid this), separate them strictly from real
changes.

NOTE: If you had to put if()@{ .. @} over a large (> 5 lines) chunk of code,
then either do NOT change the indentation of the inner part within (do not
move it to the right)! or do so in a separate commit

@item
Always fill out the commit log message. Describe in a few lines what you
changed and why. You can refer to mailing list postings if you fix a
particular bug. Comments such as "fixed!" or "Changed it." are unacceptable.
Recommended format:

@example
area changed: Short 1 line description

details describing what and why and giving references.
@end example

@item
Make sure the author of the commit is set correctly. (see git commit --author)
If you apply a patch, send an
answer to ffmpeg-devel (or wherever you got the patch from) saying that
you applied the patch.

@item
When applying patches that have been discussed (at length) on the mailing
list, reference the thread in the log message.

@item
Do NOT commit to code actively maintained by others without permission.
Send a patch to ffmpeg-devel instead. If no one answers within a reasonable
timeframe (12h for build failures and security fixes, 3 days small changes,
1 week for big patches) then commit your patch if you think it is OK.
Also note, the maintainer can simply ask for more time to review!

@item
Subscribe to the ffmpeg-cvslog mailing list. The diffs of all commits
are sent there and reviewed by all the other developers. Bugs and possible
improvements or general questions regarding commits are discussed there. 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 a patch to ffmpeg-devel, the documentation
maintainer(s) will review and commit your stuff.

@item
Try to keep important discussions and requests (also) on the public
developer mailing list, so that all developers can benefit from them.

@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.

@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). The third
component always starts at 100 to distinguish FFmpeg from Libav.

@item
Compiler warnings indicate potential bugs or code with bad style. If a type of
warning always points to correct and clean code, that warning should
be disabled, not the code changed.
Thus the remaining warnings can either be bugs or correct code.
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.

@item
Make sure that no parts of the codebase that you maintain are missing from the
@file{MAINTAINERS} file. If something that you want to maintain is missing add it with
your name after it.
If at some point you no longer want to maintain some code, then please help in
finding a new maintainer and also don't forget to update the @file{MAINTAINERS} file.
@end enumerate

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

@anchor{Submitting patches}
@section Submitting patches

First, read the @ref{Coding Rules} above if you did not yet, in particular
the rules regarding patch submission.

When you submit your patch, please use @code{git format-patch} or
@code{git send-email}. We cannot read other diffs :-).

Also 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 FFmpeg 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.

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()')

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
@uref{https://lists.ffmpeg.org/mailman/listinfo/ffmpeg-devel, ffmpeg-devel}
mailing list. Use @code{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, some developer
will pick it up and commit it to the official FFmpeg 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 AVCodecID to @file{avcodec.h}?
When adding new codec IDs, also add an entry to the codec descriptor
list in @file{libavcodec/codec_desc.c}.

@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're 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 fate} pass with the patch applied?

@item
Was the patch generated with git format-patch or send-email?

@item
Did you sign off your patch? (git commit -s)
See @url{http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob_plain;f=Documentation/SubmittingPatches} for the meaning
of sign off.

@item
Did you provide a clear git commit log message?

@item
Is the patch against latest FFmpeg git master branch?

@item
Are you subscribed to ffmpeg-devel?
(the list is subscribers only due to spam)

@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, the noise bitstream filter, and
@uref{http://caca.zoy.org/wiki/zzuf, zzuf}. Your decoder or demuxer
should not crash, end in a (near) infinite loop, or allocate ridiculous
amounts of memory when fed damaged data.

@item
Did you test your decoder or demuxer against sample files?
Samples may be obtained at @url{https://samples.ffmpeg.org}.

@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.ffmpeg.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 FFmpeg, 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
Consider adding a regression test for your code.

@item
If you added YASM code please check that things still work with --disable-yasm.

@item
Make sure you check the return values of function and return appropriate
error codes. Especially memory allocation functions like @code{av_malloc()}
are notoriously left unchecked, which is a serious problem.

@item
Test your code with valgrind and or Address Sanitizer to ensure it's free
of leaks, out of array accesses, etc.
@end enumerate

@section Patch review process

All patches posted to ffmpeg-devel 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.

If you feel that the review process is too slow and you are willing to try to
take over maintainership of the area of code you change then just clone
git master and maintain the area of code there. We will merge each area from
where its best maintained.

When resubmitting patches, please do not make any significant changes
not related to the comments received during review. Such patches will
be rejected. Instead, submit significant changes or new features as
separate patches.

Everyone is welcome to review patches. Also if you are waiting for your patch
to be reviewed, please consider helping to review other patches, that is a great
way to get everyone's patches reviewed sooner.

@anchor{Regression tests}
@section Regression tests

Before submitting a patch (or committing to the repository), you should at least
test that you did not break anything.

Running 'make fate' accomplishes this, please see @url{fate.html} for details.

[Of course, some patches may change the results of the regression tests. In
this case, the reference results of the regression tests shall be modified
accordingly].

@subsection Adding files to the fate-suite dataset

When there is no muxer or encoder available to generate test media for a
specific test then the media has to be included in the fate-suite.
First please make sure that the sample file is as small as possible to test the
respective decoder or demuxer sufficiently. Large files increase network
bandwidth and disk space requirements.
Once you have a working fate test and fate sample, provide in the commit
message or introductory message for the patch series that you post to
the ffmpeg-devel mailing list, a direct link to download the sample media.

@subsection Visualizing Test Coverage

The FFmpeg build system allows visualizing the test coverage in an easy
manner with the coverage tools @code{gcov}/@code{lcov}.  This involves
the following steps:

@enumerate
@item
    Configure to compile with instrumentation enabled:
    @code{configure --toolchain=gcov}.

@item
    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 FFmpeg, in any combination.

@item
    Run @code{make lcov} to generate coverage data in HTML format.

@item
    View @code{lcov/index.html} in your preferred HTML viewer.
@end enumerate

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

@subsection Using Valgrind

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

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

@anchor{Release process}
@section Release process

FFmpeg maintains a set of @strong{release branches}, which are the
recommended deliverable for system integrators and distributors (such as
Linux distributions, etc.). At regular times, a @strong{release
manager} prepares, tests and publishes tarballs on the
@url{https://ffmpeg.org} website.

There are two kinds of releases:

@enumerate
@item
@strong{Major releases} always include the latest and greatest
features and functionality.

@item
@strong{Point releases} are cut from @strong{release} branches,
which are named @code{release/X}, with @code{X} being the release
version number.
@end enumerate

Note that we promise to our users that shared libraries from any FFmpeg
release never break programs that have been @strong{compiled} against
previous versions of @strong{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 @strong{ffmpeg-devel} mailing list in time to allow forward planning.

@anchor{Criteria for Point Releases}
@subsection Criteria for Point Releases

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

@enumerate
@item
Fixes a security issue, preferably identified by a @strong{CVE
number} issued by @url{http://cve.mitre.org/}.

@item
Fixes a documented bug in @url{https://trac.ffmpeg.org}.

@item
Improves the included documentation.

@item
Retains both source code and binary compatibility with previous
point releases of the same release branch.
@end enumerate

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


@subsection Release Checklist

The release process involves the following steps:

@enumerate
@item
Ensure that the @file{RELEASE} file contains the version number for
the upcoming release.

@item
Add the release at @url{https://trac.ffmpeg.org/admin/ticket/versions}.

@item
Announce the intent to do a release to the mailing list.

@item
Make sure all relevant security fixes have been backported. See
@url{https://ffmpeg.org/security.html}.

@item
Ensure that the FATE regression suite still passes in the release
branch on at least @strong{i386} and @strong{amd64}
(cf. @ref{Regression tests}).

@item
Prepare the release tarballs in @code{bz2} and @code{gz} formats, and
supplementing files that contain @code{gpg} signatures

@item
Publish the tarballs at @url{https://ffmpeg.org/releases}. Create and
push an annotated tag in the form @code{nX}, with @code{X}
containing the version number.

@item
Propose and send a patch to the @strong{ffmpeg-devel} mailing list
with a news entry for the website.

@item
Publish the news entry.

@item
Send an announcement to the mailing list.
@end enumerate

@bye