FFmpeg/doc/developer.texi

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

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

@top

@contents

@chapter Introduction

This text is concerned with the development @emph{of} FFmpeg itself. Information
on using the FFmpeg libraries in other programs can be found elsewhere, e.g. in:
@itemize @bullet
@item
the installed header files
@item
@url{http://ffmpeg.org/doxygen/trunk/index.html, the Doxygen documentation}
generated from the headers
@item
the examples under @file{doc/examples}
@end itemize

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}.

If you modify FFmpeg code for your own use case, you are highly encouraged to
@emph{submit your changes back to us}, using this document as a guide. There are
both pragmatic and ideological reasons to do so:
@itemize @bullet
@item
Maintaining external changes to keep up with upstream development is
time-consuming and error-prone. With your code in the main tree, it will be
maintained by FFmpeg developers.
@item
FFmpeg developers include leading experts in the field who can find bugs or
design flaws in your code.
@item
By supporting the project you find useful you ensure it continues to be
maintained and developed.
@end itemize

All proposed code changes should be submitted for review to
@url{mailto:ffmpeg-devel@@ffmpeg.org, the development mailing list}, as
described in more detail in the @ref{Submitting patches} chapter. The code
should comply with the @ref{Development Policy} and 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}
@chapter Coding Rules

@section Language

FFmpeg is mainly programmed in the ISO C11 language, except for the public
headers which must stay C99 compatible.

Compiler-specific extensions may be used with good reason, but must not be
depended on, i.e. the code must still compile and work with compilers lacking
the extension.

The following C99 features must not be used anywhere in the codebase:
@itemize @bullet
@item
variable-length arrays;

@item
complex numbers;
@end itemize

@subsection SIMD/DSP
@anchor{SIMD/DSP}

As modern compilers are unable to generate efficient SIMD or other
performance-critical DSP code from plain C, handwritten assembly is used.
Usually such code is isolated in a separate function. Then the standard approach
is writing multiple versions of this function – a plain C one that works
everywhere and may also be useful for debugging, and potentially multiple
architecture-specific optimized implementations. Initialization code then
chooses the best available version at runtime and loads it into a function
pointer; the function in question is then always called through this pointer.

The specific syntax used for writing assembly is:
@itemize @bullet
@item
NASM on x86;

@item
GAS on ARM and RISC-V.
@end itemize

A unit testing framework for assembly called @code{checkasm} lives under
@file{tests/checkasm}. All new assembly should come with @code{checkasm} tests;
adding tests for existing assembly that lacks them is also strongly encouraged.

@subsection Other languages

Other languages than C may be used in special cases:
@itemize @bullet
@item
Compiler intrinsics or inline assembly when the code in question cannot be
written in the standard way described in the @ref{SIMD/DSP} section. This
typically applies to code that needs to be inlined.

@item
Objective-C where required for interacting with macOS-specific interfaces.
@end itemize

@section Code formatting conventions

There are the following guidelines regarding the code style 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'.

@subsection Examples
Some notable examples to illustrate common code style in FFmpeg:

@itemize @bullet

@item
Space around assignments and after
@code{if}/@code{do}/@code{while}/@code{for} keywords:

@example c, good
// Good
if (condition)
    av_foo();
@end example

@example c, good
// Good
for (size_t i = 0; i < len; i++)
    av_bar(i);
@end example

@example c, good
// Good
size_t size = 0;
@end example

However no spaces between the parentheses and condition, unless it helps
readability of complex conditions, so the following should not be done:

@example c, bad
// Bad style
if ( condition )
    av_foo();
@end example

@item
No unnecessary parentheses, unless it helps readability:

@example c, good
// Good
int fields = ilace ? 2 : 1;
@end example

@item
Don't wrap single-line blocks in braces. Use braces only if there is an accompanying else statement. This keeps future code changes easier to keep track of.

@example c, good
// Good
if (bits_pixel == 24) @{
    avctx->pix_fmt = AV_PIX_FMT_BGR24;
@} else if (bits_pixel == 8) @{
    avctx->pix_fmt = AV_PIX_FMT_GRAY8;
@} else
    return AVERROR_INVALIDDATA;

@end example

@item
Avoid assignments in conditions where it makes sense:

@example c, good
// Good
video_enc->chroma_intra_matrix = av_mallocz(sizeof(*video_enc->chroma_intra_matrix) * 64)
if (!video_enc->chroma_intra_matrix)
    return AVERROR(ENOMEM);
@end example

@example c, bad
// Bad style
if (!(video_enc->chroma_intra_matrix = av_mallocz(sizeof(*video_enc->chroma_intra_matrix) * 64)))
    return AVERROR(ENOMEM);
@end example

@example c, good
// Ok
while ((entry = av_dict_iterate(options, entry)))
    av_log(ctx, AV_LOG_INFO, "Item '%s': '%s'\n", entry->key, entry->value);
@end example

@item
When declaring a pointer variable, the @code{*} goes with the variable not the type:

@example c, good
// Good
AVStream *stream;
@end example

@example c, bad
// Bad style
AVStream* stream;
@end example

@end itemize

If you work on a file that does not follow these guidelines consistently,
change the parts that you are editing to follow these guidelines but do
not make unrelated changes in the file to make it conform to these.

@subsection Vim 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

@subsection Emacs configuration
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 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

@anchor{Naming conventions}
@section Naming conventions

Names of functions, variables, and struct members must be lowercase, using
underscores (_) to separate words. For example, @samp{avfilter_get_video_buffer}
is an acceptable function name and @samp{AVFilterGetVideo} is not.

Struct, union, enum, and typedeffed type names must use CamelCase. All structs
and unions should be typedeffed to the same name as the struct/union tag, e.g.
@code{typedef struct AVFoo @{ ... @} AVFoo;}. Enums are typically not
typedeffed.

Enumeration constants and macros must be UPPERCASE, except for macros
masquerading as functions, which should use the function naming convention.

All identifiers in the libraries should be namespaced as follows:
@itemize @bullet
@item
No namespacing for identifiers with file and lower scope (e.g. local variables,
static functions), and struct and union members,

@item
The @code{ff_} prefix must be used for variables and functions visible outside
of file scope, but only used internally within a single library, e.g.
@samp{ff_w64_demuxer}. This prevents name collisions when FFmpeg is statically
linked.

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

@item
All other internal identifiers, like private type or macro names, should be
namespaced only to avoid possible internal conflicts. E.g. @code{H264_NAL_SPS}
vs. @code{HEVC_NAL_SPS}.

@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.

@item
Other public identifiers (struct, union, enum, macro, type names) must use their
library's public prefix (@code{AV}, @code{Sws}, or @code{Swr}).
@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.

@section Miscellaneous conventions

@itemize @bullet
@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

@anchor{Development Policy}
@chapter Development Policy

@section Code behaviour

@subheading Correctness
The code must be valid. It must not crash, abort, access invalid pointers, leak
memory, cause data races or signed integer overflow, or otherwise cause
undefined behaviour. Error codes should be checked and, when applicable,
forwarded to the caller.

@subheading Thread- and library-safety
Our libraries may be called by multiple independent callers in the same process.
These calls may happen from any number of threads and the different call sites
may not be aware of each other - e.g. a user program may be calling our
libraries directly, and use one or more libraries that also call our libraries.
The code must behave correctly under such conditions.

@subheading Robustness
The code must treat as untrusted any bytestream received from a caller or read
from a file, network, etc. It must not misbehave when arbitrary data is sent to
it - typically it should print an error message and return
@code{AVERROR_INVALIDDATA} on encountering invalid input data.

@subheading Memory allocation
The code must use the @code{av_malloc()} family of functions from
@file{libavutil/mem.h} to perform all memory allocation, except in special cases
(e.g. when interacting with an external library that requires a specific
allocator to be used).

All allocations should be checked and @code{AVERROR(ENOMEM)} returned on
failure. A common mistake is that error paths leak memory - make sure that does
not happen.

@subheading stdio
Our libraries must not access the stdio streams stdin/stdout/stderr directly
(e.g. via @code{printf()} family of functions), as that is not library-safe. For
logging, use @code{av_log()}.

@section Patches/Committing
@subheading Licenses for patches must be compatible with FFmpeg.
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.

@subheading You must not commit code which breaks FFmpeg!
This means unfinished code which is enabled and breaks compilation,
or compiles but does not work/breaks the regression tests. Code which
is unfinished but disabled may be permitted under-circumstances, like
missing samples or an implementation with a small subset of features.
Always check the mailing list for any reviewers with issues and test
FATE before you push.

@subheading Commit messages
Commit messages are highly important tools for informing other developers on
what a given change does and why. Every commit must always have a properly
filled out commit message with the following format:
@example
area changed: short 1 line description

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

If the commit addresses a known bug on our bug tracker or other external issue
(e.g. CVE), the commit message should include the relevant bug ID(s) or other
external identifiers. Note that this should be done in addition to a proper
explanation and not instead of it. Comments such as "fixed!" or "Changed it."
are not acceptable.

When applying patches that have been discussed at length on the mailing list,
reference the thread in the commit message.

@subheading Testing must be adequate but not excessive.
If it works for you, others, and passes FATE then it should be OK to commit
it, provided it fits the other committing criteria. You should not worry about
over-testing things. If your code has problems (portability, triggers
compiler bugs, unusual environment etc) they will be reported and eventually
fixed.

@subheading Do not commit unrelated changes together.
They should be 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.

@subheading Cosmetic changes should be kept in separate patches.
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

@subheading Credit the author of the patch.
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.

@subheading Credit any researchers
If a commit/patch fixes an issues found by some researcher, always credit the
researcher in the commit message for finding/reporting the issue.

@subheading Always wait long enough before pushing changes
Do NOT commit to code actively maintained by others without permission.
Send a patch to ffmpeg-devel. If no one answers within a reasonable
time-frame (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!

@section Code
@subheading Warnings for correct code may be disabled if there is no other option.
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.

@section Library public interfaces
Every library in FFmpeg provides a set of public APIs in its installed headers,
which are those listed in the variable @code{HEADERS} in that library's
@file{Makefile}. All identifiers defined in those headers (except for those
explicitly documented otherwise), and corresponding symbols exported from
compiled shared or static libraries are considered public interfaces and must
comply with the API and ABI compatibility rules described in this section.

Public APIs must be backward compatible within a given major version. I.e. any
valid user code that compiles and works with a given library version must still
compile and work with any later version, as long as the major version number is
unchanged. "Valid user code" here means code that is calling our APIs in a
documented and/or intended manner and is not relying on any undefined behavior.
Incrementing the major version may break backward compatibility, but only to the
extent described in @ref{Major version bumps}.

We also guarantee backward ABI compatibility for shared and static libraries.
I.e. it should be possible to replace a shared or static build of our library
with a build of any later version (re-linking the user binary in the static
case) without breaking any valid user binaries, as long as the major version
number remains unchanged.

@subsection Adding new interfaces
Any new public identifiers in installed headers are considered new API - this
includes new functions, structs, macros, enum values, typedefs, new fields in
existing structs, new installed headers, etc. Consider the following
guidelines when adding new APIs.

@subsubheading Motivation
While new APIs can be added relatively easily, changing or removing them is much
harder due to abovementioned compatibility requirements. You should then
consider carefully whether the functionality you are adding really needs to be
exposed to our callers as new public API.

Your new API should have at least one well-established use case outside of the
library that cannot be easily achieved with existing APIs. Every library in
FFmpeg also has a defined scope - your new API must fit within it.

@subsubheading Replacing existing APIs
If your new API is replacing an existing one, it should be strictly superior to
it, so that the advantages of using the new API outweigh the cost to the
callers of changing their code. After adding the new API you should then
deprecate the old one and schedule it for removal, as described in
@ref{Removing interfaces}.

If you deem an existing API deficient and want to fix it, the preferred approach
in most cases is to add a differently-named replacement and deprecate the
existing API rather than modify it. It is important to make the changes visible
to our callers (e.g. through compile- or run-time deprecation warnings) and make
it clear how to transition to the new API (e.g. in the Doxygen documentation or
on the wiki).

@subsubheading API design
The FFmpeg libraries are used by a variety of callers to perform a wide range of
multimedia-related processing tasks. You should therefore - within reason - try
to design your new API for the broadest feasible set of use cases and avoid
unnecessarily limiting it to a specific type of callers (e.g. just media
playback or just transcoding).

@subsubheading Consistency
Check whether similar APIs already exist in FFmpeg. If they do, try to model
your new addition on them to achieve better overall consistency.

The naming of your new identifiers should follow the @ref{Naming conventions}
and be aligned with other similar APIs, if applicable.

@subsubheading Extensibility
You should also consider how your API might be extended in the future in a
backward-compatible way. If you are adding a new struct @code{AVFoo}, the
standard approach is requiring the caller to always allocate it through a
constructor function, typically named @code{av_foo_alloc()}. This way new fields
may be added to the end of the struct without breaking ABI compatibility.
Typically you will also want a destructor - @code{av_foo_free(AVFoo**)} that
frees the indirectly supplied object (and its contents, if applicable) and
writes @code{NULL} to the supplied pointer, thus eliminating the potential
dangling pointer in the caller's memory.

If you are adding new functions, consider whether it might be desirable to tweak
their behavior in the future - you may want to add a flags argument, even though
it would be unused initially.

@subsubheading Documentation
All new APIs must be documented as Doxygen-formatted comments above the
identifiers you add to the public headers. You should also briefly mention the
change in @file{doc/APIchanges}.

@subsubheading Bump the version
Backward-incompatible API or ABI changes require incrementing (bumping) the
major version number, as described in @ref{Major version bumps}. Major
bumps are significant events that happen on a schedule - so if your change
strictly requires one you should add it under @code{#if} preprocessor guards that
disable it until the next major bump happens.

New APIs that can be added without breaking API or ABI compatibility require
bumping the minor version number.

Incrementing the third (micro) version 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.

@anchor{Removing interfaces}
@subsection Removing interfaces
Due to abovementioned compatibility guarantees, removing APIs is an involved
process that should only be undertaken with good reason. Typically a deficient,
restrictive, or otherwise inadequate API is replaced by a superior one, though
it does at times happen that we remove an API without any replacement (e.g. when
the feature it provides is deemed not worth the maintenance effort, out of scope
of the project, fundamentally flawed, etc.).

The removal has two steps - first the API is deprecated and scheduled for
removal, but remains present and functional. The second step is actually
removing the API - this is described in @ref{Major version bumps}.

To deprecate an API you should signal to our users that they should stop using
it. E.g. if you intend to remove struct members or functions, you should mark
them with @code{attribute_deprecated}. When this cannot be done, it may be
possible to detect the use of the deprecated API at runtime and print a warning
(though take care not to print it too often). You should also document the
deprecation (and the replacement, if applicable) in the relevant Doxygen
documentation block.

Finally, you should define a deprecation guard along the lines of
@code{#define FF_API_<FOO> (LIBAVBAR_VERSION_MAJOR < XX)} (where XX is the major
version in which the API will be removed) in @file{libavbar/version_major.h}
(@file{version.h} in case of @code{libavutil}). Then wrap all uses of the
deprecated API in @code{#if FF_API_<FOO> .... #endif}, so that the code will
automatically get disabled once the major version reaches XX. You can also use
@code{FF_DISABLE_DEPRECATION_WARNINGS} and @code{FF_ENABLE_DEPRECATION_WARNINGS}
to suppress compiler deprecation warnings inside these guards. You should test
that the code compiles and works with the guard macro evaluating to both true
and false.

@anchor{Major version bumps}
@subsection Major version bumps
A major version bump signifies an API and/or ABI compatibility break. To reduce
the negative effects on our callers, who are required to adapt their code,
backward-incompatible changes during a major bump should be limited to:
@itemize @bullet
@item
Removing previously deprecated APIs.

@item
Performing ABI- but not API-breaking changes, like reordering struct contents.
@end itemize

@section Documentation/Other
@subheading Subscribe to the ffmpeg-devel mailing list.
It is important to be subscribed to the
@uref{https://lists.ffmpeg.org/mailman/listinfo/ffmpeg-devel, ffmpeg-devel}
mailing list. Almost any non-trivial patch is to be sent there for review.
Other developers may have comments about your contribution. We expect you see
those comments, and to improve it if requested. (N.B. Experienced committers
have other channels, and may sometimes skip review for trivial fixes.) Also,
discussion here about bug fixes and FFmpeg improvements by other developers may
be helpful information for you. Finally, by being a list subscriber, your
contribution will be posted immediately to the list, without the moderation
hold which messages from non-subscribers experience.

However, it is more important to the project that we receive your patch than
that you be subscribed to the ffmpeg-devel list. If you have a patch, and don't
want to subscribe and discuss the patch, then please do send it to the list
anyway.

@subheading Subscribe to the ffmpeg-cvslog mailing list.
Diffs of all commits are sent to the
@uref{https://lists.ffmpeg.org/mailman/listinfo/ffmpeg-cvslog, ffmpeg-cvslog}
mailing list. Some developers read this list to review all code base changes
from all sources. Subscribing to this list is not mandatory.

@subheading Keep the documentation up to date.
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.

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

@subheading Check your entries in MAINTAINERS.
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.

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

@anchor{Submitting patches}
@chapter 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. Also ensure the correct mime type is used
(text/x-diff or text/x-patch or at least text/plain) and that only one
patch is inline or attached per mail.
You can check @url{https://patchwork.ffmpeg.org}, if your patch does not show up, its mime type
likely was wrong.

@subheading How to setup git send-email?

Please see @url{https://git-send-email.io/}.
For gmail additionally see @url{https://shallowsky.com/blog/tech/email/gmail-app-passwds.html}.

@subheading Sending patches from email clients
Using @code{git send-email} might not be desirable for everyone. The
following trick allows to send patches via email clients in a safe
way. It has been tested with Outlook and Thunderbird (with X-Unsent
extension) and might work with other applications.

Create your patch like this:

@verbatim
git format-patch -s -o "outputfolder" --add-header "X-Unsent: 1" --suffix .eml --to ffmpeg-devel@ffmpeg.org -1 1a2b3c4d
@end verbatim

Now you'll just need to open the eml file with the email application
and execute 'Send'.

@subheading Reviews
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.


@chapter 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{codec_id.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_contents.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


@chapter 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? (@code{git commit -s})
See @uref{https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/process/submitting-patches.rst, Sign your work} for the meaning
of @dfn{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 @url{https://streams.videolan.org/upload/}.

@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. All new modules
should be covered by tests. That includes demuxers, muxers, decoders, encoders
filters, bitstream filters, parsers. If its not possible to do that, add
an explanation why to your patchset, its ok to not test if there's a reason.

@item
If you added NASM code please check that things still work with --disable-x86asm.

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

@chapter 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}
@chapter 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].

@section Adding files to the fate-suite dataset

If you need a sample uploaded send a mail to samples-request.

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.

@section 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.

@section 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{Maintenance}
@chapter Maintenance process

@anchor{MAINTAINERS}
@section MAINTAINERS

The developers maintaining each part of the codebase are listed in @file{MAINTAINERS}.
Being listed in @file{MAINTAINERS}, gives one the right to have git write access to
the specific repository.

@anchor{Becoming a maintainer}
@section Becoming a maintainer

People add themselves to @file{MAINTAINERS} by sending a patch like any other code
change. These get reviewed by the community like any other patch. It is expected
that, if someone has an objection to a new maintainer, she is willing to object
in public with her full name and is willing to take over maintainership for the area.


@anchor{Release process}
@chapter 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}
@section 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.


@section 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