blob: 692b0260c7270059af21e2a794bc57bdb8428c4a [file] [log] [blame]
Name: ffmpeg
License: LGPL 2.1
License File:
Upstream Git: git://
Last Upstream Merge: 5e3da256184e13502b3428f94e180888436d157b, Sep 11 2014
This file documents the layout of the Chromium copy of FFmpeg git, some common
tasks, how to produce the FFmpeg include directory, and how to create the
ffmpeg.gyp file and related configurations.
FFmpeg Git Layout:
Chromium's copy of FFmpeg is a fork of the upstream git repository, see the tags
above for up-to-date cut information. All of the chromium specific bits, minus
the gyp files, are located under the chromium/ folder off the root.
ffmpeg.gyp: See the section on ffmpeg.gyp below.
ffmpeg_generated.gypi: Pregenerated gyp listing of files necessary to build
every platform. See chromium/scripts/ for more details.
chromium/binaries/c99conv.exe: Prebuilt binary for converting FFmpeg sources
from C99 to C89 for compilation with Visual C++.
chromium/scripts: Utilities for building the gyp and config files.
chromium/config/...: Pregenerated FFmpeg config options for each platform
and architecture; e.g., ChromeOS: ARM-Neon, Linux: X64, etc.
chromium/patches/...: Chromium specific changes which haven't yet made it
upstream. See chromium/patches/README for more details on each patch.
Historically, the .patch files were staged on top of a source tarball
instead of the Git repository we have now. The .patch files are kept for
tracking purposes. The new system only requires that you add an entry to
the README file with a link to the tracking issue and code review.
Help w/ Common Tasks:
-- Submitting changes to Chromium's FFmpeg git repository.
The goal of Chromium's FFmpeg repository is to just be a mirror of the upstream
Git repository. Which means every change made must be upstreamed. If you make
a change, please add an entry to chromium/patches/README with a link to the
tracking issue and code review for your change.
Unfortunately the normal Chromium CL submission process through Rietveld does
not work with Git DEPS like FFmpeg, so you must use Gerrit to upload your change
for review:
git push ssh:// \
This will create a gerrit chanage. Make sure to "git commit --amend" and
add "Change-Id: XXX" as the last line so that future pushes do not create new
changes. You can get the "XXX" from the gerrit web interface.
Note: You'll need a Gerrit account, see:
You also need to add an ssh key at . Generate a key
following the GitHub guide linked from there, then copy the contents of your
~/.ssh/ file to that gerrit page.
If your change adds new files to the repository, you'll need to regenerate the
GYP defines by following the directions in chromium/scripts/
After all that's done and your change has landed in the Git repository, you'll
need to roll DEPS before the change will show up in Chrome.
-- Performing an upstream merge.
The upstream merge process follows the normal Git merge process:
# First, modify the origin url to enable push of merge result later.
# e.g., change remote.origin.url from:
# to:
# ssh://
# One way is to use custom_deps in .gclient. Another might be to run
# git config -e within this folder and edit the URLs if already synced,
# though this alternative could have danger.
git remote add upstream git://
git fetch upstream
git merge upstream/master
# If conflicts:
git mergetool
<Except for a different push method, follow rest of "Submitting changes to
Chromium's FFmpeg git repository.">
# Use git push for submission so commit history is kept. Might require merge
# approval on your Gerrit account. Do not commit any other way.
git push origin master
Once complete you'll need to regenerate the configuration files for every
platform (see "Short Directions" in the "Building" section below). You'll also
need to regenerate the gyp files (see chromium/scripts/ After
all that is done, you can update Chromium's root DEPS file to point to the tip
of tree commit.
-- Upstreaming a patch.
<checkout copy of upstream repo>
git checkout <hash of last Chromium FFmpeg, see tag above> -b my_patch
git apply <patch. pulled from code review or cherry-picked from this repo>
git rebase origin/master
< Follow FFmpeg guide: >
Once your change has been upstreamed, please update the chromium/patches/README
file with the status. The next time an upstream merge is done, the committer
will clear out all the patches which have been upstreamed.
-- Cherry-picking a patch from upstream.
# Tell our repo about the upstream one.
git remote add upstream git://
git fetch upstream
# Create a new branch based off of master for committing our patch.
git checkout master -b my_new_branch
# Pull the patch out of upstream.
git cherry-pick <hash of commit in upstream>
<Follow rest of "Submitting changes to Chromium's FFmpeg git repository.">
-- FFmpeg headers in the 'chromium/include' directory.
The include directory contains FFmpeg's public header files from the output of
a "make install" command. The header files are from Chromium's copy of FFmpeg.
Steps to reproduce:
1) If on Windows, refer to our MinGW/MSYS environment setup:
2) Chromium's copy of FFmpeg, should already have it if you're reading this.
3) Follow the instructions to build and install.
4) Go to your install location and copy the following into the Chromium tree:
On Windows, the libraries are linked in using /DELAYLOAD to avoid having the
DLLs present at run-time. On POSIX systems, dlopen() is used to achieve a
similar effect.
We don't use the import libraries generated from building FFmpeg because they
export every method by ordinal, which makes binary compatibility with different
builds of FFmpeg difficult if not impossible. Furthermore, it is much easier
to update a DEF file instead of rebuilding FFmpeg to generate new import
-- Recreating the ffmpeg.gyp file and populating the config directory.
The ffmpeg.gyp file is meant to be used in place of FFmpeg's
./configure && make
steps. The file was created by inspecting the build log from above.
The FFmpeg build is relatively straightforward. All files are built with
the same CFLAGS. The config.h and version.h files are the only files generated
by ./configure that are included elsewhere. They require a small bit of
Other than the configure step, FFmpeg just compiles its .c files, assembles a
few more using yasm, and that's it. Exact instructions for reproducing
ffmpeg.gyp are in the "Detailed Directions" section.
Here is a list of gotchas that have shown up.
1) FFmpeg requires special configure (--disable-optimizations) in order
to be built with -O0 successfully due to some of the hand-written
assembler using ebp. -O0 implies -fno-omit-frame-pointer which breaks
this. This will produce compiler errors like:
libavcodec/cabac.h:527: error: can't find a register in class
'GENERAL_REGS' while reloading 'asm'
cabac.h:527: error: 'asm' operand has impossible constraints
2) Sometimes, with -O0, invalid code will be exposed because dead-branch
pruning is disabled in gcc. This can manifest itself as strange link
issues or compile issues. Be careful to read all warnings in this case.
3) Since config.h is generated via ./configure, the generated file will
be sensitive to the configuration of the machine it was produced on.
In particular, yasm does not seem to always be detected if
cross-compiling for 32-bit on a 64-bit machine. Since yasm is built in
tree, make sure to force things with --enable-yasm.
4) yasm needs to be installed on mac and windows if not already there.
5) Similar to issue #3, ./configure may detect the presence of SDL and
adjust config.h accordingly. This is harmless because all the SDL
related code has been disabled in our configuration.
6) On Mac ia32, we want to be able to compile WITHOUT -fomit-frame-pointer
(so breakpad can function). To do this, we need to disable the use of the
EBP register, otherwise some of FFmpeg's inline assembly will cause
compilation errors similar to gotcha #1. For more details, see the file
comment in the This script will fix up
the generated config.h to be building without -fomit-frame-pointer.
7) On Windows, FFmpeg must first be run through a preprocessor to be compiled
via MSVC++ because it doesn't support the C99 syntax used by FFmpeg. The
converter is open source and built on libclang. The c99conv.exe in the
chromium/scripts directory is a statically linked version of the converter
from: . Compiling the dynamic linked
library is relatively easy, generating a statically linked one is not. To
build the dynamic version you first need libclang:
git clone
cd tools; git clone
Now you need to run cmake, see,
step #5. From there you can start the Visual Studio IDE with the .vcproj
or .sln files generated by cmake. Then select "Release" from the build
drop down menu. You're ready to build. Right click on the libclang target
and select build. Once complete you should have a libclang.dll in
<llvm root>\out\bin\Release. Now you can build the c99-to-c89 converter
using the packed makefile. If you're building the dynamic version you'll
need to ensure c99conv.exe and libclang.dll remain in the same directory.
Note: Building the statically linked version is painful and requires
changes to both the llvm project and the converter. Proceed at your own
risk. First, you need to modify the libclang project and change every
target's "Project Defaults" from "Dynamic Library .DLL" to "Static Library
(.lib)" and under "Code Generation" change every targets "Runtime Library"
from "Multi-threaded DLL (/MD)" to "Multi-threaded (/MT)" then rebuild the
libclang project. Afterward, you will have a lot of lib files in <llvm
root>\out\lib\Release and will need to specify all of them under LIBS in
the c99-to-c89 makefile as well as Advapi32.lib and Shell32.lib. Before
compiling c99conv.exe you will need to modify libclang a bit, open
"tools\clang\include\clang-c\Platform.h" and remove the
"__declspec(dllimport)" entry. You can now compile as normal and will only
need to package the c99conv.exe.
It's not necessary to build the statically linked version, but it's nice to
only have a single binary checked in.
8) On various platforms, ffmpeg configure may detect external iconv library
and include it by default. FFmpeg used by chromium does not depend on this
library, and the lib may not exist on target despite existing in the build
environment. Hence, we need to change CONFIG_ICONV to 0 in the config.h
(and config.asm where appropriate.) does this with
configure parameter --disable-iconv. See
Short Directions:
1) Create config.h and config.asm as needed.
On Linux (with Chromium's yasm build output in $PATH) run
./chromium/scripts/ linux ia32
./chromium/scripts/ linux x64
./chromium/scripts/ linux-noasm x64
On Linux with MIPS cross-toolchain in $PATH
./chromium/scripts/ linux mipsel
On Linux chroot run
./chromium/scripts/ linux arm
./chromium/scripts/ linux arm-neon
On Mac run
./chromium/scripts/ mac ia32
./chromium/scripts/ mac x64
On Windows run
./chromium/scripts/ win ia32
./chromium/scripts/ win x64
2) Finally, collect all these directories and copy all config files
into the source tree using
Detailed Directions:
1) Run the configure in a directory out of the tree with the arguments you
want. To see what was used before, find the config.h for the platform
of interest in:
The value of the FFMPEG_CONFIGURATION macro should have the configure
commandline that generated the file.
Note that if you are trying to build a 32-bit FFmpeg for linux on a
64-bit box, the extra flags you want to pass to ./configure are
--arch=i686 --extra-cflags=-m32 --extra-ldflags=-m32
Also, as noted in gotcha #4, explicitly setting --enable-yasm is
a good idea. (These flags have been added to
2) Copy the newly generated config.h and version.h into the correct platform
Make sure to double-check that config.h and version.h are the only files
of interest. By that, I mean check that the other generated files are
makefiles, documentation, .pc files, or something else that is not
relevant to our build.
TODO(ajwong): Check if we can modify version.h to tag our builds.
3) If on ia32, handle gotcha #6 by munging the geneated config.h file to
disable use of EBP. Call the script on
the config.h for each ia32 variant. (This has been implemented in
** This script is not idempotent. Don't run it twice **
Remember, this is only necessary for ia32 config.h files. Running this
on config.h files for other platforms (in particular, for x64) will
likely result in unecessarily slow code, or compile failures.
4) Handle gotcha #8 by using --disable-iconv configure option.
This is implemented in
5) Next, capture all the output from a build of and We will use the build log as a reference for making
the ffmpeg.gyp file.
make libavcodec/ libavformat/ \
> ffmpeg_build_log 2> ffmpeg_build_err
For Mac, replace the ".so" in the files above with ".dylib".
To get detailed output you might have to comment in common.mak
#$(foreach VAR,$(BRIEF), \
# $(eval override $(VAR) = $($(VAR))))
6) Check ffmpeg_build_err to see if there are any significant
anomalies. FFmpeg source generates a lot of compiler warnings; it
is safe to ignore those.
7) Examine all non-gcc commands to see if we're missing anything
grep -v '^gcc' ffmpeg_build_log
There should be yasm commands for assembling two yasm files, but nothing
else. Include those yasm files in the sources list for gyp. That means
grep -v '^gcc\|^yasm'
should generate nothing beyond "cd" and "ln" commands.
8) Verify that the all the gcc commands have the same compiler flags.
Do that with the following "one-liner":
grep - '^gcc' ffmpeg_build_log |
grep -v ' -MM ' |
grep -v ' -shared ' |
sed -e 's/ -MF .*$//' |
sort | uniq -c
This should find all gcc commands, exclude the dependency generation
lines, the link lines, and strip the output/input file names leaving
just the compiler flags + invocation. You should only see one "line"
of output. If there is more than one, figure out if the differences
in compiler flags are significant, and then use your best judgment.
9) Examine the output from step 7 and update the compiler flags in
ffmpeg.gyp. For easier cut/paste, append the following to the previous
command line to isolate each flag on its own line and add
tr -s ' ' | tr ' ' '\n' | sed -e "s/\(.*\)/'\1',/" | sort -u
10) Next, examine the link flags to see if anything interesting appears.
grep ' -shared ' ffmpeg_build_log |
tr ' ' '\n' |
grep -Ev '^[^-].*' |
grep -v rpath |
grep -Ev '^-L' |
sort -u
This should find all link lines, move each flag to its own line,
remove any argument that isn't a flag, remove all the rpaths (not
useful for us anyways), and remove all the -L lines (also not useful
for us).
The most interesting will likely be the -Wl,.* lines. Update the
ldflags section in ffmpeg.gyp accordingly.
11) Lastly, Find all the build .c files and update the sources line (this is
very similar to step 7):
grep -E '^gcc' ffmpeg_build_log |
grep -v ' -MM ' |
grep -v ' -shared ' |
sed -e "s|.* -o .* \(.*\)$|'source/patched-ffmpeg/\1',|" |
12) Attempt to build. :)
*13) Update the the sources! clause to exclude files that should only be built
for Chromium. For this, you basically need to do the steps above once
with the configure options for Chrome, then once with the options for
Chromium and diff the list of .c and .asm source files.