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06fef1e9f11a45df809e7cc4522f7174f48a664f
Before c63c303a1f
(the commit which
introduced a typedef for the type of the buffer of a PutBitContext)
skip_put_bits() was as follows:
static inline void skip_put_bits(PutBitContext *s, int n)
{
s->bit_left -= n;
s->buf_ptr -= 4 * (s->bit_left >> 5);
s->bit_left &= 31;
}
If s->bit_left was negative after the first subtraction, then the next
line will divide this by 32 with rounding towards -inf and multiply by
four; the result will be negative, of course.
The aforementioned commit changed this to:
static inline void skip_put_bits(PutBitContext *s, int n)
{
s->bit_left -= n;
s->buf_ptr -= sizeof(BitBuf) * ((unsigned)s->bit_left / BUF_BITS);
s->bit_left &= (BUF_BITS - 1);
}
Casting s->bit_left to unsigned meant that the rounding is still towards
-inf; yet the right side is now always positive (it transformed the
arithmetic shift into a logical shift), so that s->buf_ptr will always
be decremented (by about UINT_MAX / 8 unless n is huge) which leads to
segfaults on further usage and is already undefined pointer arithmetic
before that. This can be reproduced with the mpeg4 encoder with the
AV_CODEC_FLAG2_NO_OUTPUT flag set.
Furthermore, the earlier version as well as the new version share
another bug: s->bit_left will be in the range of 0..(BUF_BITS - 1)
afterwards, although the assumption throughout the other PutBitContext
functions is that it is in the range of 1..BUF_BITS. This might lead to
a shift by BUF_BITS in little-endian mode. This has been fixed, too.
The new version is furthermore able to skip zero bits, too.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
…
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FFmpeg README
FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.
Libraries
libavcodec
provides implementation of a wider range of codecs.libavformat
implements streaming protocols, container formats and basic I/O access.libavutil
includes hashers, decompressors and miscellaneous utility functions.libavfilter
provides a mean to alter decoded Audio and Video through chain of filters.libavdevice
provides an abstraction to access capture and playback devices.libswresample
implements audio mixing and resampling routines.libswscale
implements color conversion and scaling routines.
Tools
- ffmpeg is a command line toolbox to manipulate, convert and stream multimedia content.
- ffplay is a minimalistic multimedia player.
- ffprobe is a simple analysis tool to inspect multimedia content.
- Additional small tools such as
aviocat
,ismindex
andqt-faststart
.
Documentation
The offline documentation is available in the doc/ directory.
The online documentation is available in the main website and in the wiki.
Examples
Coding examples are available in the doc/examples directory.
License
FFmpeg codebase is mainly LGPL-licensed with optional components licensed under GPL. Please refer to the LICENSE file for detailed information.
Contributing
Patches should be submitted to the ffmpeg-devel mailing list using
git format-patch
or git send-email
. Github pull requests should be
avoided because they are not part of our review process and will be ignored.
Languages
C
90.1%
Assembly
8%
Makefile
1.3%
C++
0.2%
Objective-C
0.1%
Other
0.1%