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06fef1e9f1
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>
FFmpeg README
FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.
Libraries
libavcodecprovides implementation of a wider range of codecs.libavformatimplements streaming protocols, container formats and basic I/O access.libavutilincludes hashers, decompressors and miscellaneous utility functions.libavfilterprovides a mean to alter decoded Audio and Video through chain of filters.libavdeviceprovides an abstraction to access capture and playback devices.libswresampleimplements audio mixing and resampling routines.libswscaleimplements 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,ismindexandqt-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.