[FFmpeg-devel] [PATCH 4/4] lavf/matroskaenc: don't try to end the segment when the output isn't seekable
rodger.combs at gmail.com
Sun Mar 29 19:54:59 CEST 2015
In the case of most calls to end_ebml_master, the target is within the current segment, but in this case, it's in the first segment or header file, so if the context was marked as non-seekable manually (as in segment.c), avio_seek will ignore that and execute it anyway. This is fine in other uses of end_ebml_master, since both the current and target position are within the current segment, but breaks when ending mkv->segment, since `master.pos` refers to an offset in the first segment. So instead of failing the seek and returning, we end up seeking to an early point in the last segment, overwriting 8 unrelated bytes, and jumping back.
> On Mar 29, 2015, at 11:29, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Sat, Mar 28, 2015 at 07:25:21PM -0600, Rodger Combs wrote:
>> libavformat/matroskaenc.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
>> index 6b2e390..c242a0e 100644
>> --- a/libavformat/matroskaenc.c
>> +++ b/libavformat/matroskaenc.c
>> @@ -1948,9 +1948,9 @@ static int mkv_write_trailer(AVFormatContext *s)
>> put_ebml_float(pb, MATROSKA_ID_DURATION, mkv->duration);
>> avio_seek(pb, currentpos, SEEK_SET);
>> + end_ebml_master(pb, mkv->segment);
>> - end_ebml_master(pb, mkv->segment);
> the change should be ok but
> why exactly does this fail ?
> if the output isnt seekable avio_seek() would fail and end_ebml_master
> would return with just the failed seek.
> why does this cause a problem ?
> Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
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