[FFmpeg-trac] #2773(avformat:new): Seeking past the end of an mp3 file has different behavior than other formats

[FFmpeg]

#2773: Seeking past the end of an mp3 file has different behavior than other
formats
----------------------------------+--------------------------------------
             Reporter:  wadams    |                     Type:  defect
               Status:  new       |                 Priority:  normal
            Component:  avformat  |                  Version:  git-master
             Keywords:  mp3 seek  |               Blocked By:
             Blocking:            |  Reproduced by developer:  0
Analyzed by developer:  0         |
----------------------------------+--------------------------------------
 I am trying to determine with complete accuracy the length of audio files
 of various formats by seeking to the near the end and reading packets.
 This has worked since version 1.2 but since I pulled the latest master
 branch from git this method no longer works for mp3 files. After I call
 av_seek_frame() on particular files, the next call to av_read_frame()
 returns EOF even if I use AVSEEK_FLAG_BACKWARD. Other formats will
 generally seek to the last sync point in the file when seeking past the
 end with the backward flag.

 I looked through the git history and found the the commit that changed
 this behavior is
 
[http://git.videolan.org/?p=ffmpeg.git;a=commit;h=e096283ea55bc36a637b47329e19ddb26fb1440b
 e096283ea55bc36a637b47329e19ddb26fb1440b].



 Here is some code that will reproduce the problem with the attached file.
 It executes as expected when I revert the above mentioned commit.

 {{{
 int main(int argc, char** argv)
 {
         int err, i, streamIndex;
         AVFormatContext* fmtCtx;
         AVStream* stream;
         AVPacket pkt;

         av_register_all();

         fmtCtx = avformat_alloc_context();

         err = avformat_open_input(&fmtCtx, "test.mp3", NULL, NULL);
         if (err < 0)
         {
                 fprintf(stderr, "Failed to open input\n");
                 return 1;
         }

         avformat_find_stream_info(fmtCtx, NULL);

         streamIndex = -1;
         for (i = 0; i < fmtCtx->nb_streams; i++)
                 if (fmtCtx->streams[i]->codec->codec_type ==
 AVMEDIA_TYPE_AUDIO)
                 {
                         streamIndex = i;
                         break;
                 }

         if (streamIndex < 0)
         {
                 fprintf(stderr, "No audio stream\n");
                 return 1;
         }

         stream = fmtCtx->streams[streamIndex];

         err = av_seek_frame(fmtCtx, streamIndex, stream->duration,
 AVSEEK_FLAG_BACKWARD);
         if (err < 0)
         {
                 fprintf(stderr, "Failed to seek stream\n");
                 return 1;
         }


         av_init_packet(&pkt);
         pkt.data = NULL;
         pkt.size = 0;

         err = av_read_frame(fmtCtx, &pkt);
         if (err < 0)
         {
                 char buf[1024];
                 av_strerror(err, buf, sizeof(buf));

                 fprintf(stderr, "Failed to read frame. Error: %d, %s\n",
 err, buf);
                 return 1;
         }

         av_free_packet(&pkt);
         avformat_close_input(&fmtCtx);

         return 0;
 }
 }}}

 Thanks,
 Bill

-- 
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2773>
FFmpeg <http://ffmpeg.org>
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