[FFmpeg-trac] #1831(undetermined:new): Seeking by byte returns success but does not seek

[FFmpeg]

#1831: Seeking by byte returns success but does not seek
-------------------------------------+-------------------------------------
               Reporter:  mbradshaw  |                  Owner:
                   Type:  defect     |                 Status:  new
               Priority:  normal     |              Component:
                Version:  1.0        |  undetermined
             Blocked By:             |               Keywords:
Reproduced by developer:  0          |               Blocking:
                                     |  Analyzed by developer:  0
-------------------------------------+-------------------------------------
 '''Summary of the bug:'''
 When calling `av_seek_frame()` (or `avformat_seek_file()`) with
 `AVFMT_NO_BYTE_SEEK`, it returns 0 indicating success, however, the seek
 is not performed. I have attached a file that exposes this failure, though
 I have many files for which this problem occurs.

 '''How to reproduce:'''
 I created the below quick sample code to show that seeking by byte does
 not work. If you change the seeking flag to `AVSEEK_FLAG_BACKWARD` then it
 works as expected. In both cases, however, `av_seek_frame()` returns 0,
 indicating success.
 {{{
 #include <iostream>

 extern "C"
 {
 #include <avcodec.h>
 #include <avformat.h>
 #include <swscale.h>
 };

 int main()
 {
     av_register_all();

     AVFrame* frame = avcodec_alloc_frame();
     if (!frame)
     {
                 std::cout << "Error allocating the frame" << std::endl;
         return 1;
     }

     AVFormatContext* formatContext = NULL;
     if (avformat_open_input(&formatContext,
 "C:/Users/mbradshaw/Desktop/b/MP4-MC264-AAC.mp4", NULL, NULL) != 0)
     {
         av_free(frame);
                 std::cout << "Error opening the file" << std::endl;
         return 1;
     }

     if (avformat_find_stream_info(formatContext, NULL) < 0)
     {
         av_free(frame);
         av_close_input_file(formatContext);
                 std::cout << "Error finding the stream info" << std::endl;
         return 1;
     }

         AVStream* audioStream = NULL;
         for (unsigned int i = 0; i < formatContext->nb_streams; ++i)
         {
                 if (formatContext->streams[i]->codec->codec_type ==
 AVMEDIA_TYPE_AUDIO)
                 {
                         audioStream = formatContext->streams[i];
                         break;
                 }
         }

     if (audioStream == NULL)
     {
         av_free(frame);
         av_close_input_file(formatContext);
                 std::cout << "Could not find any audio stream in the file"
 << std::endl;
         return 1;
     }

     AVCodecContext* codecContext = audioStream->codec;

     codecContext->codec = avcodec_find_decoder(codecContext->codec_id);
     if (codecContext->codec == NULL)
     {
         av_free(frame);
         av_close_input_file(formatContext);
                 std::cout << "Couldn't find a proper decoder" <<
 std::endl;
         return 1;
     }
     else if (avcodec_open2(codecContext, codecContext->codec, NULL) != 0)
     {
         av_free(frame);
         av_close_input_file(formatContext);
                 std::cout << "Couldn't open the context with the decoder"
 << std::endl;
         return 1;
     }

     AVPacket packet;
     av_init_packet(&packet);

         int packetCount = 0;
         int decodedFrameCount = 0;
     while (av_read_frame(formatContext, &packet) == 0)
     {
                 ++packetCount;
         if (packet.stream_index == audioStream->index)
         {
             int frameFinished = 0;
             avcodec_decode_audio4(codecContext, frame, &frameFinished,
 &packet);

             if (frameFinished)
             {
                                 ++decodedFrameCount;
                         }
         }

                 av_free_packet(&packet);
     }

         std::cout << "packetCount: " << packetCount <<
 "\tdecodedFrameCount: " << decodedFrameCount << std::endl;
         std::cout << "formatContext->iformat->flags: " <<
 formatContext->iformat->flags << "\tno byte seeking: " <<
 (formatContext->iformat->flags & AVFMT_NO_BYTE_SEEK) << std::endl;

         avcodec_flush_buffers(codecContext);
     av_init_packet(&packet);
         std::cout << "av_seek_frame() returned: " <<
 av_seek_frame(formatContext, -1, 0, AVSEEK_FLAG_BYTE) << std::endl;

         packetCount = 0;
         decodedFrameCount = 0;
     while (av_read_frame(formatContext, &packet) == 0)
     {
                 ++packetCount;
         if (packet.stream_index == audioStream->index)
         {
                         // Try to decode the packet into a frame
             int frameFinished = 0;
             avcodec_decode_audio4(codecContext, frame, &frameFinished,
 &packet);

             if (frameFinished)
             {
                                 ++decodedFrameCount;
                         }
         }

                 av_free_packet(&packet);
     }

         std::cout << "packetCount: " << packetCount <<
 "\tdecodedFrameCount: " << decodedFrameCount << std::endl;

     av_free(frame);
     avcodec_close(codecContext);
     av_close_input_file(formatContext);
 }
 }}}

 '''Actual results:'''
 {{{
 packetCount: 526        decodedFrameCount: 259
 formatContext->iformat->flags: 0        no byte seeking: 0
 av_seek_frame() returned: 0
 packetCount: 0  decodedFrameCount: 0
 }}}

 '''Expected results:''' (if seeking really does succeed)
 {{{
 packetCount: 526        decodedFrameCount: 259
 formatContext->iformat->flags: 0        no byte seeking: 0
 av_seek_frame() returned: 0
 packetCount: 526        decodedFrameCount: 259
 }}}

 '''Alternative expected results:''' (if seeking really does fail)
 {{{
 packetCount: 526        decodedFrameCount: 259
 formatContext->iformat->flags: 0        no byte seeking: 0
 av_seek_frame() returned: -1
 packetCount: 0        decodedFrameCount: 0
 }}}

-- 
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/1831>
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