ffmpeg decode/encode subtitle

void saveSubtitle( AVFormatContext *context, Stream stream )
{
  stringstream outfile;
  outfile << "/tmp/subtitle_" << stream.index << ".srt";
  string filename = outfile.str();
  AVStream *avstream = context->streams[stream.index];
  AVCodec *codec = avcodec_find_decoder( avstream->codec->codec_id );
  int result = avcodec_open2( avstream->codec, codec, NULL );
  checkResult( result == 0, "Error opening codec" );
  cerr << "found codec: " << codec << ", open result= " << result << endl;
  AVOutputFormat *outFormat = av_guess_format( NULL, filename.c_str(), NULL );
  checkResult( outFormat != NULL, "Error finding format" );
  cerr << "Found output format: " << outFormat->name << " (" << outFormat->long_name << ")" << endl;

  AVFormatContext *outFormatContext;
  avformat_alloc_output_context2( &outFormatContext, NULL, NULL, filename.c_str() );
  AVCodec *encoder = avcodec_find_encoder( outFormat->subtitle_codec );
  checkResult( encoder != NULL, "Error finding encoder" );
  cerr << "Found encoder: " << encoder->name << endl;


  AVStream *outStream = avformat_new_stream( outFormatContext, encoder );
  checkResult( outStream != NULL, "Error allocating out stream" );
  AVCodecContext *c = outStream->codec;

  result = avcodec_get_context_defaults3( c, encoder );
  checkResult( result == 0, "error on get context default" );



  cerr << "outstream codec: " << outStream->codec << endl;
  cerr << "Opened stream " << outStream->id << ", codec=" << outStream->codec->codec_id << endl;
  result = avio_open( &outFormatContext->pb, filename.c_str(), AVIO_FLAG_WRITE );
  checkResult( result == 0, "Error opening out file" );
  cerr << "out file opened correctly" << endl;
  result = avformat_write_header( outFormatContext, NULL );
  checkResult(result==0, "Error writing header");
  cerr << "header wrote correctly" << endl;
  result = 0;
  AVPacket pkt;
  av_init_packet( &pkt );
  pkt.data = NULL;
  pkt.size = 0;

  cerr << "srt codec id: " << AV_CODEC_ID_SUBRIP << endl;
  while( av_read_frame( context, &pkt ) >= 0 )
  {
    if(pkt.stream_index != stream.index)
      continue;
    int gotSubtitle = 0;
    AVSubtitle subtitle;
    result = avcodec_decode_subtitle2( avstream->codec, &subtitle, &gotSubtitle, &pkt );
    uint64_t bufferSize = 1024 * 1024 ;
    uint8_t *buffer = new uint8_t[bufferSize];
    memset(buffer, 0, bufferSize);
    if( result >= 0 )
    {
      result = avcodec_encode_subtitle( outStream->codec, buffer, bufferSize, &subtitle );
      cerr << "Encode subtitle result: " << result << endl;
    }

    cerr << "Encoded subtitle: " << buffer << endl;
    delete [] buffer;
  }
}