带你写一个Mp文件解析器-Mp3文件结构全解析(二)

typedef FrameHeader
{
unsigned int sync:11;                        //同步信息
unsigned int version:2;                      //版本
unsigned int layer: 2;                           //层
unsigned int error protection:1;           // CRC校验
unsigned int bitrate_index:4;              //位率
unsigned int sampling_frequency:2;         //采样频率
unsigned int padding:1;                    //帧长调节
unsigned int private:1;                       //保留字
unsigned int mode:2;                         //声道模式
unsigned int mode extension:2;        //扩充模式
unsigned int copyright:1;                           // 版权
unsigned int original:1;                      //原版标志
unsigned int emphasis:2;                  //强调模式
}HEADER, *LPHEADER;

position = lseek(fd, tagsize-10, 0);
  printf("seek to get audio frame , position = %d\n", position);
  int nFrames, nFileSampleRate;
  unsigned char ucHeaderByte1, ucHeaderByte2, ucHeaderByte3, ucHeaderByte4;
  float fBitRateSum=0;
syncWordSearch:
  while( position < file_size)
  {
    if (read(fd, &ucHeaderByte1, sizeof(ucHeaderByte1)) < 0) {
      perror("Read File: ");
      exit(1);
    }
    position ++;
    //printf("111:%d\n", ucHeaderByte1);
    if( ucHeaderByte1 == 0xFF )
    {
      if (read(fd, &ucHeaderByte2, sizeof(ucHeaderByte2)) < 0) {
        perror("Read File: ");
        exit(1);
      }
      position ++;
      unsigned char ucByte2LowerNibble = ucHeaderByte2 & 0xF0;
      if( ucByte2LowerNibble == 0xF0 || ucByte2LowerNibble == 0xE0 )
      {
          ++nFrames;
          printf("Found frame %d at offset = %ld B\n",nFrames, position);
          //printf("Header Bits:\n");
          //get the rest of the header:
          if (read(fd, &ucHeaderByte3, sizeof(ucHeaderByte3)) < 0) {
            perror("Read File: ");
            exit(1);
          }
          position ++;
          if (read(fd, &ucHeaderByte4, sizeof(ucHeaderByte4)) < 0) {
            perror("Read File: ");
            exit(1);
          }
          position ++;
          //print the header:
          //printBits(sizeof(ucHeaderByte1),&ucHeaderByte1);
          //printBits(sizeof(ucHeaderByte2),&ucHeaderByte2);
          //printBits(sizeof(ucHeaderByte3),&ucHeaderByte3);
          //printBits(sizeof(ucHeaderByte4),&ucHeaderByte4);
          //get header info:
          int nFrameSamplingFrequency = findFrameSamplingFrequency(ucHeaderByte3);
          int nFrameBitRate = findFrameBitRate(ucHeaderByte3);
          int nMpegVersionAndLayer = findMpegVersionAndLayer(ucHeaderByte2);
          if( nFrameBitRate==0 || nFrameSamplingFrequency == 0 || nMpegVersionAndLayer==0 )
          {//if this happens then we must have found the sync word but it was not actually part of the header
              --nFrames;
              printf("Error: not a header\n\n");
              goto syncWordSearch;
          }
          fBitRateSum += nFrameBitRate;
          if(nFrames==1){ nFileSampleRate = nFrameSamplingFrequency; }
          int nFramePadded = findFramePadding(ucHeaderByte3);
          //calculate frame size:
          int nFrameLength = (144 * (float)nFrameBitRate /
                                            (float)nFrameSamplingFrequency ) + nFramePadded;
          printf("\tFrame Length: %d Bytes \n\n", nFrameLength);
          //lnPreviousFramePosition=ftell(ifMp3)-4; //the position of the first byte of this frame
          //move file position by forward by frame length to bring it to next frame:
          position = lseek(fd, position + nFrameLength-4, 0);
      }
    }
  }
  float fFileAveBitRate= fBitRateSum/nFrames;
  printmp3details(nFrames,nFileSampleRate,fFileAveBitRate);