JM8.6中NALU（此处指VCL式的NALU)是如何写进码流的？

       在lencod.c的main函数中调用了encode_one_slice函数，在encode_one_slice函数中调用了frame_picture函数，调用了frame_picture函数后(frame_picture还会继续调用其他重要函数) 就实现了对第一帧的编码，这样全局变量frame_pic就发生了变化，得到了它想得到的码流，现在要写码流，怎么写呢？

      在encode_one_frame中接着就调用到了writeout_picture函数，调用形式是：

writeout_picture (frame_pic);

      可见，把代码码流信息的frame_pic穿进去了，进入writeout_picture函数看看：

static int writeout_picture(Picture *pic){  Bitstream *currStream;  int partition, slice;  Slice *currSlice;  img->currentPicture=pic;  // 遍历slice,通常，一个frame对应一个slice  for (slice=0; slice<pic->no_slices; slice++)  {    currSlice = pic->slices[slice];    // 目前都发现currSlice->max_part_nr为1for (partition=0; partition<currSlice->max_part_nr; partition++)    {      currStream = (currSlice->partArr[partition]).bitstream;        // currStream->bits_to_go相当于标记，必须为8  assert (currStream->bits_to_go == 8);    //! should always be the case, the                                                //! byte alignment is done in terminate_slice    // writeUnit函数的第一参数类型是Bitstream*  writeUnit (currSlice->partArr[partition].bitstream,partition);    }           // partition loop  }           // slice loop    return 0;   }

      可以看到，把一个picture对应的码流分成slice和数据分块来传给writeUnit的形参（一般认为：一个slice对应一个数据分块），进入到writeUnit函数看看：

static void writeUnit(Bitstream* currStream,int partition){  NALU_t *nalu;  // 实际上currStream->bits_to_go就是一个标记  // 如果不为8, 则表明往下继续执行会有错误  assert (currStream->bits_to_go == 8);  // 分配堆空间， *4是为了安全起见  nalu = AllocNALU(img->width*img->height*4);  nalu->startcodeprefix_len = 2+(img->current_mb_nr == 0?ZEROBYTES_SHORTSTARTCODE+1:ZEROBYTES_SHORTSTARTCODE);//printf ("nalu->startcodeprefix_len %d\n", nalu->startcodeprefix_len);    nalu->len = currStream->byte_pos +1;            // add one for the first byte of the NALU//printf ("nalu->len %d\n", nalu->len);  // 内存数据复制，这个是最重要的.  memcpy (&nalu->buf[1], currStream->streamBuffer, nalu->len-1);    // 下面对nalu的一些参数进行赋值  // 其实就是对nalu header的8个比特赋值  if (img->currentPicture->idr_flag)  {    nalu->nal_unit_type = NALU_TYPE_IDR;    nalu->nal_reference_idc = NALU_PRIORITY_HIGHEST;  }  else if (img->type == B_SLICE)  {//different nal header for different partitions    if(input->partition_mode == 0){      nalu->nal_unit_type = NALU_TYPE_SLICE;}else{      nalu->nal_unit_type = NALU_TYPE_DPA +  partition;}        if (img->nal_reference_idc !=0){nalu->nal_reference_idc = NALU_PRIORITY_HIGH;}    else    {      nalu->nal_reference_idc = NALU_PRIORITY_DISPOSABLE;    }  }  else   // non-b frame, non IDR slice  {//different nal header for different partitions    if(input->partition_mode == 0)    {       nalu->nal_unit_type = NALU_TYPE_SLICE;}else{       nalu->nal_unit_type = NALU_TYPE_DPA +  partition;}    if (img->nal_reference_idc !=0)    {       nalu->nal_reference_idc = NALU_PRIORITY_HIGH;    }    else    {       nalu->nal_reference_idc = NALU_PRIORITY_DISPOSABLE;    }  }  nalu->forbidden_bit = 0; // 必须为0  // WriteNALU是指向函数的全局指针（提前赋值了）,  // 所以要想用鼠标进入WriteNALU“函数”是不可能的  // 真正调用到的函数是：WriteAnnexbNALU  // 而WriteAnnexbNALU函数就是写码流函数  stat->bit_ctr += WriteNALU (nalu); // 写码流    FreeNALU(nalu);}

       不用再进入WriteAnnexbNALU函数了，因为，对这个函数太熟悉了.