3.AVPacket使用

结构体如下:

typedef struct AVPacket{
/**
* A reference to the reference-counted buffer where the packet data is
* stored.
* May be NULL, then the packet data is not reference-counted.
*/
AVBufferRef *buf;    
//用来管理data指针引用的数据缓存,通过av_packet_ref() 和 av_packet_unref()
//来使buf->buffer->refcount成员引用计数+-,如果引用计数为0,则释放buffer. 
/**
* Presentation timestamp in AVStream->time_base units; the time at which
* the decompressed packet will be presented to the user.
* Can be AV_NOPTS_VALUE if it is not stored in the file.
* pts MUST be larger or equal to dts as presentation cannot happen before
* decompression, unless one wants to view hex dumps. Some formats misuse
* the terms dts and pts/cts to mean something different. Such timestamps
* must be converted to true pts/dts before they are stored in AVPacket.
*/
int64_t pts;    
//显示时间戳,需要所属媒体流AVStream的time_base时基来换算出当前显示的标准时间(时分秒)
//比如dpts = av_q2d(AVStream->time_base) * AVPacket->pts;

int64_t dts;       //解码时间戳,需要所属媒体流AVStream的time_base时基来换算出当前显示的标准时间(时分秒)
uint8_t *data;    //压缩编码的数据。
int size;     //data的大小
int stream_index;    //标识该AVPacket所属的视频/音频流

int flags;    //标识,结合AV_PKT_FLAG使用，比如:
//#define AV_PKT_FLAG_KEY 0x0001 //关键帧
//#define AV_PKT_FLAG_CORRUPT 0x0002 //损坏的数据
//#define AV_PKT_FLAG_DISCARD 0x0004 /丢弃的数据

/**
* Additional packet data that can be provided by the container.
* Packet can contain several types of side information.
*/
AVPacketSideData *side_data;    //容器提供的一些附加数据
int side_data_elems;         //边缘数据元数个数

/**
* Duration of this packet in AVStream->time_base units, 0 if unknown.
* Equals next_pts - this_pts in presentation order.
*/
int64_t duration;    //数据的时长，需要所属媒体流AVStream的time_base时基来换算出当前的标准时间，未知则值为默认值0

int64_t pos;        //数据在流媒体中的位置，未知则值为默认值-1

}AVPacket;

AVPacket常用函数如下:

AVPacket *av_packet_alloc(void)
{
   AVPacket *pkt = av_mallocz(sizeof(AVPacket));
   if (!pkt)
    return pkt;

   av_packet_unref(pkt);

   return pkt;
}

创建一个AVPacket的实例，但该函数并不会为数据分配空间，其指向数据域的指针为NULL。

void av_packet_free(AVPacket **pkt)
{
  if (!pkt || !*pkt)
    return;

  av_packet_unref(*pkt);
  av_freep(pkt);
}

首先将AVPacket->buf->buffer->refcount成员减1（数据域的引用技术减为0时会自动释放）,然后再释放为AVPacket分配的空间。

int av_packet_ref(AVPacket *dst, const AVPacket *src)
{
  int ret;

  ret = av_packet_copy_props(dst, src); //复制部分成员(比如:pts,dts,pos,duration,side_data)到dst
  if (ret < 0)
    return ret;

  if (!src->buf) { //如果src->buf为空,则为dst新分配一个数据域,并将src->data复制到dst->buf->data
    ret = packet_alloc(&dst->buf, src->size);
  if (ret < 0)
   goto fail;
  av_assert1(!src->size || src->data);
  if (src->size)
    memcpy(dst->buf->data, src->data, src->size);

  dst->data = dst->buf->data;
 } else { //不为空,则调用av_buffer_ref()来使引用+1,并将dst->buf指向src->buf,
  dst->buf = av_buffer_ref(src->buf);
  if (!dst->buf) {
  ret = AVERROR(ENOMEM);
  goto fail;
 }
  dst->data = src->data;    //然后将src->data复制到dst->data中
 }

  dst->size = src->size;

  return 0;
  fail:
    av_packet_free_side_data(dst);
  return ret;
}

av_packet_ref根据src来创建一个新的AVPacket

void av_packet_unref(AVPacket *pkt)
{
  av_packet_free_side_data(pkt);
  av_buffer_unref(&pkt->buf);
  av_init_packet(pkt);
  pkt->data = NULL;
  pkt->size = 0;
}

将AVPacket->buf->buffer->refcount成员减1（数据域的引用技术减为0时会自动释放）,替代了旧api(av_free_packet)

AVPacket解码示例：

AVPacket *packet = av_packet_alloc(); // 创建一个packet

while(av_read_frame(pFormatCtx,packet))
{
  if(packet->stream_index == audio_index)
  {
    ...
  }
  else if(packet->stream_index == video_index)
  {
   ...
  }

  av_packet_unref(packet); // 也可以使用旧api:av_free_packet
}
av_packet_free(packet);    //释放packet,如果还想使用,则需要重新alloc