求助FFmpeg av_dup_packet(packet)函数

int packet_queue_put(PacketQueue *queue, AVPacket *packet)
{
     AVPacketList   *pkt_list;
     // ???
     if (av_dup_packet(packet) < 0)
     {
         return -1;
     }

     pkt_list = (AVPacketList *)av_malloc(sizeof(AVPacketList));
     if (pkt_list == NULL)
     {
          return -1;
     }

     pkt_list->pkt   = *packet;
     pkt_list->next  = NULL;

     //上锁
     SDL_LockMutex(queue->mutex);

     if (queue->last_pkt == NULL)    //空队
     {
          queue->first_pkt = pkt_list;
     }
     else
     {
         queue->last_pkt->next = pkt_list;
     }

     queue->last_pkt = pkt_list;  //这里queue->last_pkt = queue->last_pkt->next 的意思，但是，处理了更多的情况。
     queue->nb_packets++;
     queue->size += packet->size;
     SDL_CondSignal(queue->cond);      //???

     SDL_UnlockMutex(queue->mutex);

     return 0;
}