一、av_packet_alloc

AVPacket* av_packet_alloc(void )

• Allocate an AVPacket and set its fields to default values.
  The resulting struct must be freed using av_packet_free().
• 开辟packet空间并初始化默认值，需要ac_packet_free()释放空间

二、ac_packet_free

| void av_packet_free ( AVPacket ** pkt )

• Free the packet, if the packet is reference counted, it will be unreferenced first.
• 释放AVpacket空间，当引用有引用的时候会释放引用

三、av_init_packet

void av_init_packet(AVPacket * pkt)

• Initialize optional fields of a packet with default values.
• Note, this does not touch the data and size members, which have to be initialized separately.
• 初始化packet的值为默认值，注意：该函数不会影响data引用的数据缓存空间和size，需要单独处理。

四、av_packet_unref

void av_packet_unref ( AVPacket **pkt)

• Wipe the packet.
• Unreference the buffer referenced by the packet and reset the remaining packet fields to their default values.
• 将缓存空间的引用计数-1，并将Packet中的其他字段设为初始值。如果引用计数为0，自动的释放缓存空间。

https://www.ffmpeg.org/doxygen/trunk/group__lavc__packet.html#gaaf85aa950695631e0217a16062289b66