本文为作者原创，转载请注明出处：https://www.cnblogs.com/leisure_chn/p/10410320.html
本文基于FFmpeg 4.1版本。
1. 数据结构定义
struct AVPacket定义于<libavcodec/avcodec.h>
struct AVPacket packet;
AVPacket中存储的是经过编码的压缩数据。在解码中，AVPacket由解复用器输出到解码器；在编码中，AVPacket由编码器输出到复用器。下图中，解复用器(demuxer)的输出和复用器(muxer)的输入“encoded data packets”的数据类型就是AVPacket：
_______ ______________
| | | |
| input | demuxer | encoded data | decoder
| file | ---------> | packets | -----+
|_______| |______________| |
v
_________
| |
| decoded |
| frames |
|_________|
________ ______________ |
| | | | |
| output | <-------- | encoded data | <----+
| file | muxer | packets | encoder
|________| |______________|
对于视频而言，一个AVPacket通常只包含一个压缩视频帧。而对于音频而言，一个AVPacket可能包含多个完整的音频压缩帧。AVPacket也可以不包含压缩编码数据，而只包含side data，这种包可以称为空packet。例如，编码结束后只需要更新一些参数时就可以发空packet。
AVPacket对象可以在栈上分配，注意此处指的是AVPacket对象本身。而AVPacket中包含的数据缓冲区是通过av_malloc()在堆上分配的。
TODO: AVPacket对象在栈上分配，原理不清楚，待研究
/**
* This structure stores compressed data. It is typically exported by demuxers
* and then passed as input to decoders, or received as output from encoders and
* then passed to muxers.
*
* For video, it should typically contain one compressed frame. For audio it may
* contain several compressed frames. Encoders are allowed to output empty
* packets, with no compressed data, containing only side data
* (e.g. to update some stream parameters at the end of encoding).
*
* AVPacket is one of the few structs in FFmpeg, whose size is a part of public
* ABI. Thus it may be allocated on stack and no new fields can be added to it
* without libavcodec and libavformat major bump.
*
* The semantics of data ownership depends on the buf field.
* If it is set, the packet data is dynamically allocated and is
* valid indefinitely until a call to av_packet_unref() reduces the
* reference count to 0.
*
* If the buf field is not set av_packet_ref() would make a copy instead
* of increasing the reference count.
*
* The side data is always allocated with av_malloc(), copied by
* av_packet_ref() and freed by av_packet_unref().
*
* @see av_packet_ref
* @see av_packet_unref
*/
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;
/**
* 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;
/**
* Decompression timestamp in AVStream->time_base units; the time at which
* the packet is decompressed.
* Can be AV_NOPTS_VALUE if it is not stored in the file.
*/
int64_t dts;
uint8_t *data;
int size;
int stream_index;
/**
* A combination of AV_PKT_FLAG values
*/
int flags;
/**
* Additional packet data that can be provided by the container.
* Packet can contain several types of side information.
*/
AVPacketSideData *side_data;
int side_data_elems;
/**
* Duration of this packet in AVStream->time_base units, 0 if unknown.
* Equals next_pts - this_pts in presentation order.
*/
int64_t duration;
int64_t pos; ///< byte position in stream, -1 if unknown
#if FF_API_CONVERGENCE_DURATION
/**
* @deprecated Same as the duration field, but as int64_t. This was required
* for Matroska subtitles, whose duration values could overflow when the
* duration field was still an int.
*/
attribute_deprecated
int64_t convergence_duration;
#endif
} AVPacket;
音视频数据缓冲区
uint8_t *data:int size:
数据缓冲区地址与大小。音视频编码压缩数据存储于此片内存区域。此内存区域由AVBufferRef *buf管理。AVBufferRef *buf:
数据缓冲区引用，也可叫引用计数缓冲区。对上一字段uint8_t *data指向的内存区域提供引用计数等管理机制。
AVBufferRef对数据缓冲区提供了管理机制，用户不应直接访问数据缓冲区。参考“FFmpeg数据结构AVBuffer”
如果buf值为NULL，则data指向的数据缓冲区不使用引用计数机制。av_packet_ref(dst, src)将执行数据缓冲区的拷贝，而非仅仅增加缓冲区引用计数。
如果buf值非NULL，则data指向的数据缓冲区使用引用计数机制。av_packet_ref(dst, src)将不拷贝缓冲区，而仅增加缓冲区引用计数。av_packet_unref()将数据缓冲区引用计数减1，当缓冲区引用计数为0时，缓冲区内存被FFmpeg回收。
对于struct AVPacket pkt对象，如果pkt.buf值非NULL，则有pkt.data == pkt.buf->data == pkt.buf->buffer.data
额外类型数据
AVPacketSideData *side_dataint side_data_elems
由容器(container)提供的额外包数据。TODO: 待研究
packet属性
int64_t pts:
显示时间戳。单位time_base，帧率的倒数。int64_t dts:
解码时间戳。单位time_base，帧率的倒数。int stream_index:
当前包(packet)所有流(stream)的索引(index)。int flags:
packet标志位。比如是否关键帧等。int64_t duration:
当前包解码后的帧播放持续的时长。单位timebase。值等于下一帧pts减当前帧pts。int64_t pos:
当前包在流中的位置，单位字节。
2. 关键函数实现
这里列出的几个关键函数，主要是为了帮助理解struct AVPacket数据结构
2.1 av_packet_ref()
int av_packet_ref(AVPacket *dst, const AVPacket *src)
{
int ret;
ret = av_packet_copy_props(dst, src);
if (ret < 0)
return ret;
if (!src->buf) {
ret = packet_alloc(&dst->buf, src->size);
if (ret < 0)
goto fail;
if (src->size)
memcpy(dst->buf->data, src->data, src->size);
dst->data = dst->buf->data;
} else {
dst->buf = av_buffer_ref(src->buf);
if (!dst->buf) {
ret = AVERROR(ENOMEM);
goto fail;
}
dst->data = src->data;
}
dst->size = src->size;
return 0;
fail:
av_packet_free_side_data(dst);
return ret;
}
av_packet_ref()作了处理如下:
a) 如果src->buf为NULL，则将src缓冲区拷贝到新创建的dst缓冲区，注意src缓冲区不支持引用计数，但新建的dst缓冲区是支持引用计数的，因为dst->buf不为NULL。
b) 如果src->buf不为空，则dst与src共用缓冲区，调用av_buffer_ref()增加缓冲区引用计数即可。av_buffer_ref()分析参考“FFmpeg数据结构AVBuffer”
2.2 av_packet_unref()
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;
}
av_packet_unref()注销AVPacket *pkt对象，并调用av_buffer_unref(&pkt->buf);将缓冲区引用计数减1。av_buffer_unref()中将缓冲区引用计数减1后，若缓冲区引用计数变成0，则回收缓冲区内存。av_buffer_unref()分析参考“FFmpeg数据结构AVBuffer”
3. 参考资料
[1] FFmpeg数据结构：AVPacket解析, https://www.cnblogs.com/wangguchangqing/p/5790705.html
4. 修改记录
2018-12-14 V1.0 初稿
