本文為作者原創，轉載請注明出處：https://www.cnblogs.com/leisure_chn/p/10410320.html
本文基於FFmpeg 4.1版本。
1. 數據結構定義
struct AVPacket定義於<libavcodec/avcodec.h>
struct AVPacket packet;
AVPacket中存儲的是經過編碼的壓縮數據。在解碼中，AVPacket由解復用器輸出到解碼器；在編碼中，AVPacket由編碼器輸出到復用器。下圖中，解復用器(demuxer)的輸出和復用器(muxer)的輸入“encoded data packets”的數據類型就是AVPacket：
_______ ______________
| | | |
| input | demuxer | encoded data | decoder
| file | ---------> | packets | -----+
|_______| |______________| |
v
_________
| |
| decoded |
| frames |
|_________|
________ ______________ |
| | | | |
| output | <-------- | encoded data | <----+
| file | muxer | packets | encoder
|________| |______________|
對於視頻而言，一個AVPacket通常只包含一個壓縮視頻幀。而對於音頻而言，一個AVPacket可能包含多個完整的音頻壓縮幀。AVPacket也可以不包含壓縮編碼數據，而只包含side data，這種包可以稱為空packet。例如，編碼結束后只需要更新一些參數時就可以發空packet。
AVPacket對象可以在棧上分配，注意此處指的是AVPacket對象本身。而AVPacket中包含的數據緩沖區是通過av_malloc()在堆上分配的。
TODO: AVPacket對象在棧上分配，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;
音視頻數據緩沖區
uint8_t *data:int size:
數據緩沖區地址與大小。音視頻編碼壓縮數據存儲於此片內存區域。此內存區域由AVBufferRef *buf管理。AVBufferRef *buf:
數據緩沖區引用，也可叫引用計數緩沖區。對上一字段uint8_t *data指向的內存區域提供引用計數等管理機制。
AVBufferRef對數據緩沖區提供了管理機制，用戶不應直接訪問數據緩沖區。參考“FFmpeg數據結構AVBuffer”
如果buf值為NULL，則data指向的數據緩沖區不使用引用計數機制。av_packet_ref(dst, src)將執行數據緩沖區的拷貝，而非僅僅增加緩沖區引用計數。
如果buf值非NULL，則data指向的數據緩沖區使用引用計數機制。av_packet_ref(dst, src)將不拷貝緩沖區，而僅增加緩沖區引用計數。av_packet_unref()將數據緩沖區引用計數減1，當緩沖區引用計數為0時，緩沖區內存被FFmpeg回收。
對於struct AVPacket pkt對象，如果pkt.buf值非NULL，則有pkt.data == pkt.buf->data == pkt.buf->buffer.data
額外類型數據
AVPacketSideData *side_dataint side_data_elems
由容器(container)提供的額外包數據。TODO: 待研究
packet屬性
int64_t pts:
顯示時間戳。單位time_base，幀率的倒數。int64_t dts:
解碼時間戳。單位time_base，幀率的倒數。int stream_index:
當前包(packet)所有流(stream)的索引(index)。int flags:
packet標志位。比如是否關鍵幀等。int64_t duration:
當前包解碼后的幀播放持續的時長。單位timebase。值等於下一幀pts減當前幀pts。int64_t pos:
當前包在流中的位置，單位字節。
2. 關鍵函數實現
這里列出的幾個關鍵函數，主要是為了幫助理解struct AVPacket數據結構
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()作了處理如下:
a) 如果src->buf為NULL，則將src緩沖區拷貝到新創建的dst緩沖區，注意src緩沖區不支持引用計數，但新建的dst緩沖區是支持引用計數的，因為dst->buf不為NULL。
b) 如果src->buf不為空，則dst與src共用緩沖區，調用av_buffer_ref()增加緩沖區引用計數即可。av_buffer_ref()分析參考“FFmpeg數據結構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()注銷AVPacket *pkt對象，並調用av_buffer_unref(&pkt->buf);將緩沖區引用計數減1。
av_buffer_unref()中將緩沖區引用計數減1后，若緩沖區引用計數變成0，則回收緩沖區內存。av_buffer_unref()分析參考“FFmpeg數據結構AVBuffer”
3. 參考資料
[1] FFmpeg數據結構：AVPacket解析, https://www.cnblogs.com/wangguchangqing/p/5790705.html
4. 修改記錄
2018-12-14 V1.0 初稿