ffmpeg下载地址 http://www.ffmpeg.club/
AVPacket是ffmpeg用来存放编码后的视频帧数据，我们来分析一下这个结构体，先贴出ffmpeg3.2中AVPacket声明的源代码：
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;
我们依次进行分析
AVBufferRef *buf;
用来存放引用计数的数据，如果没有使用引用计数，值就是NULL，当你多个packet对象引用同一帧数据的时候用到。
int64_t pts;
本帧数据显示的时间，比较关键的数据，在做seek和播放进度的时候都要用到它，pts只是一个数量，对应于AVStream->time_base，要根据time_base才能转换为具体的时间,音频和视频一般有不同的time_base，所以在做音视频同步一定要做转换，不能直接拿pts做。
转换方式，比如转为毫秒
AVFormatContext *ic = NULL;
static double r2d(AVRational r)
{
return r.num == 0 || r.den == 0 ? 0. : (double)r.num / (double)r.den;
}
//。。。
int pts = (pkt->pts *r2d(ic->streams[pkt->stream_index]->time_base)) * 1000;
int64_t dts;
基本属性等同于pts，区别就是dts对应的是解码时间不是显示时间，解码后会放入缓冲，比如h264，如果有b帧，则要先解码后面的b帧，再解码之前的帧。
uint8_t *data; int size;
帧的数据和数据大小
int stream_index;
帧数据所属流的索引，用来区分音频，视频，和字幕数据。
int flags;
标志，其中为1表示该数据是一个关键帧
AV_PKT_FLAG_KEY 0x0001 关键帧
AVPacketSideData *side_data;
int side_data_elems;
容器提供的一些附加数据
int64_t duration;
下一帧pts - 当前帧pts ，也就表示两帧时间间隔。
int64_t pos;
当前帧数据在文件中的位置（字节为单位），如果做文件移位用到，如果rtsp就没有此数据。
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