注：写了一系列的结构体的分析的文章，在这里列一个列表：
FFMPEG结构体分析：AVStream
FFMPEG结构体分析：AVPacket
FFMPEG有几个最重要的结构体，包含了解协议，解封装，解码操作，此前已经进行过分析：
在此不再详述，其中AVStream是存储每一个视频/音频流信息的结构体。本文将会分析一下该结构体里重要变量的含义和作用。
首先看一下结构体的定义（位于avformat.h文件中）：
/* 雷霄骅
* 中国传媒大学/数字电视技术
* leixiaohua1020@126.com
*
*/
/**
* Stream structure.
* New fields can be added to the end with minor version bumps.
* Removal, reordering and changes to existing fields require a major
* version bump.
* sizeof(AVStream) must not be used outside libav*.
*/
typedef struct AVStream {
int index; /**< stream index in AVFormatContext */
/**
* Format-specific stream ID.
* decoding: set by libavformat
* encoding: set by the user
*/
int id;
AVCodecContext *codec; /**< codec context */
/**
* Real base framerate of the stream.
* This is the lowest framerate with which all timestamps can be
* represented accurately (it is the least common multiple of all
* framerates in the stream). Note, this value is just a guess!
* For example, if the time base is 1/90000 and all frames have either
* approximately 3600 or 1800 timer ticks, then r_frame_rate will be 50/1.
*/
AVRational r_frame_rate;
void *priv_data;
/**
* encoding: pts generation when outputting stream
*/
struct AVFrac pts;
/**
* This is the fundamental unit of time (in seconds) in terms
* of which frame timestamps are represented. For fixed-fps content,
* time base should be 1/framerate and timestamp increments should be 1.
* decoding: set by libavformat
* encoding: set by libavformat in av_write_header
*/
AVRational time_base;
/**
* Decoding: pts of the first frame of the stream in presentation order, in stream time base.
* Only set this if you are absolutely 100% sure that the value you set
* it to really is the pts of the first frame.
* This may be undefined (AV_NOPTS_VALUE).
* @note The ASF header does NOT contain a correct start_time the ASF
* demuxer must NOT set this.
*/
int64_t start_time;
/**
* Decoding: duration of the stream, in stream time base.
* If a source file does not specify a duration, but does specify
* a bitrate, this value will be estimated from bitrate and file size.
*/
int64_t duration;
int64_t nb_frames; ///< number of frames in this stream if known or 0
int disposition; /**< AV_DISPOSITION_* bit field */
enum AVDiscard discard; ///< Selects which packets can be discarded at will and do not need to be demuxed.
/**
* sample aspect ratio (0 if unknown)
* - encoding: Set by user.
* - decoding: Set by libavformat.
*/
AVRational sample_aspect_ratio;
AVDictionary *metadata;
/**
* Average framerate
*/
AVRational avg_frame_rate;
/**
* For streams with AV_DISPOSITION_ATTACHED_PIC disposition, this packet
* will contain the attached picture.
*
* decoding: set by libavformat, must not be modified by the caller.
* encoding: unused
*/
AVPacket attached_pic;
/*****************************************************************
* All fields below this line are not part of the public API. They
* may not be used outside of libavformat and can be changed and
* removed at will.
* New public fields should be added right above.
*****************************************************************
*/
/**
* Stream information used internally by av_find_stream_info()
*/
#define MAX_STD_TIMEBASES (60*12+5)
struct {
int64_t last_dts;
int64_t duration_gcd;
int duration_count;
double duration_error[2][2][MAX_STD_TIMEBASES];
int64_t codec_info_duration;
int nb_decoded_frames;
int found_decoder;
} *info;
int pts_wrap_bits; /**< number of bits in pts (used for wrapping control) */
// Timestamp generation support:
/**
* Timestamp corresponding to the last dts sync point.
*
* Initialized when AVCodecParserContext.dts_sync_point >= 0 and
* a DTS is received from the underlying container. Otherwise set to
* AV_NOPTS_VALUE by default.
*/
int64_t reference_dts;
int64_t first_dts;
int64_t cur_dts;
int64_t last_IP_pts;
int last_IP_duration;
/**
* Number of packets to buffer for codec probing
*/
#define MAX_PROBE_PACKETS 2500
int probe_packets;
/**
* Number of frames that have been demuxed during av_find_stream_info()
*/
int codec_info_nb_frames;
/**
* Stream Identifier
* This is the MPEG-TS stream identifier +1
* 0 means unknown
*/
int stream_identifier;
int64_t interleaver_chunk_size;
int64_t interleaver_chunk_duration;
/* av_read_frame() support */
enum AVStreamParseType need_parsing;
struct AVCodecParserContext *parser;
/**
* last packet in packet_buffer for this stream when muxing.
*/
struct AVPacketList *last_in_packet_buffer;
AVProbeData probe_data;
#define MAX_REORDER_DELAY 16
int64_t pts_buffer[MAX_REORDER_DELAY+1];
AVIndexEntry *index_entries; /**< Only used if the format does not
support seeking natively. */
int nb_index_entries;
unsigned int index_entries_allocated_size;
/**
* flag to indicate that probing is requested
* NOT PART OF PUBLIC API
*/
int request_probe;
} AVStream;
AVStream重要的变量如下所示：
int index：标识该视频/音频流
AVCodecContext *codec：指向该视频/音频流的AVCodecContext（它们是一一对应的关系）
AVRational time_base：时基。通过该值可以把PTS，DTS转化为真正的时间。FFMPEG其他结构体中也有这个字段，但是根据我的经验，只有AVStream中的time_base是可用的。PTS*time_base=真正的时间
int64_t duration：该视频/音频流长度
AVDictionary *metadata：元数据信息
AVRational avg_frame_rate：帧率（注：对视频来说，这个挺重要的）
AVPacket attached_pic：附带的图片。比如说一些MP3，AAC音频文件附带的专辑封面。
该结构体其他字段的作用目前还有待于探索。