FFMPEG结构体分析：AVFormatContext

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：AVFrame
FFMPEG结构体分析：AVFormatContext
FFMPEG结构体分析：AVCodecContext
FFMPEG结构体分析：AVIOContext
FFMPEG结构体分析：AVCodec
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：
FFMPEG中最关键的结构体之间的关系

在此不再详述，其中AVFormatContext是包含码流参数较多的结构体。本文将会详细分析一下该结构体里每个变量的含义和作用。
首先看一下结构体的定义（位于avformat.h）：
/* 雷霄骅
* 中国传媒大学/数字电视技术
* leixiaohua1020@126.com
*
*/
/**
* Format I/O context.
* 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(AVFormatContext) must not be used outside libav*, use
* avformat_alloc_context() to create an AVFormatContext.
*/
typedef struct AVFormatContext {
/**
* A class for logging and AVOptions. Set by avformat_alloc_context().
* Exports (de)muxer private options if they exist.
*/
const AVClass *av_class;

/**
* Can only be iformat or oformat, not both at the same time.
*
* decoding: set by avformat_open_input().
* encoding: set by the user.
*/
struct AVInputFormat *iformat;
struct AVOutputFormat *oformat;

/**
* Format private data. This is an AVOptions-enabled struct
* if and only if iformat/oformat.priv_class is not NULL.
*/
void *priv_data;

/*
* I/O context.
*
* decoding: either set by the user before avformat_open_input() (then
* the user must close it manually) or set by avformat_open_input().
* encoding: set by the user.
*
* Do NOT set this field if AVFMT_NOFILE flag is set in
* iformat/oformat.flags. In such a case, the (de)muxer will handle
* I/O in some other way and this field will be NULL.
*/
AVIOContext *pb;

/* stream info */
int ctx_flags; /**< Format-specific flags, see AVFMTCTX_xx */

/**
* A list of all streams in the file. New streams are created with
* avformat_new_stream().
*
* decoding: streams are created by libavformat in avformat_open_input().
* If AVFMTCTX_NOHEADER is set in ctx_flags, then new streams may also
* appear in av_read_frame().
* encoding: streams are created by the user before avformat_write_header().
*/
unsigned int nb_streams;
AVStream **streams;

char filename[1024]; /**< input or output filename */

/**
* Decoding: position of the first frame of the component, in
* AV_TIME_BASE fractional seconds. NEVER set this value directly:
* It is deduced from the AVStream values.
*/
int64_t start_time;

/**
* Decoding: duration of the stream, in AV_TIME_BASE fractional
* seconds. Only set this value if you know none of the individual stream
* durations and also do not set any of them. This is deduced from the
* AVStream values if not set.
*/
int64_t duration;

/**
* Decoding: total stream bitrate in bit/s, 0 if not
* available. Never set it directly if the file_size and the
* duration are known as FFmpeg can compute it automatically.
*/
int bit_rate;

unsigned int packet_size;
int max_delay;

int flags;
#define AVFMT_FLAG_GENPTS 0x0001 ///< Generate missing pts even if it requires parsing future frames.
#define AVFMT_FLAG_IGNIDX 0x0002 ///< Ignore index.
#define AVFMT_FLAG_NONBLOCK 0x0004 ///< Do not block when reading packets from input.
#define AVFMT_FLAG_IGNDTS 0x0008 ///< Ignore DTS on frames that contain both DTS & PTS
#define AVFMT_FLAG_NOFILLIN 0x0010 ///< Do not infer any values from other values, just return what is stored in the container
#define AVFMT_FLAG_NOPARSE 0x0020 ///< Do not use AVParsers, you also must set AVFMT_FLAG_NOFILLIN as the fillin code works on frames and no parsing -> no frames. Also seeking to frames can not work if parsing to find frame boundaries has been disabled
#define AVFMT_FLAG_CUSTOM_IO 0x0080 ///< The caller has supplied a custom AVIOContext, don't avio_close() it.
#define AVFMT_FLAG_DISCARD_CORRUPT 0x0100 ///< Discard frames marked corrupted
#define AVFMT_FLAG_MP4A_LATM 0x8000 ///< Enable RTP MP4A-LATM payload
#define AVFMT_FLAG_SORT_DTS 0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)
#define AVFMT_FLAG_PRIV_OPT 0x20000 ///< Enable use of private options by delaying codec open (this could be made default once all code is converted)
#define AVFMT_FLAG_KEEP_SIDE_DATA 0x40000 ///< Dont merge side data but keep it separate.

/**
* decoding: size of data to probe; encoding: unused.
*/
unsigned int probesize;

/**
* decoding: maximum time (in AV_TIME_BASE units) during which the input should
* be analyzed in avformat_find_stream_info().
*/
int max_analyze_duration;

const uint8_t *key;
int keylen;

unsigned int nb_programs;
AVProgram **programs;

/**
* Forced video codec_id.
* Demuxing: Set by user.
*/
enum CodecID video_codec_id;

/**
* Forced audio codec_id.
* Demuxing: Set by user.
*/
enum CodecID audio_codec_id;

/**
* Forced subtitle codec_id.
* Demuxing: Set by user.
*/
enum CodecID subtitle_codec_id;

/**
* Maximum amount of memory in bytes to use for the index of each stream.
* If the index exceeds this size, entries will be discarded as
* needed to maintain a smaller size. This can lead to slower or less
* accurate seeking (depends on demuxer).
* Demuxers for which a full in-memory index is mandatory will ignore
* this.
* muxing : unused
* demuxing: set by user
*/
unsigned int max_index_size;

/**
* Maximum amount of memory in bytes to use for buffering frames
* obtained from realtime capture devices.
*/
unsigned int max_picture_buffer;

unsigned int nb_chapters;
AVChapter **chapters;

AVDictionary *metadata;

/**
* Start time of the stream in real world time, in microseconds
* since the unix epoch (00:00 1st January 1970). That is, pts=0
* in the stream was captured at this real world time.
* - encoding: Set by user.
* - decoding: Unused.
*/
int64_t start_time_realtime;

/**
* decoding: number of frames used to probe fps
*/
int fps_probe_size;

/**
* Error recognition; higher values will detect more errors but may
* misdetect some more or less valid parts as errors.
* - encoding: unused
* - decoding: Set by user.
*/
int error_recognition;

/**
* Custom interrupt callbacks for the I/O layer.
*
* decoding: set by the user before avformat_open_input().
* encoding: set by the user before avformat_write_header()
* (mainly useful for AVFMT_NOFILE formats). The callback
* should also be passed to avio_open2() if it's used to
* open the file.
*/
AVIOInterruptCB interrupt_callback;

/**
* Flags to enable debugging.
*/
int debug;
#define FF_FDEBUG_TS 0x0001

/**
* Transport stream id.
* This will be moved into demuxer private options. Thus no API/ABI compatibility
*/
int ts_id;

/**
* Audio preload in microseconds.
* Note, not all formats support this and unpredictable things may happen if it is used when not supported.
* - encoding: Set by user via AVOptions (NO direct access)
* - decoding: unused
*/
int audio_preload;

/**
* Max chunk time in microseconds.
* Note, not all formats support this and unpredictable things may happen if it is used when not supported.
* - encoding: Set by user via AVOptions (NO direct access)
* - decoding: unused
*/
int max_chunk_duration;

/**
* Max chunk size in bytes
* Note, not all formats support this and unpredictable things may happen if it is used when not supported.
* - encoding: Set by user via AVOptions (NO direct access)
* - decoding: unused
*/
int max_chunk_size;

/*****************************************************************
* 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.
*****************************************************************
*/

/**
* This buffer is only needed when packets were already buffered but
* not decoded, for example to get the codec parameters in MPEG
* streams.
*/
struct AVPacketList *packet_buffer;
struct AVPacketList *packet_buffer_end;

/* av_seek_frame() support */
int64_t data_offset; /**< offset of the first packet */

/**
* Raw packets from the demuxer, prior to parsing and decoding.
* This buffer is used for buffering packets until the codec can
* be identified, as parsing cannot be done without knowing the
* codec.
*/
struct AVPacketList *raw_packet_buffer;
struct AVPacketList *raw_packet_buffer_end;
/**
* Packets split by the parser get queued here.
*/
struct AVPacketList *parse_queue;
struct AVPacketList *parse_queue_end;
/**
* Remaining size available for raw_packet_buffer, in bytes.
*/
#define RAW_PACKET_BUFFER_SIZE 2500000
int raw_packet_buffer_remaining_size;
} AVFormatContext;


在使用FFMPEG进行开发的时候，AVFormatContext是一个贯穿始终的数据结构，很多函数都要用到它作为参数。它是FFMPEG解封装（flv，mp4，rmvb，avi）功能的结构体。下面看几个主要变量的作用（在这里考虑解码的情况）：

struct AVInputFormat *iformat：输入数据的封装格式
AVIOContext *pb：输入数据的缓存
unsigned int nb_streams：视音频流的个数
AVStream **streams：视音频流
char filename[1024]：文件名
int64_t duration：时长（单位：微秒us，转换为秒需要除以1000000）
int bit_rate：比特率（单位bps，转换为kbps需要除以1000）
AVDictionary *metadata：元数据

视频的时长可以转换成HH:MM:SS的形式，示例代码如下：
AVFormatContext *pFormatCtx;
CString timelong;
...
//duration是以微秒为单位
//转换成hh:mm:ss形式
int tns, thh, tmm, tss;
tns = (pFormatCtx->duration)/1000000;
thh = tns / 3600;
tmm = (tns % 3600) / 60;
tss = (tns % 60);
timelong.Format("%02d:%02d:%02d",thh,tmm,tss);

视频的原数据（metadata）信息可以通过AVDictionary获取。元数据存储在AVDictionaryEntry结构体中，如下所示
typedef struct AVDictionaryEntry {
char *key;
char *value;
} AVDictionaryEntry;每一条元数据分为key和value两个属性。

在ffmpeg中通过av_dict_get()函数获得视频的原数据。
下列代码显示了获取元数据并存入meta字符串变量的过程，注意每一条key和value之间有一个"\t:"，value之后有一个"\r\n"
//MetaData------------------------------------------------------------
//从AVDictionary获得
//需要用到AVDictionaryEntry对象
//CString author,copyright,description;
CString meta=NULL,key,value;
AVDictionaryEntry *m = NULL;
//不用一个一个找出来
/* m=av_dict_get(pFormatCtx->metadata,"author",m,0);
author.Format("作者：%s",m->value);
m=av_dict_get(pFormatCtx->metadata,"copyright",m,0);
copyright.Format("版权：%s",m->value);
m=av_dict_get(pFormatCtx->metadata,"description",m,0);
description.Format("描述：%s",m->value);
*/
//使用循环读出
//(需要读取的数据，字段名称，前一条字段（循环时使用），参数)
while(m=av_dict_get(pFormatCtx->metadata,"",m,AV_DICT_IGNORE_SUFFIX)){
key.Format(m->key);
value.Format(m->value);
meta+=key+"\t:"+value+"\r\n" ;
}