/**

 * Format I/O context. 这个结构体是在解封装的过程中用的，其中有很多重要的信息，所以一定要搞清楚其中的成员。  * 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 @ref avoptions. Set by avformat_alloc_context().      * Exports (de)muxer private options if they exist.      */  

const AVClass *av_class; //由avformat_malloc_context()函数来设置，我们知道AVFormatContext就是由avformat_malloc_context()申请的,申请的时候就设置了AVClass，主要是导出muxer或者demuxer的私有选项

    /**

     * The input container format.      *      * Demuxing only, set by avformat_open_input().      */     struct AVInputFormat *iformat;  //表示输入的封装格式，在avformat_open_input()中设置，解封装中设置

    /**

     * 输出容器格式The output container format.      *      * Muxing only, must be set by the caller before avformat_write_header().      */     struct AVOutputFormat *oformat; //表示输出的封装格式，必须在avformat_write_header()之前被调用者设置，封装中设置

    /**

     * Format private data. This is an AVOptions-enabled struct      * if and only if iformat/oformat.priv_class is not NULL.      *      * - muxing: set by avformat_write_header()     * - demuxing: set by avformat_open_input()      */     void *priv_data; //格式私有的数据，解封装的过程中，由avformat_open_input()函数设置，

记住avformat_open_input（)打开一个输入流并且读取头部，这时候编解码器还没有打开 ，这个打开的流必须用avformat_close_input()函数来关闭

    /**

     * I/O context.      *      * - demuxing: either set by the user before avformat_open_input() (then      *             the user must close it manually) or set by avformat_open_input().      * - muxing: set by the user before avformat_write_header(). The caller must      *           take care of closing / freeing the IO context.      *      * 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; //输入输出上下文，输入数据的缓存

解封装中：要么由用户在avformat_open_input()之前设置(用户手动释放)，要么就由avformat_open_input()来设置

封装中：由用户在avformat_write_header()之前设置，调用者必须当心关闭和释放IO上下文

    /* stream info */

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

    /**

     * Number of elements in AVFormatContext.streams.      *      * Set by avformat_new_stream(), must not be modified by any other code.      */     unsigned int nb_streams;   //AVFormatContext.streams数组中元素的个数，由avformat_new_stream()设置，必须不能被其他代码修改     /**      * A list of all streams in the file. New streams are created with      * avformat_new_stream().      *      * - demuxing: 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().      * - muxing: streams are created by the user before avformat_write_header().      *      * Freed by libavformat in avformat_free_context().      */     AVStream **streams;  //文件中所有流的列表，新流是用avformat_new_stream()函数创建的，由avformat_free_context()函数来释放

    /**

     * input or output filename      *      * - demuxing: set by avformat_open_input()      * - muxing: may be set by the caller before avformat_write_header()      */     char filename[1024]; //流总要打开文件吧，这个数组就是文件名

    /**

     * 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.      *      * Demuxing only, set by libavformat.      */     int64_t start_time; //第一帧的位置，单位为 AV_TIME_BASE fractional seconds，它是由AVStream的值中推导出来的，不要直接设置

    /**

     * 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.      *      * Demuxing only, set by libavformat.      */     int64_t duration;    //以AV_TIME_BASE小数秒为单位的流的持续时间，如果完全不知道任何一个流的持续时间的时候才设置，不要设置他们中的任何一个。这种情况下，这个是从AVStream的值中推导出来的

    /**

     * 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; //以b/s为单位的平均码率，不可获取时为0.如果文件的大小和持续时间可以被FFmpeg自动计算出来则绝不要设置

    unsigned int packet_size;   //pakcet的大小

    int max_delay;   //最大的时延

    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_NOBUFFER     0x0040 ///< Do not buffer frames when possible #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_FLUSH_PACKETS    0x0200 ///< Flush the AVIOContext every packet. //这就是值为512的对应的flag，每一个包都刷新AVIOContext结构体 #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 ///< Don't merge side data but keep it separate.

    /**

     * Maximum size of the data read from input for determining      * the input container format.      * Demuxing only, set by the caller before avformat_open_input().      */     unsigned int probesize;  //用于决定输入容器格式的，从输入读取的数据的最大大小，仅用于解封装，由调用者在avformat_open_input()之前设置

    /**

     * Maximum duration (in AV_TIME_BASE units) of the data read      * from input in avformat_find_stream_info().      * Demuxing only, set by the caller before avformat_find_stream_info().      */     int max_analyze_duration; //在avformat_find_stream_info()中的input读取的数据的最大的持续时间（以AV_TIME_BASE为单位）,仅用于解封装，由调用者在avformat_find_stream_info()之前设置。

    const uint8_t *key; 

    int keylen; 

    unsigned int nb_programs; 

    AVProgram **programs; 

    /**

     * Forced video codec_id.      * Demuxing: Set by user.      */     enum AVCodecID video_codec_id;   //强制视频编解码器的ID，在解封装的时候可由用户设定

    /**

     * Forced audio codec_id.      * Demuxing: Set by user.      */     enum AVCodecID audio_codec_id;  //强制音频编解码器的ID，在解封装的时候可由用户设定

    /**

     * Forced subtitle codec_id.      * Demuxing: Set by user.      */     enum AVCodecID subtitle_codec_id; //强制副编解码器的ID，在解封装的时候可由用户设定

    /**

     * 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; //每一个流的索引使用的最大数量的内存（以字节为单位）

//如果索引超过了这个大小，条目将会被丢弃来维持一个更小的尺寸,这能够导致更慢或者定位精度降低。一个完全在内存中的索引的解封装器将强制忽略这个。封装的时候不用，解封装的时候由用户设置

    /**

     * Maximum amount of memory in bytes to use for buffering frames      * obtained from realtime capture devices.      */     unsigned int max_picture_buffer;  //从实时捕捉设备中获取的那些缓冲帧使用的内存的最大字节数

    /**

     * Number of chapters in AVChapter array.      * When muxing, chapters are normally written in the file header,      * so nb_chapters should normally be initialized before write_header      * is called. Some muxers (e.g. mov and mkv) can also write chapters      * in the trailer.  To write chapters in the trailer, nb_chapters      * must be zero when write_header is called and non-zero when      * write_trailer is called.      * - muxing: set by user      * - demuxing: set by libavformat      */    unsigned int nb_chapters;  //     AVChapter **chapters;      //

    /**

     * Metadata that applies to the whole file.      *      * - demuxing: set by libavformat in avformat_open_input()      * - muxing: may be set by the caller before avformat_write_header()      *      * Freed by libavformat in avformat_free_context().      */     AVDictionary *metadata;   //应用到整个文件的元数据，由Libavformat在avformat_free_context()函数释放

//解封装的时候：由libavformat库在函数avformat_open_input()中设置

//封装的时候：可以由用户在avformat_write_header()之前设置

    /**

     * 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.      * - muxing: Set by the caller before avformat_write_header(). If set to      *           either 0 or AV_NOPTS_VALUE, then the current wall-time will      *           be used.      * - demuxing: Set by libavformat. AV_NOPTS_VALUE if unknown. Note that      *             the value may become known after some number of frames      *             have been received.      */     int64_t start_time_realtime;   //以从1970年开始的真实时间的us数来计算的起始时间，也就是说流中pts=0的是在这个真实时间捕获的

//封装的时候：由调用者在avformat_write_header()之前设置，设为0，则使用当前的墙钟时间

//解封装的时候：由libavformat设置

    /**

     * The number of frames used for determining the framerate in      * avformat_find_stream_info().      * Demuxing only, set by the caller before avformat_find_stream_info().      */     int fps_probe_size; //在avformat_find_stream_info()函数中使用的用于计算帧率的帧数，仅用于解封装，caller在avformat_find_stream_info()函数之前设置

    /**

     * Error recognition; higher values will detect more errors but may      * misdetect some more or less valid parts as errors.      * Demuxing only, set by the caller before avformat_open_input().      */     int error_recognition; //错误识别，更高的值将检测更多的错误也会把多少有用的部分当做错误，仅用于解封装，caller在avformat_find_stream_info()函数之前设置

    /**

     * Custom interrupt callbacks for the I/O layer.      *      * demuxing: set by the user before avformat_open_input().      * muxing: 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; //定制IO层的中断回调函数

    /**

     * Flags to enable debugging.      */     int debug;   //使能调试的标志位 #define FF_FDEBUG_TS        0x0001

    /**

     * Maximum buffering duration for interleaving.      *      * To ensure all the streams are interleaved correctly,      * av_interleaved_write_frame() will wait until it has at least one packet      * for each stream before actually writing any packets to the output file.      * When some streams are "sparse" (i.e. there are large gaps between      * successive packets), this can result in excessive buffering.      *      * This field specifies the maximum difference between the timestamps of the      * first and the last packet in the muxing queue, above which libavformat      * will output a packet regardless of whether it has queued a packet for all      * the streams.      *      * Muxing only, set by the caller before avformat_write_header().      */     int64_t max_interleave_delta;

    /**

     * Transport stream id.      * This will be moved into demuxer private options. Thus no API/ABI compatibility      */     int ts_id; //转换流id，这个将改为解封装器私有选项，故没有兼容性

    /**

     * 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; //以us为单位的音频预加载

    /**

     * 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; //以字节为单位的chunk的最大尺寸

    /**

     * forces the use of wallclock timestamps as pts/dts of packets      * This has undefined results in the presence of B frames.      * - encoding: unused      * - decoding: Set by user via AVOptions (NO direct access)      */     int use_wallclock_as_timestamps; //强制packet的pts/dts使用墙钟时间戳

    /**

     * Avoid negative timestamps during muxing.      *  0 -> allow negative timestamps      *  1 -> avoid negative timestamps      * -1 -> choose automatically (default)      * Note, this only works when interleave_packet_per_dts is in use.      * - encoding: Set by user via AVOptions (NO direct access)      * - decoding: unused      */     int avoid_negative_ts;

    /**

     * avio flags, used to force AVIO_FLAG_DIRECT.      * - encoding: unused      * - decoding: Set by user via AVOptions (NO direct access)      */     int avio_flags;

    /**

     * The duration field can be estimated through various ways, and this field can be used      * to know how the duration was estimated.      * - encoding: unused      * - decoding: Read by user via AVOptions (NO direct access)      */    enum AVDurationEstimationMethod duration_estimation_method;//因为持续时间可以通过很多方法估算出来，这个字段用于指导持续时间是如何计算的

    /**

     * Skip initial bytes when opening stream      * - encoding: unused      * - decoding: Set by user via AVOptions (NO direct access)      */     unsigned int skip_initial_bytes;//打开流时跳过最初的字节数

    /**

     * Correct single timestamp overflows      * - encoding: unused      * - decoding: Set by user via AVOptions (NO direct access)      */     unsigned int correct_ts_overflow;//改正单个时间戳溢出

    /**

     * Force seeking to any (also non key) frames.      * - encoding: unused      * - decoding: Set by user via AVOptions (NO direct access)      */     int seek2any; //强制定位到任何帧（非关键帧也一样）

    /**

     * Flush the I/O context after each packet.      * - encoding: Set by user via AVOptions (NO direct access)      * - decoding: unused      */     int flush_packets;//在每一个packet之后刷新IO上下文

    /**

     * format probing score.      * The maximal score is AVPROBE_SCORE_MAX, its set when the demuxer probes      * the format.      * - encoding: unused      * - decoding: set by avformat, read by user via av_format_get_probe_score() (NO direct access)      */     int probe_score;  //格式探索分数，当解封装器探测出格式的时候设为最大AVPROBE_SCORE_MAX，解封装的时候，由libavforamt设置，用户通过av_format_get_probe_score()读取

    /*****************************************************************华丽的分割线

     * 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;

    /**

     * Offset to remap timestamps to be non-negative.      * Expressed in timebase units.      * @see AVStream.mux_ts_offset      */     int64_t offset;

    /**

     * Timebase for the timestamp offset.      */     AVRational offset_timebase;

    /**

     * An opaque field for libavformat internal usage.      * Must not be accessed in any way by callers.      */     AVFormatInternal *internal;

    /**

     * IO repositioned flag.      * This is set by avformat when the underlaying IO context read pointer      * is repositioned, for example when doing byte based seeking.      * Demuxers can use the flag to detect such changes.      */     int io_repositioned;

    /**

     * Forced video codec.      * This allows forcing a specific decoder, even when there are multiple with      * the same codec_id.      * Demuxing: Set by user via av_format_set_video_codec (NO direct access).      */     AVCodec *video_codec;

    /**

     * Forced audio codec.      * This allows forcing a specific decoder, even when there are multiple with      * the same codec_id.      * Demuxing: Set by user via av_format_set_audio_codec (NO direct access).      */     AVCodec *audio_codec;

    /**

     * Forced subtitle codec.      * This allows forcing a specific decoder, even when there are multiple with      * the same codec_id.      * Demuxing: Set by user via av_format_set_subtitle_codec (NO direct access).      */     AVCodec *subtitle_codec;

    /**

     * Number of bytes to be written as padding in a metadata header.      * Demuxing: Unused.      * Muxing: Set by user via av_format_set_metadata_header_padding.      */     int metadata_header_padding;

    /**

     * User data.      * This is a place for some private data of the user.      * Mostly usable with control_message_cb or any future callbacks in device's context.      */     void *opaque;

    /**

     * Callback used by devices to communicate with application.      */     av_format_control_message control_message_cb;

    /**

     * Output timestamp offset, in microseconds.      * Muxing: set by user via AVOptions (NO direct access)      */     int64_t output_ts_offset; } AVFormatContext;