---

  团队博客: 汽车电子社区

---

一、概述

  avformat中实现了目前多媒体领域中的几乎所有封装格式，可以封装，可以解封装（也叫解复用），根据需求不同，所支持的也有所不同，ffmpeg能否支持一种封装格式的视频的封装与解封装，完全取决于这个库，例如mp4、flv、mkv等容器的封装与解封装；又或者RTMP、RTSP、TCP、UDP等协议的封装与解封装；关于封装与解封装的操作，对CPU的消耗不会特别严重，因为封装与解封装不涉及到复杂的计算，更多的是I/O，如果希望增加自己定制的封装格式的话，需要在libavformat中增加自己的对应的format模块。通过代码的文件结构可以看得出来。

二、解封装流程

  1、avformat_alloc_context()：负责申请一个AVFormatContext结构的内存,并进行简单初始化。
  2、avformat_open_input()：打开输入音/视频文件。
  3、avformat_find_stream_info()：获取音视频文件信息。
  4、av_read_frame()：读取音视频包。
  5、avformat_seek_file()：定位文件。
  6、av_seek_frame()：定位文件。
  7、avformat_free_context()：释放该结构里的所有东西以及该结构本身。
  8、avformat_close_input()：关闭解复用器。关闭后就不再需要使用avformat_free_context 进行释放。

三、重要结构体

3.1、AVFormatContext

  AVFormatContext是FFmpeg中一个非常重要的结构体，它包含了一个完整的媒体文件或媒体流的所有信息。在FFmpeg中，每一个AVFormatContext结构体都对应着一个媒体文件或媒体流。当我们使用FFmpe对媒体文件或者媒体流的解码、编码或者转换时，都是基于AVFormatContext结构体进行操作的。
  AVFormatContext结构体包含的信息很多，其中最重要的时AVInputFormat和AVOutputFormat，它们分表代表输入和输出的格式，包含了所有支持的编解码器、封装格式、容器格式等信息。在使用FFmpeg进行媒体文件或者媒体流的解码、编码或者转换时，我们需要根据不同的输入和输出格式来选择合适的VInputFormat和AVOutputFormat。
  除了AVInputFormat和AVOutputFormat以外，AVFormatContext还包含了一些其它的信息，比如AVStream，它表示一个媒体流的所有信息，包括媒体流的类型、编码方式、采样率、帧率等；AVCodecContext表示一个编解码器的所有信息，包括编解码器的类型、编解码方式、采样率、帧率等等；AVDictionary它是一个字典类型的结构体，用于存储一些额外的元数据信息，比如媒体文件的标题、作者、描述等等。

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;

    /**
     * The input container format.
     *
     * Demuxing only, set by avformat_open_input().
     */
    const struct AVInputFormat *iformat;

    /**
     * The output container format.
     *
     * Muxing only, must be set by the caller before avformat_write_header().
     */
    const struct AVOutputFormat *oformat;

    /**
     * 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;

    /**
     * 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;

    /* stream info */
    /**
     * Flags signalling stream properties. A combination of AVFMTCTX_*.
     * Set by libavformat.
     */
    int ctx_flags;

    /**
     * Number of elements in AVFormatContext.streams.
     *
     * Set by avformat_new_stream(), must not be modified by any other code.
     */
    unsigned int nb_streams;
    /**
     * 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;

    /**
     * input or output URL. Unlike the old filename field, this field has no
     * length restriction.
     *
     * - demuxing: set by avformat_open_input(), initialized to an empty
     *             string if url parameter was NULL in avformat_open_input().
     * - muxing: may be set by the caller before calling avformat_write_header()
     *           (or avformat_init_output() if that is called first) to a string
     *           which is freeable by av_free(). Set to an empty string if it
     *           was NULL in avformat_init_output().
     *
     * Freed by libavformat in avformat_free_context().
     */
    char *url;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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.
     */
    int64_t bit_rate;

    unsigned int packet_size;
    int max_delay;

    /**
     * Flags modifying the (de)muxer behaviour. A combination of AVFMT_FLAG_*.
     * Set by the user before avformat_open_input() / avformat_write_header().
     */
    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.
/**
 * When muxing, try to avoid writing any random/volatile data to the output.
 * This includes any random IDs, real-time timestamps/dates, muxer version, etc.
 *
 * This flag is mainly intended for testing.
 */
#define AVFMT_FLAG_BITEXACT         0x0400
#define AVFMT_FLAG_SORT_DTS    0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)
#define AVFMT_FLAG_FAST_SEEK   0x80000 ///< Enable fast, but inaccurate seeks for some formats
#if FF_API_LAVF_SHORTEST
#define AVFMT_FLAG_SHORTEST   0x100000 ///< Stop muxing when the shortest stream stops.
#endif
#define AVFMT_FLAG_AUTO_BSF   0x200000 ///< Add bitstream filters as requested by the muxer

    /**
     * Maximum number of bytes read from input in order to determine stream
     * properties. Used when reading the global header and in
     * avformat_find_stream_info().
     *
     * Demuxing only, set by the caller before avformat_open_input().
     *
     * @note this is \e not  used for determining the \ref AVInputFormat
     *       "input format"
     * @sa format_probesize
     */
    int64_t probesize;

    /**
     * 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().
     * Can be set to 0 to let avformat choose using a heuristic.
     */
    int64_t max_analyze_duration;

    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;

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

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

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * Flags to enable debugging.
     */
    int debug;
#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;

    /**
     * Allow non-standard and experimental extension
     * @see AVCodecContext.strict_std_compliance
     */
    int strict_std_compliance;

    /**
     * Flags indicating events happening on the file, a combination of
     * AVFMT_EVENT_FLAG_*.
     *
     * - demuxing: may be set by the demuxer in avformat_open_input(),
     *   avformat_find_stream_info() and av_read_frame(). Flags must be cleared
     *   by the user once the event has been handled.
     * - muxing: may be set by the user after avformat_write_header() to
     *   indicate a user-triggered event.  The muxer will clear the flags for
     *   events it has handled in av_[interleaved]_write_frame().
     */
    int event_flags;
/**
 * - demuxing: the demuxer read new metadata from the file and updated
 *   AVFormatContext.metadata accordingly
 * - muxing: the user updated AVFormatContext.metadata and wishes the muxer to
 *   write it into the file
 */
#define AVFMT_EVENT_FLAG_METADATA_UPDATED 0x0001

    /**
     * Maximum number of packets to read while waiting for the first timestamp.
     * Decoding only.
     */
    int max_ts_probe;

    /**
     * Avoid negative timestamps during muxing.
     * Any value of the AVFMT_AVOID_NEG_TS_* constants.
     * Note, this works better when using av_interleaved_write_frame().
     * - muxing: Set by user
     * - demuxing: unused
     */
    int avoid_negative_ts;
#define AVFMT_AVOID_NEG_TS_AUTO             -1 ///< Enabled when required by target format
#define AVFMT_AVOID_NEG_TS_DISABLED          0 ///< Do not shift timestamps even when they are negative.
#define AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE 1 ///< Shift timestamps so they are non negative
#define AVFMT_AVOID_NEG_TS_MAKE_ZERO         2 ///< Shift timestamps so that they start at 0

    /**
     * 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
     * - 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
     * - 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
     * - decoding: unused
     */
    int max_chunk_size;

    /**
     * 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
     */
    int use_wallclock_as_timestamps;

    /**
     * avio flags, used to force AVIO_FLAG_DIRECT.
     * - encoding: unused
     * - decoding: Set by user
     */
    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
     */
    enum AVDurationEstimationMethod duration_estimation_method;

    /**
     * Skip initial bytes when opening stream
     * - encoding: unused
     * - decoding: Set by user
     */
    int64_t skip_initial_bytes;

    /**
     * Correct single timestamp overflows
     * - encoding: unused
     * - decoding: Set by user
     */
    unsigned int correct_ts_overflow;

    /**
     * Force seeking to any (also non key) frames.
     * - encoding: unused
     * - decoding: Set by user
     */
    int seek2any;

    /**
     * Flush the I/O context after each packet.
     * - encoding: Set by user
     * - decoding: unused
     */
    int flush_packets;

    /**
     * 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
     */
    int probe_score;

    /**
     * Maximum number of bytes read from input in order to identify the
     * \ref AVInputFormat "input format". Only used when the format is not set
     * explicitly by the caller.
     *
     * Demuxing only, set by the caller before avformat_open_input().
     *
     * @sa probesize
     */
    int format_probesize;

    /**
     * ',' separated list of allowed decoders.
     * If NULL then all are allowed
     * - encoding: unused
     * - decoding: set by user
     */
    char *codec_whitelist;

    /**
     * ',' separated list of allowed demuxers.
     * If NULL then all are allowed
     * - encoding: unused
     * - decoding: set by user
     */
    char *format_whitelist;

    /**
     * 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
     */
    const struct 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
     */
    const struct 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
     */
    const struct AVCodec *subtitle_codec;

    /**
     * Forced data codec.
     * This allows forcing a specific decoder, even when there are multiple with
     * the same codec_id.
     * Demuxing: Set by user
     */
    const struct AVCodec *data_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.
     */
    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
     */
    int64_t output_ts_offset;

    /**
     * dump format separator.
     * can be ", " or "\n      " or anything else
     * - muxing: Set by user.
     * - demuxing: Set by user.
     */
    uint8_t *dump_separator;

    /**
     * Forced Data codec_id.
     * Demuxing: Set by user.
     */
    enum AVCodecID data_codec_id;

    /**
     * ',' separated list of allowed protocols.
     * - encoding: unused
     * - decoding: set by user
     */
    char *protocol_whitelist;

    /**
     * A callback for opening new IO streams.
     *
     * Whenever a muxer or a demuxer needs to open an IO stream (typically from
     * avformat_open_input() for demuxers, but for certain formats can happen at
     * other times as well), it will call this callback to obtain an IO context.
     *
     * @param s the format context
     * @param pb on success, the newly opened IO context should be returned here
     * @param url the url to open
     * @param flags a combination of AVIO_FLAG_*
     * @param options a dictionary of additional options, with the same
     *                semantics as in avio_open2()
     * @return 0 on success, a negative AVERROR code on failure
     *
     * @note Certain muxers and demuxers do nesting, i.e. they open one or more
     * additional internal format contexts. Thus the AVFormatContext pointer
     * passed to this callback may be different from the one facing the caller.
     * It will, however, have the same 'opaque' field.
     */
    int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,
                   int flags, AVDictionary **options);

#if FF_API_AVFORMAT_IO_CLOSE
    /**
     * A callback for closing the streams opened with AVFormatContext.io_open().
     *
     * @deprecated use io_close2
     */
    attribute_deprecated
    void (*io_close)(struct AVFormatContext *s, AVIOContext *pb);
#endif

    /**
     * ',' separated list of disallowed protocols.
     * - encoding: unused
     * - decoding: set by user
     */
    char *protoctypedef 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;

    /**
     * The input container format.
     *
     * Demuxing only, set by avformat_open_input().
     */
    const struct AVInputFormat *iformat;

    /**
     * The output container format.
     *
     * Muxing only, must be set by the caller before avformat_write_header().
     */
    const struct AVOutputFormat *oformat;

    /**
     * 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;

    /**
     * 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;

    /* stream info */
    /**
     * Flags signalling stream properties. A combination of AVFMTCTX_*.
     * Set by libavformat.
     */
    int ctx_flags;

    /**
     * Number of elements in AVFormatContext.streams.
     *
     * Set by avformat_new_stream(), must not be modified by any other code.
     */
    unsigned int nb_streams;
    /**
     * 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;

    /**
     * input or output URL. Unlike the old filename field, this field has no
     * length restriction.
     *
     * - demuxing: set by avformat_open_input(), initialized to an empty
     *             string if url parameter was NULL in avformat_open_input().
     * - muxing: may be set by the caller before calling avformat_write_header()
     *           (or avformat_init_output() if that is called first) to a string
     *           which is freeable by av_free(). Set to an empty string if it
     *           was NULL in avformat_init_output().
     *
     * Freed by libavformat in avformat_free_context().
     */
    char *url;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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.
     */
    int64_t bit_rate;

    unsigned int packet_size;
    int max_delay;

    /**
     * Flags modifying the (de)muxer behaviour. A combination of AVFMT_FLAG_*.
     * Set by the user before avformat_open_input() / avformat_write_header().
     */
    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.
/**
 * When muxing, try to avoid writing any random/volatile data to the output.
 * This includes any random IDs, real-time timestamps/dates, muxer version, etc.
 *
 * This flag is mainly intended for testing.
 */
#define AVFMT_FLAG_BITEXACT         0x0400
#define AVFMT_FLAG_SORT_DTS    0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)
#define AVFMT_FLAG_FAST_SEEK   0x80000 ///< Enable fast, but inaccurate seeks for some formats
#if FF_API_LAVF_SHORTEST
#define AVFMT_FLAG_SHORTEST   0x100000 ///< Stop muxing when the shortest stream stops.
#endif
#define AVFMT_FLAG_AUTO_BSF   0x200000 ///< Add bitstream filters as requested by the muxer

    /**
     * Maximum number of bytes read from input in order to determine stream
     * properties. Used when reading the global header and in
     * avformat_find_stream_info().
     *
     * Demuxing only, set by the caller before avformat_open_input().
     *
     * @note this is \e not  used for determining the \ref AVInputFormat
     *       "input format"
     * @sa format_probesize
     */
    int64_t probesize;

    /**
     * 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().
     * Can be set to 0 to let avformat choose using a heuristic.
     */
    int64_t max_analyze_duration;

    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;

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

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

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * Flags to enable debugging.
     */
    int debug;
#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;

    /**
     * Allow non-standard and experimental extension
     * @see AVCodecContext.strict_std_compliance
     */
    int strict_std_compliance;

    /**
     * Flags indicating events happening on the file, a combination of
     * AVFMT_EVENT_FLAG_*.
     *
     * - demuxing: may be set by the demuxer in avformat_open_input(),
     *   avformat_find_stream_info() and av_read_frame(). Flags must be cleared
     *   by the user once the event has been handled.
     * - muxing: may be set by the user after avformat_write_header() to
     *   indicate a user-triggered event.  The muxer will clear the flags for
     *   events it has handled in av_[interleaved]_write_frame().
     */
    int event_flags;
/**
 * - demuxing: the demuxer read new metadata from the file and updated
 *   AVFormatContext.metadata accordingly
 * - muxing: the user updated AVFormatContext.metadata and wishes the muxer to
 *   write it into the file
 */
#define AVFMT_EVENT_FLAG_METADATA_UPDATED 0x0001

    /**
     * Maximum number of packets to read while waiting for the first timestamp.
     * Decoding only.
     */
    int max_ts_probe;

    /**
     * Avoid negative timestamps during muxing.
     * Any value of the AVFMT_AVOID_NEG_TS_* constants.
     * Note, this works better when using av_interleaved_write_frame().
     * - muxing: Set by user
     * - demuxing: unused
     */
    int avoid_negative_ts;
#define AVFMT_AVOID_NEG_TS_AUTO             -1 ///< Enabled when required by target format
#define AVFMT_AVOID_NEG_TS_DISABLED          0 ///< Do not shift timestamps even when they are negative.
#define AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE 1 ///< Shift timestamps so they are non negative
#define AVFMT_AVOID_NEG_TS_MAKE_ZERO         2 ///< Shift timestamps so that they start at 0

    /**
     * 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
     * - 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
     * - 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
     * - decoding: unused
     */
    int max_chunk_size;

    /**
     * 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
     */
    int use_wallclock_as_timestamps;

    /**
     * avio flags, used to force AVIO_FLAG_DIRECT.
     * - encoding: unused
     * - decoding: Set by user
     */
    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
     */
    enum AVDurationEstimationMethod duration_estimation_method;

    /**
     * Skip initial bytes when opening stream
     * - encoding: unused
     * - decoding: Set by user
     */
    int64_t skip_initial_bytes;

    /**
     * Correct single timestamp overflows
     * - encoding: unused
     * - decoding: Set by user
     */
    unsigned int correct_ts_overflow;

    /**
     * Force seeking to any (also non key) frames.
     * - encoding: unused
     * - decoding: Set by user
     */
    int seek2any;

    /**
     * Flush the I/O context after each packet.
     * - encoding: Set by user
     * - decoding: unused
     */
    int flush_packets;

    /**
     * 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
     */
    int probe_score;

    /**
     * Maximum number of bytes read from input in order to identify the
     * \ref AVInputFormat "input format". Only used when the format is not set
     * explicitly by the caller.
     *
     * Demuxing only, set by the caller before avformat_open_input().
     *
     * @sa probesize
     */
    int format_probesize;

    /**
     * ',' separated list of allowed decoders.
     * If NULL then all are allowed
     * - encoding: unused
     * - decoding: set by user
     */
    char *codec_whitelist;

    /**
     * ',' separated list of allowed demuxers.
     * If NULL then all are allowed
     * - encoding: unused
     * - decoding: set by user
     */
    char *format_whitelist;

    /**
     * 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
     */
    const struct 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
     */
    const struct 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
     */
    const struct AVCodec *subtitle_codec;

    /**
     * Forced data codec.
     * This allows forcing a specific decoder, even when there are multiple with
     * the same codec_id.
     * Demuxing: Set by user
     */
    const struct AVCodec *data_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.
     */
    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
     */
    int64_t output_ts_offset;

    /**
     * dump format separator.
     * can be ", " or "\n      " or anything else
     * - muxing: Set by user.
     * - demuxing: Set by user.
     */
    uint8_t *dump_separator;

    /**
     * Forced Data codec_id.
     * Demuxing: Set by user.
     */
    enum AVCodecID data_codec_id;

    /**
     * ',' separated list of allowed protocols.
     * - encoding: unused
     * - decoding: set by user
     */
    char *protocol_whitelist;

    /**
     * A callback for opening new IO streams.
     *
     * Whenever a muxer or a demuxer needs to open an IO stream (typically from
     * avformat_open_input() for demuxers, but for certain formats can happen at
     * other times as well), it will call this callback to obtain an IO context.
     *
     * @param s the format context
     * @param pb on success, the newly opened IO context should be returned here
     * @param url the url to open
     * @param flags a combination of AVIO_FLAG_*
     * @param options a dictionary of additional options, with the same
     *                semantics as in avio_open2()
     * @return 0 on success, a negative AVERROR code on failure
     *
     * @note Certain muxers and demuxers do nesting, i.e. they open one or more
     * additional internal format contexts. Thus the AVFormatContext pointer
     * passed to this callback may be different from the one facing the caller.
     * It will, however, have the same 'opaque' field.
     */
    int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,
                   int flags, AVDictionary **options);

#if FF_API_AVFORMAT_IO_CLOSE
    /**
     * A callback for closing the streams opened with AVFormatContext.io_open().
     *
     * @deprecated use io_close2
     */
    attribute_deprecated
    void (*io_close)(struct AVFormatContext *s, AVIOContext *pb);
#endif

    /**
     * ',' separated list of disallowed protocols.
     * - encoding: unused
     * - decoding: set by user
     */
    char *protocol_blacklist;

    /**
     * The maximum number of streams.
     * - encoding: unused
     * - decoding: set by user
     */
    int max_streams;

    /**
     * Skip duration calcuation in estimate_timings_from_pts.
     * - encoding: unused
     * - decoding: set by user
     */
    int skip_estimate_duration_from_pts;

    /**
     * Maximum number of packets that can be probed
     * - encoding: unused
     * - decoding: set by user
     */
    int max_probe_packets;

    /**
     * A callback for closing the streams opened with AVFormatContext.io_open().
     *
     * Using this is preferred over io_close, because this can return an error.
     * Therefore this callback is used instead of io_close by the generic
     * libavformat code if io_close is NULL or the default.
     *
     * @param s the format context
     * @param pb IO context to be closed and freed
     * @return 0 on success, a negative AVERROR code on failure
     */
    int (*io_close2)(struct AVFormatContext *s, AVIOContext *pb);
} 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;

    /**
     * The input container format.
     *
     * Demuxing only, set by avformat_open_input().
     */
    const struct AVInputFormat *iformat;

    /**
     * The output container format.
     *
     * Muxing only, must be set by the caller before avformat_write_header().
     */
    const struct AVOutputFormat *oformat;

    /**
     * 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;

    /**
     * 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;

    /* stream info */
    /**
     * Flags signalling stream properties. A combination of AVFMTCTX_*.
     * Set by libavformat.
     */
    int ctx_flags;

    /**
     * Number of elements in AVFormatContext.streams.
     *
     * Set by avformat_new_stream(), must not be modified by any other code.
     */
    unsigned int nb_streams;
    /**
     * 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;

    /**
     * input or output URL. Unlike the old filename field, this field has no
     * length restriction.
     *
     * - demuxing: set by avformat_open_input(), initialized to an empty
     *             string if url parameter was NULL in avformat_open_input().
     * - muxing: may be set by the caller before calling avformat_write_header()
     *           (or avformat_init_output() if that is called first) to a string
     *           which is freeable by av_free(). Set to an empty string if it
     *           was NULL in avformat_init_output().
     *
     * Freed by libavformat in avformat_free_context().
     */
    char *url;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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.
     */
    int64_t bit_rate;

    unsigned int packet_size;
    int max_delay;

    /**
     * Flags modifying the (de)muxer behaviour. A combination of AVFMT_FLAG_*.
     * Set by the user before avformat_open_input() / avformat_write_header().
     */
    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.
/**
 * When muxing, try to avoid writing any random/volatile data to the output.
 * This includes any random IDs, real-time timestamps/dates, muxer version, etc.
 *
 * This flag is mainly intended for testing.
 */
#define AVFMT_FLAG_BITEXACT         0x0400
#define AVFMT_FLAG_SORT_DTS    0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)
#define AVFMT_FLAG_FAST_SEEK   0x80000 ///< Enable fast, but inaccurate seeks for some formats
#if FF_API_LAVF_SHORTEST
#define AVFMT_FLAG_SHORTEST   0x100000 ///< Stop muxing when the shortest stream stops.
#endif
#define AVFMT_FLAG_AUTO_BSF   0x200000 ///< Add bitstream filters as requested by the muxer

    /**
     * Maximum number of bytes read from input in order to determine stream
     * properties. Used when reading the global header and in
     * avformat_find_stream_info().
     *
     * Demuxing only, set by the caller before avformat_open_input().
     *
     * @note this is \e not  used for determining the \ref AVInputFormat
     *       "input format"
     * @sa format_probesize
     */
    int64_t probesize;

    /**
     * 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().
     * Can be set to 0 to let avformat choose using a heuristic.
     */
    int64_t max_analyze_duration;

    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;

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

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

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * Flags to enable debugging.
     */
    int debug;
#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;

    /**
     * Allow non-standard and experimental extension
     * @see AVCodecContext.strict_std_compliance
     */
    int strict_std_compliance;

    /**
     * Flags indicating events happening on the file, a combination of
     * AVFMT_EVENT_FLAG_*.
     *
     * - demuxing: may be set by the demuxer in avformat_open_input(),
     *   avformat_find_stream_info() and av_read_frame(). Flags must be cleared
     *   by the user once the event has been handled.
     * - muxing: may be set by the user after avformat_write_header() to
     *   indicate a user-triggered event.  The muxer will clear the flags for
     *   events it has handled in av_[interleaved]_write_frame().
     */
    int event_flags;
/**
 * - demuxing: the demuxer read new metadata from the file and updated
 *   AVFormatContext.metadata accordingly
 * - muxing: the user updated AVFormatContext.metadata and wishes the muxer to
 *   write it into the file
 */
#define AVFMT_EVENT_FLAG_METADATA_UPDATED 0x0001

    /**
     * Maximum number of packets to read while waiting for the first timestamp.
     * Decoding only.
     */
    int max_ts_probe;

    /**
     * Avoid negative timestamps during muxing.
     * Any value of the AVFMT_AVOID_NEG_TS_* constants.
     * Note, this works better when using av_interleaved_write_frame().
     * - muxing: Set by user
     * - demuxing: unused
     */
    int avoid_negative_ts;
#define AVFMT_AVOID_NEG_TS_AUTO             -1 ///< Enabled when required by target format
#define AVFMT_AVOID_NEG_TS_DISABLED          0 ///< Do not shift timestamps even when they are negative.
#define AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE 1 ///< Shift timestamps so they are non negative
#define AVFMT_AVOID_NEG_TS_MAKE_ZERO         2 ///< Shift timestamps so that they start at 0

    /**
     * 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
     * - 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
     * - 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
     * - decoding: unused
     */
    int max_chunk_size;

    /**
     * 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
     */
    int use_wallclock_as_timestamps;

    /**
     * avio flags, used to force AVIO_FLAG_DIRECT.
     * - encoding: unused
     * - decoding: Set by user
     */
    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
     */
    enum AVDurationEstimationMethod duration_estimation_method;

    /**
     * Skip initial bytes when opening stream
     * - encoding: unused
     * - decoding: Set by user
     */
    int64_t skip_initial_bytes;

    /**
     * Correct single timestamp overflows
     * - encoding: unused
     * - decoding: Set by user
     */
    unsigned int correct_ts_overflow;

    /**
     * Force seeking to any (also non key) frames.
     * - encoding: unused
     * - decoding: Set by user
     */
    int seek2any;

    /**
     * Flush the I/O context after each packet.
     * - encoding: Set by user
     * - decoding: unused
     */
    int flush_packets;

    /**
     * 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
     */
    int probe_score;

    /**
     * Maximum number of bytes read from input in order to identify the
     * \ref AVInputFormat "input format". Only used when the format is not set
     * explicitly by the caller.
     *
     * Demuxing only, set by the caller before avformat_open_input().
     *
     * @sa probesize
     */
    int format_probesize;

    /**
     * ',' separated list of allowed decoders.
     * If NULL then all are allowed
     * - encoding: unused
     * - decoding: set by user
     */
    char *codec_whitelist;

    /**
     * ',' separated list of allowed demuxers.
     * If NULL then all are allowed
     * - encoding: unused
     * - decoding: set by user
     */
    char *format_whitelist;

    /**
     * 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
     */
    const struct 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
     */
    const struct 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
     */
    const struct AVCodec *subtitle_codec;

    /**
     * Forced data codec.
     * This allows forcing a specific decoder, even when there are multiple with
     * the same codec_id.
     * Demuxing: Set by user
     */
    const struct AVCodec *data_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.
     */
    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
     */
    int64_t output_ts_offset;

    /**
     * dump format separator.
     * can be ", " or "\n      " or anything else
     * - muxing: Set by user.
     * - demuxing: Set by user.
     */
    uint8_t *dump_separator;

    /**
     * Forced Data codec_id.
     * Demuxing: Set by user.
     */
    enum AVCodecID data_codec_id;

    /**
     * ',' separated list of allowed protocols.
     * - encoding: unused
     * - decoding: set by user
     */
    char *protocol_whitelist;

    /**
     * A callback for opening new IO streams.
     *
     * Whenever a muxer or a demuxer needs to open an IO stream (typically from
     * avformat_open_input() for demuxers, but for certain formats can happen at
     * other times as well), it will call this callback to obtain an IO context.
     *
     * @param s the format context
     * @param pb on success, the newly opened IO context should be returned here
     * @param url the url to open
     * @param flags a combination of AVIO_FLAG_*
     * @param options a dictionary of additional options, with the same
     *                semantics as in avio_open2()
     * @return 0 on success, a negative AVERROR code on failure
     *
     * @note Certain muxers and demuxers do nesting, i.e. they open one or more
     * additional internal format contexts. Thus the AVFormatContext pointer
     * passed to this callback may be different from the one facing the caller.
     * It will, however, have the same 'opaque' field.
     */
    int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,
                   int flags, AVDictionary **options);

#if FF_API_AVFORMAT_IO_CLOSE
    /**
     * A callback for closing the streams opened with AVFormatContext.io_open().
     *
     * @deprecated use io_close2
     */
    attribute_deprecated
    void (*io_close)(struct AVFormatContext *s, AVIOContext *pb);
#endif

    /**
     * ',' separated list of disallowed protocols.
     * - encoding: unused
     * - decoding: set by user
     */
    char *protocol_blacklist;

    /**
     * The maximum number of streams.
     * - encoding: unused
     * - decoding: set by user
     */
    int max_streams;

    /**
     * Skip duration calcuation in estimate_timings_from_pts.
     * - encoding: unused
     * - decoding: set by user
     */
    int skip_estimate_duration_from_pts;

    /**
     * Maximum number of packets that can be probed
     * - encoding: unused
     * - decoding: set by user
     */
    int max_probe_packets;

    /**
     * A callback for closing the streams opened with AVFormatContext.io_open().
     *
     * Using this is preferred over io_close, because this can return an error.
     * Therefore this callback is used instead of io_close by the generic
     * libavformat code if io_close is NULL or the default.
     *
     * @param s the format context
     * @param pb IO context to be closed and freed
     * @return 0 on success, a negative AVERROR code on failure
     */
    int (*io_close2)(struct AVFormatContext *s, AVIOContext *pb);
} 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;

    /**
     * The input container format.
     *
     * Demuxing only, set by avformat_open_input().
     */
    const struct AVInputFormat *iformat;

    /**
     * The output container format.
     *
     * Muxing only, must be set by the caller before avformat_write_header().
     */
    const struct AVOutputFormat *oformat;

    /**
     * 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;

    /**
     * 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;

    /* stream info */
    /**
     * Flags signalling stream properties. A combination of AVFMTCTX_*.
     * Set by libavformat.
     */
    int ctx_flags;

    /**
     * Number of elements in AVFormatContext.streams.
     *
     * Set by avformat_new_stream(), must not be modified by any other code.
     */
    unsigned int nb_streams;
    /**
     * 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;

    /**
     * input or output URL. Unlike the old filename field, this field has no
     * length restriction.
     *
     * - demuxing: set by avformat_open_input(), initialized to an empty
     *             string if url parameter was NULL in avformat_open_input().
     * - muxing: may be set by the caller before calling avformat_write_header()
     *           (or avformat_init_output() if that is called first) to a string
     *           which is freeable by av_free(). Set to an empty string if it
     *           was NULL in avformat_init_output().
     *
     * Freed by libavformat in avformat_free_context().
     */
    char *url;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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.
     */
    int64_t bit_rate;

    unsigned int packet_size;
    int max_delay;

    /**
     * Flags modifying the (de)muxer behaviour. A combination of AVFMT_FLAG_*.
     * Set by the user before avformat_open_input() / avformat_write_header().
     */
    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.
/**
 * When muxing, try to avoid writing any random/volatile data to the output.
 * This includes any random IDs, real-time timestamps/dates, muxer version, etc.
 *
 * This flag is mainly intended for testing.
 */
#define AVFMT_FLAG_BITEXACT         0x0400
#define AVFMT_FLAG_SORT_DTS    0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)
#define AVFMT_FLAG_FAST_SEEK   0x80000 ///< Enable fast, but inaccurate seeks for some formats
#if FF_API_LAVF_SHORTEST
#define AVFMT_FLAG_SHORTEST   0x100000 ///< Stop muxing when the shortest stream stops.
#endif
#define AVFMT_FLAG_AUTO_BSF   0x200000 ///< Add bitstream filters as requested by the muxer

    /**
     * Maximum number of bytes read from input in order to determine stream
     * properties. Used when reading the global header and in
     * avformat_find_stream_info().
     *
     * Demuxing only, set by the caller before avformat_open_input().
     *
     * @note this is \e not  used for determining the \ref AVInputFormat
     *       "input format"
     * @sa format_probesize
     */
    int64_t probesize;

    /**
     * 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().
     * Can be set to 0 to let avformat choose using a heuristic.
     */
    int64_t max_analyze_duration;

    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;

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

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

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * 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;

    /**
     * Flags to enable debugging.
     */
    int debug;
#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;

    /**
     * Allow non-standard and experimental extension
     * @see AVCodecContext.strict_std_compliance
     */
    int strict_std_compliance;

    /**
     * Flags indicating events happening on the file, a combination of
     * AVFMT_EVENT_FLAG_*.
     *
     * - demuxing: may be set by the demuxer in avformat_open_input(),
     *   avformat_find_stream_info() and av_read_frame(). Flags must be cleared
     *   by the user once the event has been handled.
     * - muxing: may be set by the user after avformat_write_header() to
     *   indicate a user-triggered event.  The muxer will clear the flags for
     *   events it has handled in av_[interleaved]_write_frame().
     */
    int event_flags;
/**
 * - demuxing: the demuxer read new metadata from the file and updated
 *   AVFormatContext.metadata accordingly
 * - muxing: the user updated AVFormatContext.metadata and wishes the muxer to
 *   write it into the file
 */
#define AVFMT_EVENT_FLAG_METADATA_UPDATED 0x0001

    /**
     * Maximum number of packets to read while waiting for the first timestamp.
     * Decoding only.
     */
    int max_ts_probe;

    /**
     * Avoid negative timestamps during muxing.
     * Any value of the AVFMT_AVOID_NEG_TS_* constants.
     * Note, this works better when using av_interleaved_write_frame().
     * - muxing: Set by user
     * - demuxing: unused
     */
    int avoid_negative_ts;
#define AVFMT_AVOID_NEG_TS_AUTO             -1 ///< Enabled when required by target format
#define AVFMT_AVOID_NEG_TS_DISABLED          0 ///< Do not shift timestamps even when they are negative.
#define AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE 1 ///< Shift timestamps so they are non negative
#define AVFMT_AVOID_NEG_TS_MAKE_ZERO         2 ///< Shift timestamps so that they start at 0

    /**
     * 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
     * - 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
     * - 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
     * - decoding: unused
     */
    int max_chunk_size;

    /**
     * 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
     */
    int use_wallclock_as_timestamps;

    /**
     * avio flags, used to force AVIO_FLAG_DIRECT.
     * - encoding: unused
     * - decoding: Set by user
     */
    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
     */
    enum AVDurationEstimationMethod duration_estimation_method;

    /**
     * Skip initial bytes when opening stream
     * - encoding: unused
     * - decoding: Set by user
     */
    int64_t skip_initial_bytes;

    /**
     * Correct single timestamp overflows
     * - encoding: unused
     * - decoding: Set by user
     */
    unsigned int correct_ts_overflow;

    /**
     * Force seeking to any (also non key) frames.
     * - encoding: unused
     * - decoding: Set by user
     */
    int seek2any;

    /**
     * Flush the I/O context after each packet.
     * - encoding: Set by user
     * - decoding: unused
     */
    int flush_packets;

    /**
     * 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
     */
    int probe_score;

    /**
     * Maximum number of bytes read from input in order to identify the
     * \ref AVInputFormat "input format". Only used when the format is not set
     * explicitly by the caller.
     *
     * Demuxing only, set by the caller before avformat_open_input().
     *
     * @sa probesize
     */
    int format_probesize;

    /**
     * ',' separated list of allowed decoders.
     * If NULL then all are allowed
     * - encoding: unused
     * - decoding: set by user
     */
    char *codec_whitelist;

    /**
     * ',' separated list of allowed demuxers.
     * If NULL then all are allowed
     * - encoding: unused
     * - decoding: set by user
     */
    char *format_whitelist;

    /**
     * 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
     */
    const struct 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
     */
    const struct 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
     */
    const struct AVCodec *subtitle_codec;

    /**
     * Forced data codec.
     * This allows forcing a specific decoder, even when there are multiple with
     * the same codec_id.
     * Demuxing: Set by user
     */
    const struct AVCodec *data_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.
     */
    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
     */
    int64_t output_ts_offset;

    /**
     * dump format separator.
     * can be ", " or "\n      " or anything else
     * - muxing: Set by user.
     * - demuxing: Set by user.
     */
    uint8_t *dump_separator;

    /**
     * Forced Data codec_id.
     * Demuxing: Set by user.
     */
    enum AVCodecID data_codec_id;

    /**
     * ',' separated list of allowed protocols.
     * - encoding: unused
     * - decoding: set by user
     */
    char *protocol_whitelist;

    /**
     * A callback for opening new IO streams.
     *
     * Whenever a muxer or a demuxer needs to open an IO stream (typically from
     * avformat_open_input() for demuxers, but for certain formats can happen at
     * other times as well), it will call this callback to obtain an IO context.
     *
     * @param s the format context
     * @param pb on success, the newly opened IO context should be returned here
     * @param url the url to open
     * @param flags a combination of AVIO_FLAG_*
     * @param options a dictionary of additional options, with the same
     *                semantics as in avio_open2()
     * @return 0 on success, a negative AVERROR code on failure
     *
     * @note Certain muxers and demuxers do nesting, i.e. they open one or more
     * additional internal format contexts. Thus the AVFormatContext pointer
     * passed to this callback may be different from the one facing the caller.
     * It will, however, have the same 'opaque' field.
     */
    int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url,
                   int flags, AVDictionary **options);

#if FF_API_AVFORMAT_IO_CLOSE
    /**
     * A callback for closing the streams opened with AVFormatContext.io_open().
     *
     * @deprecated use io_close2
     */
    attribute_deprecated
    void (*io_close)(struct AVFormatContext *s, AVIOContext *pb);
#endif

    /**
     * ',' separated list of disallowed protocols.
     * - encoding: unused
     * - decoding: set by user
     */
    char *protocol_blacklist;

    /**
     * The maximum number of streams.
     * - encoding: unused
     * - decoding: set by user
     */
    int max_streams;

    /**
     * Skip duration calcuation in estimate_timings_from_pts.
     * - encoding: unused
     * - decoding: set by user
     */
    int skip_estimate_duration_from_pts;

    /**
     * Maximum number of packets that can be probed
     * - encoding: unused
     * - decoding: set by user
     */
    int max_probe_packets;

    /**
     * A callback for closing the streams opened with AVFormatContext.io_open().
     *
     * Using this is preferred over io_close, because this can return an error.
     * Therefore this callback is used instead of io_close by the generic
     * libavformat code if io_close is NULL or the default.
     *
     * @param s the format context
     * @param pb IO context to be closed and freed
     * @return 0 on success, a negative AVERROR code on failure
     */
    int (*io_close2)(struct AVFormatContext *s, AVIOContext *pb);
} AVFormatContext;ol_blacklist;

    /**
     * The maximum number of streams.
     * - encoding: unused
     * - decoding: set by user
     */
    int max_streams;

    /**
     * Skip duration calcuation in estimate_timings_from_pts.
     * - encoding: unused
     * - decoding: set by user
     */
    int skip_estimate_duration_from_pts;

    /**
     * Maximum number of packets that can be probed
     * - encoding: unused
     * - decoding: set by user
     */
    int max_probe_packets;

    /**
     * A callback for closing the streams opened with AVFormatContext.io_open().
     *
     * Using this is preferred over io_close, because this can return an error.
     * Therefore this callback is used instead of io_close by the generic
     * libavformat code if io_close is NULL or the default.
     *
     * @param s the format context
     * @param pb IO context to be closed and freed
     * @return 0 on success, a negative AVERROR code on failure
     */
    int (*io_close2)(struct AVFormatContext *s, AVIOContext *pb);
} AVFormatContext;

  1. nb_streams：表示容器中流的数量。
  2. streams：AVStream类型指针数组，每个元素代表一个流。每个AVStream结构体包含有该流的相信信息。
  3. duration：表示该容器的总时长，单位为微妙（可用的话）。
  4. iformat/oformat：分别表示输入格式和输出格式的AVInputFormat/AVOutFormat结构体指针。
  5. flags：包含有关容器选项的标志。常用标志包括AVFMT_NOFILE（不需要打开文件），AVFMT_FLOBALHEADER（向输出流写入全局标题），AVFMT——NOBINEARCH（禁用二进制搜索等）。
  6. pb：AVIOContest结构体类型，代表文件输入/输出流。它与AVFormatContext跟踪同一个文件，并提供对该文件的读写操作。pb由avion_open2()打开。
  7. metadata：包含元数据信息的AVDictionary类型指针，其中包含有关容器的任何其他信息，例如标题、作家、发行商等。
  8. filename：表示输入/输出文件名的字符串、通常只给调试和打印统计信息使用。

3.2、AVInputFormat

  AVInputFormat 是类似COM 接口的数据结构，表示输入文件容器格式，是解复用器（解封装）作用时读取媒体文件并将其拆分为数据块（数据包）。每个数据包，包含一个或者多个编码帧。着重于功能函数，一种文件容器格式对应一个AVInputFormat 结构，在程序运行时有多个实例，位于avoformat.h文件中。

typedef struct AVInputFormat {
    /**
     * A comma separated list of short names for the format. New names
     * may be appended with a minor bump.
     */
    const char *name;

    /**
     * Descriptive name for the format, meant to be more human-readable
     * than name. You should use the NULL_IF_CONFIG_SMALL() macro
     * to define it.
     */
    const char *long_name;

    /**
     * Can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_SHOW_IDS,
     * AVFMT_NOTIMESTAMPS, AVFMT_GENERIC_INDEX, AVFMT_TS_DISCONT, AVFMT_NOBINSEARCH,
     * AVFMT_NOGENSEARCH, AVFMT_NO_BYTE_SEEK, AVFMT_SEEK_TO_PTS.
     */
    int flags;

    /**
     * If extensions are defined, then no probe is done. You should
     * usually not use extension format guessing because it is not
     * reliable enough
     */
    const char *extensions;

    const struct AVCodecTag * const *codec_tag;

    const AVClass *priv_class; ///< AVClass for the private context

    /**
     * Comma-separated list of mime types.
     * It is used check for matching mime types while probing.
     * @see av_probe_input_format2
     */
    const char *mime_type;

    /*****************************************************************
     * No fields below this line are 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.
     *****************************************************************
     */
    /**
     * Raw demuxers store their codec ID here.
     */
    int raw_codec_id;

    /**
     * Size of private data so that it can be allocated in the wrapper.
     */
    int priv_data_size;

    /**
     * Internal flags. See FF_FMT_FLAG_* in internal.h.
     */
    int flags_internal;

    /**
     * Tell if a given file has a chance of being parsed as this format.
     * The buffer provided is guaranteed to be AVPROBE_PADDING_SIZE bytes
     * big so you do not have to check for that unless you need more.
     */
    int (*read_probe)(const AVProbeData *);

    /**
     * Read the format header and initialize the AVFormatContext
     * structure. Return 0 if OK. 'avformat_new_stream' should be
     * called to create new streams.
     */
    int (*read_header)(struct AVFormatContext *);

    /**
     * Read one packet and put it in 'pkt'. pts and flags are also
     * set. 'avformat_new_stream' can be called only if the flag
     * AVFMTCTX_NOHEADER is used and only in the calling thread (not in a
     * background thread).
     * @return 0 on success, < 0 on error.
     *         Upon returning an error, pkt must be unreferenced by the caller.
     */
    int (*read_packet)(struct AVFormatContext *, AVPacket *pkt);

    /**
     * Close the stream. The AVFormatContext and AVStreams are not
     * freed by this function
     */
    int (*read_close)(struct AVFormatContext *);

    /**
     * Seek to a given timestamp relative to the frames in
     * stream component stream_index.
     * @param stream_index Must not be -1.
     * @param flags Selects which direction should be preferred if no exact
     *              match is available.
     * @return >= 0 on success (but not necessarily the new offset)
     */
    int (*read_seek)(struct AVFormatContext *,
                     int stream_index, int64_t timestamp, int flags);

    /**
     * Get the next timestamp in stream[stream_index].time_base units.
     * @return the timestamp or AV_NOPTS_VALUE if an error occurred
     */
    int64_t (*read_timestamp)(struct AVFormatContext *s, int stream_index,
                              int64_t *pos, int64_t pos_limit);

    /**
     * Start/resume playing - only meaningful if using a network-based format
     * (RTSP).
     */
    int (*read_play)(struct AVFormatContext *);

    /**
     * Pause playing - only meaningful if using a network-based format
     * (RTSP).
     */
    int (*read_pause)(struct AVFormatContext *);

    /**
     * Seek to timestamp ts.
     * Seeking will be done so that the point from which all active streams
     * can be presented successfully will be closest to ts and within min/max_ts.
     * Active streams are all streams that have AVStream.discard < AVDISCARD_ALL.
     */
    int (*read_seek2)(struct AVFormatContext *s, int stream_index, int64_t min_ts, int64_t ts, int64_t max_ts, int flags);

    /**
     * Returns device list with it properties.
     * @see avdevice_list_devices() for more details.
     */
    int (*get_device_list)(struct AVFormatContext *s, struct AVDeviceInfoList *device_list);

} AVInputFormat;

  1. const char *name：格式名列表.也可以分配一个新名字。
  2. const char *long_name：格式的描述性名称，意味着比名称更易于阅读。
  3. int flags：可用的flag有: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_SHOW_IDS,AVFMT_GENERIC_INDEX, AVFMT_TS_DISCONT, AVFMT_NOBINSEARCH,AVFMT_NOGENSEARCH, AVFMT_NO_BYTE_SEEK, AVFMT_SEEK_TO_PTS。
  4. const char *extensions：文件扩展名。
  5. const AVClass *priv_class：一个模拟类型列表.用来在probe的时候check匹配的类型。
  6. struct AVInputFormat *next：用于把所有支持的输入文件容器格式连接成链表，便于遍历查找。
  7. int priv_data_size：标示具体的文件容器格式对应的Context 的大小。
  8. int (*read_probe)(AVProbeData *)：判断一个给定的文件是否有可能被解析为这种格式。 给定的buf足够大，所以你没有必要去检查它，除非你需要更多 。
  9. int (*read_header)(struct AVFormatContext *)：读取format头并初始化AVFormatContext结构体，如果成功，返回0。创建新的流需要调用avformat_new_stream。
  10. int (*read_header2)(struct AVFormatContext *, AVDictionary **options)：新加的函数指针，用于打开进一步嵌套输入的格式。
  11. int (*read_packet)(struct AVFormatContext *, AVPacket *pkt)：读取一个数据包并将其放在“pkt”中。 pts和flag也被设置。
  12. int (*read_close)(struct AVFormatContext *)：关闭流。AVFormatContext和Streams不会被此函数释放。
  13. int (*read_seek)(struct AVFormatContext *, int stream_index, int64_t timestamp, int flags)
  14. int64_t (*read_timestamp)(struct AVFormatContext *s, int stream_index, int64_t *pos, int64_t pos_limit)：获取stream [stream_index] .time_base单位中的下一个时间戳。
  15. int (*read_play)(struct AVFormatContext *)：开始/继续播放 - 仅当使用基于网络的（RTSP）格式时才有意义。
  16. int (*read_pause)(struct AVFormatContext *)：暂停播放 - 仅当使用基于网络的（RTSP）格式时才有意义。
  17. int (*read_seek2)(struct AVFormatContext *s, int stream_index, int64_t min_ts, int64_t ts, int64_t max_ts, int flags)：寻求时间戳ts。
  18. int (*get_device_list)(struct AVFormatContext *s, struct AVDeviceInfoList *device_list)：返回设备列表及其属性。
  19. int (*create_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps)：初始化设备能力子模块。
  20. int (*free_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps)：释放设备能力子模块

3.3、AVOutputFormat

  AVOutpufFormat与AVInputFormat类似，是类似COM 接口的数据结构，表示输出文件容器格式，着重于功能函数，位于Avoformat.h文件中。

typedef struct AVOutputFormat {
    const char *name;
    /**
     * Descriptive name for the format, meant to be more human-readable
     * than name. You should use the NULL_IF_CONFIG_SMALL() macro
     * to define it.
     */
    const char *long_name;
    const char *mime_type;
    const char *extensions; /**< comma-separated filename extensions */
    /* output support */
    enum AVCodecID audio_codec;    /**< default audio codec */
    enum AVCodecID video_codec;    /**< default video codec */
    enum AVCodecID subtitle_codec; /**< default subtitle codec */
    /**
     * can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER,
     * AVFMT_GLOBALHEADER, AVFMT_NOTIMESTAMPS, AVFMT_VARIABLE_FPS,
     * AVFMT_NODIMENSIONS, AVFMT_NOSTREAMS,
     * AVFMT_TS_NONSTRICT, AVFMT_TS_NEGATIVE
     */
    int flags;

    /**
     * List of supported codec_id-codec_tag pairs, ordered by "better
     * choice first". The arrays are all terminated by AV_CODEC_ID_NONE.
     */
    const struct AVCodecTag * const *codec_tag;

    const AVClass *priv_class; ///< AVClass for the private context
} AVOutputFormat;

3.4、AVStream

  AVStream是存储每一个视频/音频流信息的结构体。本文分析一下该结构体里重要变量的含义和作用。该结构体位于avformat.h中，首先看一下结构体的定义：

typedef struct AVStream {
    /**
     * A class for @ref avoptions. Set on stream creation.
     */
    const AVClass *av_class;

    int index;    /**< stream index in AVFormatContext */
    /**
     * Format-specific stream ID.
     * decoding: set by libavformat
     * encoding: set by the user, replaced by libavformat if left unset
     */
    int id;

    /**
     * Codec parameters associated with this stream. Allocated and freed by
     * libavformat in avformat_new_stream() and avformat_free_context()
     * respectively.
     *
     * - demuxing: filled by libavformat on stream creation or in
     *             avformat_find_stream_info()
     * - muxing: filled by the caller before avformat_write_header()
     */
    AVCodecParameters *codecpar;

    void *priv_data;

    /**
     * This is the fundamental unit of time (in seconds) in terms
     * of which frame timestamps are represented.
     *
     * decoding: set by libavformat
     * encoding: May be set by the caller before avformat_write_header() to
     *           provide a hint to the muxer about the desired timebase. In
     *           avformat_write_header(), the muxer will overwrite this field
     *           with the timebase that will actually be used for the timestamps
     *           written into the file (which may or may not be related to the
     *           user-provided one, depending on the format).
     */
    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.
     *
     * Encoding: May be set by the caller before avformat_write_header() to
     * provide a hint to the muxer about the estimated duration.
     */
    int64_t duration;

    int64_t nb_frames;                 ///< number of frames in this stream if known or 0

    /**
     * Stream disposition - a combination of AV_DISPOSITION_* flags.
     * - demuxing: set by libavformat when creating the stream or in
     *             avformat_find_stream_info().
     * - muxing: may be set by the caller before avformat_write_header().
     */
    int disposition;

    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
     *
     * - demuxing: May be set by libavformat when creating the stream or in
     *             avformat_find_stream_info().
     * - muxing: May be set by the caller before avformat_write_header().
     */
    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;

#if FF_API_AVSTREAM_SIDE_DATA
    /**
     * An array of side data that applies to the whole stream (i.e. the
     * container does not allow it to change between packets).
     *
     * There may be no overlap between the side data in this array and side data
     * in the packets. I.e. a given side data is either exported by the muxer
     * (demuxing) / set by the caller (muxing) in this array, then it never
     * appears in the packets, or the side data is exported / sent through
     * the packets (always in the first packet where the value becomes known or
     * changes), then it does not appear in this array.
     *
     * - demuxing: Set by libavformat when the stream is created.
     * - muxing: May be set by the caller before avformat_write_header().
     *
     * Freed by libavformat in avformat_free_context().
     *
     * @deprecated use AVStream's @ref AVCodecParameters.coded_side_data
     *             "codecpar side data".
     */
    attribute_deprecated
    AVPacketSideData *side_data;
    /**
     * The number of elements in the AVStream.side_data array.
     *
     * @deprecated use AVStream's @ref AVCodecParameters.nb_coded_side_data
     *             "codecpar side data".
     */
    attribute_deprecated
    int            nb_side_data;
#endif

    /**
     * Flags indicating events happening on the stream, a combination of
     * AVSTREAM_EVENT_FLAG_*.
     *
     * - demuxing: may be set by the demuxer in avformat_open_input(),
     *   avformat_find_stream_info() and av_read_frame(). Flags must be cleared
     *   by the user once the event has been handled.
     * - muxing: may be set by the user after avformat_write_header(). to
     *   indicate a user-triggered event.  The muxer will clear the flags for
     *   events it has handled in av_[interleaved]_write_frame().
     */
    int event_flags;
/**
 * - demuxing: the demuxer read new metadata from the file and updated
 *     AVStream.metadata accordingly
 * - muxing: the user updated AVStream.metadata and wishes the muxer to write
 *     it into the file
 */
#define AVSTREAM_EVENT_FLAG_METADATA_UPDATED 0x0001
/**
 * - demuxing: new packets for this stream were read from the file. This
 *   event is informational only and does not guarantee that new packets
 *   for this stream will necessarily be returned from av_read_frame().
 */
#define AVSTREAM_EVENT_FLAG_NEW_PACKETS (1 << 1)

    /**
     * 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;

    /**
     * Number of bits in timestamps. Used for wrapping control.
     *
     * - demuxing: set by libavformat
     * - muxing: set by libavformat
     *
     */
    int pts_wrap_bits;
} AVStream;

  分析：
  1. int index：在AVFormatContext中的流索引。
  2. int id：流id，例如音频流id 视频流id等，解封装的时候，由libavformat模块来赋值；加封装的时候，由用户设置，如果未设置，则用libavformat替换。、
  3. AVRational time_base：这是表示帧时间戳的基本时间单位(以秒为单位) 。
  4. int64_t duration：解码时:流的持续时间，以流的时间为基数。如果源文件没有指定持续时间，但指定了比特率，则该值将根据比特率和文件大小进行估计。使用该数值得出视频流的持续时间，计算方式 duration *(time_base.num/time_base.den) 秒。duration的数值有可能为空，所以一般情况下使用AVFormatContext的duration来计算时长。
  5. int64_t nb_frames：帧数。
  6. AVRational sample_aspect_ratio：样本比特率。
  7. AVRational avg_frame_rate：平均帧速率。
  8. AVCodecParameters *codecpar：与此流相关的编解码器参数。分别在avformat_new_stream()和avformat_free_context()中由libavformat分配和释放。

四、重要函数分析

4.1、avformat_alloc_context

  源码位于：libavformat/options.c
  函数用来申请AVFormatContext类型变量并初始化默认参数。
  该函数用于分配空间创建一个AVFormatContext对象，并且强调使用  avformat_free_context方法来清理并释放该对象的空间。

static const AVClass av_format_context_class = {
    /**
     * 类的名字，通常它与 AVClass 关联的上下文结构体类型的名字相同。
     * 这里类名为"AVFormatContext"，说明该 AVClass 关联的上下文结构体为
     * AVFormatContext.
     */
    .class_name     = "AVFormatContext",
    /** 
     * 这里这个函数返回的是 AVFormatContext 中 iformat->name 
     * 或者 oformat->name 字符串.
     */
    .item_name      = format_to_name,
    /**
     * avformat_options 是一个 AVOption 结构体类的数组，
     * 该数组中的元素为 AVFormatContext 中的一个个字段，
     * 以后可以通过 av_opt_* 系列函数对 AVFormatContext
     * 中的字段进行操作。
     */
    .option         = avformat_options,
    .version        = LIBAVUTIL_VERSION_INT,
    /**
     * 返回下一个启用 AVOption 的子项
     */
    .child_next     = format_child_next,
    .child_class_iterate = format_child_class_iterate,
    .category       = AV_CLASS_CATEGORY_MUXER,
    .get_category   = get_category,
};

static int io_open_default(AVFormatContext *s, AVIOContext **pb,
                           const char *url, int flags, AVDictionary **options)
{
    int loglevel;

    if (!strcmp(url, s->url) ||
        s->iformat && !strcmp(s->iformat->name, "image2") ||
        s->oformat && !strcmp(s->oformat->name, "image2")
    ) {
        loglevel = AV_LOG_DEBUG;
    } else
        loglevel = AV_LOG_INFO;

    av_log(s, loglevel, "Opening \'%s\' for %s\n", url, flags & AVIO_FLAG_WRITE ? "writing" : "reading");

    return ffio_open_whitelist(pb, url, flags, &s->interrupt_callback, options, s->protocol_whitelist, s->protocol_blacklist);
}

#if FF_API_AVFORMAT_IO_CLOSE
void ff_format_io_close_default(AVFormatContext *s, AVIOContext *pb)
{
    avio_close(pb);
}
#endif

static int io_close2_default(AVFormatContext *s, AVIOContext *pb)
{
    return avio_close(pb);
}

AVFormatContext *avformat_alloc_context(void)
{
    FFFormatContext *const si = av_mallocz(sizeof(*si));    //使用av_malloc分配空间，分配空间的作用是存储数据
    AVFormatContext *s;    //创建一个AVFormatContext对象为s

    if (!si)
        return NULL; //判断si是否为NILL，为空返回NULL

    s = &si->pub;
    s->av_class = &av_format_context_class;
    s->io_open  = io_open_default;
#if FF_API_AVFORMAT_IO_CLOSE
FF_DISABLE_DEPRECATION_WARNINGS
    s->io_close = ff_format_io_close_default;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    s->io_close2= io_close2_default;

    av_opt_set_defaults(s);

    si->pkt = av_packet_alloc();
    si->parse_pkt = av_packet_alloc();
    if (!si->pkt || !si->parse_pkt) {
        avformat_free_context(s);
        return NULL;
    }

#if FF_API_LAVF_SHORTEST
    /* shortest_end:
     * 最短的流结束的时间戳 */
    si->shortest_end = AV_NOPTS_VALUE;
#endif

    return s;
}

  1. 创建AVFormatContext，他可用于由框架分配的在上下文和所有内容 ，即分配解复用器上下文。
  2. 初始化io_open等函数指针，为init_input做准备。
  3. 并初始化option相关参数给s。

4.2、avformat_open_input

int avformat_open_input(AVFormatContext **ps, const char *filename,
                        const AVInputFormat *fmt, AVDictionary **options)
{
    AVFormatContext *s = *ps;
    FFFormatContext *si;
    AVDictionary *tmp = NULL;
    ID3v2ExtraMeta *id3v2_extra_meta = NULL;
    int ret = 0;

    if (!s && !(s = avformat_alloc_context()))
        return AVERROR(ENOMEM);
    si = ffformatcontext(s);
    if (!s->av_class) {
        av_log(NULL, AV_LOG_ERROR, "Input context has not been properly allocated by avformat_alloc_context() and is not NULL either\n");
        return AVERROR(EINVAL);
    }
    if (fmt)
        s->iformat = fmt;

    if (options)
        av_dict_copy(&tmp, *options, 0);

    if (s->pb) // must be before any goto fail
        s->flags |= AVFMT_FLAG_CUSTOM_IO;

    if ((ret = av_opt_set_dict(s, &tmp)) < 0)
        goto fail;

    if (!(s->url = av_strdup(filename ? filename : ""))) {
        ret = AVERROR(ENOMEM);
        goto fail;
    }

    if ((ret = init_input(s, filename, &tmp)) < 0)
        goto fail;
    s->probe_score = ret;

    if (!s->protocol_whitelist && s->pb && s->pb->protocol_whitelist) {
        s->protocol_whitelist = av_strdup(s->pb->protocol_whitelist);
        if (!s->protocol_whitelist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

    if (!s->protocol_blacklist && s->pb && s->pb->protocol_blacklist) {
        s->protocol_blacklist = av_strdup(s->pb->protocol_blacklist);
        if (!s->protocol_blacklist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

    if (s->format_whitelist && av_match_list(s->iformat->name, s->format_whitelist, ',') <= 0) {
        av_log(s, AV_LOG_ERROR, "Format not on whitelist \'%s\'\n", s->format_whitelist);
        ret = AVERROR(EINVAL);
        goto fail;
    }

    avio_skip(s->pb, s->skip_initial_bytes);

    /* Check filename in case an image number is expected. */
    if (s->iformat->flags & AVFMT_NEEDNUMBER) {
        if (!av_filename_number_test(filename)) {
            ret = AVERROR(EINVAL);
            goto fail;
        }
    }

    s->duration = s->start_time = AV_NOPTS_VALUE;

    /* Allocate private data. */
    if (s->iformat->priv_data_size > 0) {
        if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        if (s->iformat->priv_class) {
            *(const AVClass **) s->priv_data = s->iformat->priv_class;
            av_opt_set_defaults(s->priv_data);
            if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)
                goto fail;
        }
    }

    /* e.g. AVFMT_NOFILE formats will not have an AVIOContext */
    if (s->pb)
        ff_id3v2_read_dict(s->pb, &si->id3v2_meta, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);

    if (s->iformat->read_header)
        if ((ret = s->iformat->read_header(s)) < 0) {
            if (s->iformat->flags_internal & FF_FMT_INIT_CLEANUP)
                goto close;
            goto fail;
        }

    if (!s->metadata) {
        s->metadata    = si->id3v2_meta;
        si->id3v2_meta = NULL;
    } else if (si->id3v2_meta) {
        av_log(s, AV_LOG_WARNING, "Discarding ID3 tags because more suitable tags were found.\n");
        av_dict_free(&si->id3v2_meta);
    }

    if (id3v2_extra_meta) {
        if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
            !strcmp(s->iformat->name, "tta") || !strcmp(s->iformat->name, "wav")) {
            if ((ret = ff_id3v2_parse_apic(s, id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_chapters(s, id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_priv(s, id3v2_extra_meta)) < 0)
                goto close;
        } else
            av_log(s, AV_LOG_DEBUG, "demuxer does not support additional id3 data, skipping\n");
        ff_id3v2_free_extra_meta(&id3v2_extra_meta);
    }

    if ((ret = avformat_queue_attached_pictures(s)) < 0)
        goto close;

    if (s->pb && !si->data_offset)
        si->data_offset = avio_tell(s->pb);

    si->raw_packet_buffer_size = 0;

    update_stream_avctx(s);

    if (options) {
        av_dict_free(options);
        *options = tmp;
    }
    *ps = s;
    return 0;

close:
    if (s->iformat->read_close)
        s->iformat->read_close(s);
fail:
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);
    av_dict_free(&tmp);
    if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
        avio_closep(&s->pb);
    avformat_free_context(s);
    *ps = NULL;
    return ret;
}

  函数的形参：
    ps：函数调用成功之后处理过的AVFormatContext结构体。
    file：打开的视音频流的URL。
    fmt：强制指定AVFormatContext中AVInputFormat的。这个参数一般情况下可以设置为NULL，这样FFmpeg可以自动检测AVInputFormat。
    options：附加的一些选项，一般情况下可以设置为NULL。
  在该函数中，FFMPEG完成了：
    1. 输入输出结构体AVIOContext的初始化；
    2. 输入数据的协议（例如RTMP，或者file）的识别（通过一套评分机制）：
      1. 判断文件名的后缀。
      2. 读取文件头的数据进行比对。
    3. 使用获得最高分的文件协议对应的URLProtocol，通过函数指针的方式，与FFMPEG连接（非专业用词）。
    4. 剩下的就是调用该URLProtocol的函数进行open,read等操作了 没看到相关的代码。

4.2.1、init_input

static int init_input(AVFormatContext *s, const char *filename,
                      AVDictionary **options)
{
    int ret;
    AVProbeData pd = { filename, NULL, 0 };
    int score = AVPROBE_SCORE_RETRY;

    if (s->pb) {
        s->flags |= AVFMT_FLAG_CUSTOM_IO;
        if (!s->iformat)
            return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                          s, 0, s->format_probesize);
        else if (s->iformat->flags & AVFMT_NOFILE)
            av_log(s, AV_LOG_WARNING, "Custom AVIOContext makes no sense and "
                                      "will be ignored with AVFMT_NOFILE format.\n");
        return 0;
    }

    if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
        (!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
        return score;

    if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
        return ret;

    if (s->iformat)
        return 0;
    return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                  s, 0, s->format_probesize);
}

  这个函数在短短的几行代码中包含了好几个return，因此逻辑还是有点复杂的，我们可以梳理一下：在函数的开头的score变量是一个判决AVInputFormat的分数的门限值，如果最后得到的AVInputFormat的分数低于该门限值，就认为没有找到合适的AVInputFormat。
  FFmpeg内部判断封装格式的原理实际上是对每种AVInputFormat给出一个分数，满分是100分，越有可能正确的AVInputFormat给出的分数就越高。最后选择分数最高的AVInputFormat作为推测结果。
  score的值是一个宏定义AVPROBE_SCORE_RETRY，我们可以看一下它的定义：

#define AVPROBE_SCORE_RETRY (AVPROBE_SCORE_MAX/4)  

  其中AVPROBE_SCORE_MAX是score的最大值，取值是100：

 #define AVPROBE_SCORE_MAX       100 ///< maximum score  

  由此我们可以得出score取值是25，即如果推测后得到的最佳AVInputFormat的分值低于25，就认为没有找到合适的AVInputFormat。
  整个函数的逻辑大体如下：
    1. 当使用了自定义的AVIOContext的时候（AVFormatContext中的AVIOContext不为空，即s->pb!=NULL），如果指定了AVInputFormat就直接返回，如果没有指定就调用av_probe_input_buffer2()推测AVInputFormat。这一情况出现的不算很多，但是当我们从内存中读取数据的时候（需要初始化自定义的AVIOContext），就会执行这一步骤。
    2. 在更一般的情况下，如果已经指定了AVInputFormat，就直接返回；如果没有指定AVInputFormat，就调用av_probe_input_format(NULL,…)根据文件路径判断文件格式。这里特意把av_probe_input_format()的第1个参数写成“NULL”，是为了强调这个时候实际上并没有给函数提供输入数据，此时仅仅通过文件路径推测AVInputFormat。
    3. 如果发现通过文件路径判断不出来文件格式，那么就需要打开文件探测文件格式了，这个时候会首先调用io_open()打开文件，然后调用av_probe_input_buffer2()推测AVInputFormat。

4.2.2、av_probe_input_format2

const AVInputFormat *av_probe_input_format2(const AVProbeData *pd,
                                            int is_opened, int *score_max)
{
    int score_ret;
    const AVInputFormat *fmt = av_probe_input_format3(pd, is_opened, &score_ret);
    if (score_ret > *score_max) {
        *score_max = score_ret;
        return fmt;
    } else
        return NULL;
}

  该函数用于根据输入数据查找合适的AVInputFormat。参数含义如下所示：
    pd：存储输入数据信息的AVProbeData结构体。
    is_opened：文件是否打开。
    score_max：判决AVInputFormat的门限值。只有某格式判决分数大于该门限值的时候，函数才会返回该封装格式，否则返回NULL。
  从函数中可以看出，av_probe_input_format2()调用了av_probe_input_format3()，并且增加了一个判断，当av_probe_input_format3()返回的分数大于score_max的时候，才会返回AVInputFormat，否则返回NULL。
  下面我们看一下av_probe_input_format3()
  从函数声明中可以看出，av_probe_input_format3()和av_probe_input_format2()的区别是函数的第3个参数不同：av_probe_input_format2()是一个分数的门限值，而av_probe_input_format3()是一个探测后的最匹配的格式的分数值。
  av_probe_input_format3函数最主要的部分是循环。该循环调用av_iformat_next()遍历（av_demuxer_iterate函数替代了av_iformat_next）FFmpeg中所有的AVInputFormat，并根据以下规则确定AVInputFormat和输入媒体数据的匹配分数（score，反应匹配程度）：
    1. 如果AVInputFormat中包含read_probe()，就调用read_probe()函数获取匹配分数（这一方法如果结果匹配的话，一般会获得AVPROBE_SCORE_MAX的分值，即100分）。如果不包含该函数，就使用av_match_ext()函数比较输入媒体的扩展名和AVInputFormat的扩展名是否匹配，如果匹配的话，设定匹配分数为AVPROBE_SCORE_EXTENSION（AVPROBE_SCORE_EXTENSION取值为50，即50分）。
    2. 使用av_match_name()比较输入媒体的mime_type和AVInputFormat的mime_type，如果匹配的话，设定匹配分数为AVPROBE_SCORE_MIME（AVPROBE_SCORE_MIME取值为75，即75分）。
    3. 如果该AVInputFormat的匹配分数大于此前的最大匹配分数，则记录当前的匹配分数为最大匹配分数，并且记录当前的AVInputFormat为最佳匹配的AVInputFormat。

4.3、avformat_find_stream_info

int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
{
    FFFormatContext *const si = ffformatcontext(ic);
    int count = 0, ret = 0;
    int64_t read_size;
    AVPacket *pkt1 = si->pkt;
    int64_t old_offset  = avio_tell(ic->pb);
    // new streams might appear, no options for those
    int orig_nb_streams = ic->nb_streams;
    int flush_codecs;
    int64_t max_analyze_duration = ic->max_analyze_duration;
    int64_t max_stream_analyze_duration;
    int64_t max_subtitle_analyze_duration;
    int64_t probesize = ic->probesize;
    int eof_reached = 0;
    int *missing_streams = av_opt_ptr(ic->iformat->priv_class, ic->priv_data, "missing_streams");

    flush_codecs = probesize > 0;

    av_opt_set_int(ic, "skip_clear", 1, AV_OPT_SEARCH_CHILDREN);

    max_stream_analyze_duration = max_analyze_duration;
    max_subtitle_analyze_duration = max_analyze_duration;
    if (!max_analyze_duration) {
        max_stream_analyze_duration =
        max_analyze_duration        = 5*AV_TIME_BASE;
        max_subtitle_analyze_duration = 30*AV_TIME_BASE;
        if (!strcmp(ic->iformat->name, "flv"))
            max_stream_analyze_duration = 90*AV_TIME_BASE;
        if (!strcmp(ic->iformat->name, "mpeg") || !strcmp(ic->iformat->name, "mpegts"))
            max_stream_analyze_duration = 7*AV_TIME_BASE;
    }

    if (ic->pb) {
        FFIOContext *const ctx = ffiocontext(ic->pb);
        av_log(ic, AV_LOG_DEBUG, "Before avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d nb_streams:%d\n",
               avio_tell(ic->pb), ctx->bytes_read, ctx->seek_count, ic->nb_streams);
    }

    for (unsigned i = 0; i < ic->nb_streams; i++) {
        const AVCodec *codec;
        AVDictionary *thread_opt = NULL;
        AVStream *const st  = ic->streams[i];
        FFStream *const sti = ffstream(st);
        AVCodecContext *const avctx = sti->avctx;

        /* check if the caller has overridden the codec id */
        // only for the split stuff
        if (!sti->parser && !(ic->flags & AVFMT_FLAG_NOPARSE) && sti->request_probe <= 0) {
            sti->parser = av_parser_init(st->codecpar->codec_id);
            if (sti->parser) {
                if (sti->need_parsing == AVSTREAM_PARSE_HEADERS) {
                    sti->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
                } else if (sti->need_parsing == AVSTREAM_PARSE_FULL_RAW) {
                    sti->parser->flags |= PARSER_FLAG_USE_CODEC_TS;
                }
            } else if (sti->need_parsing) {
                av_log(ic, AV_LOG_VERBOSE, "parser not found for codec "
                       "%s, packets or times may be invalid.\n",
                       avcodec_get_name(st->codecpar->codec_id));
            }
        }

        ret = avcodec_parameters_to_context(avctx, st->codecpar);
        if (ret < 0)
            goto find_stream_info_err;
        if (sti->request_probe <= 0)
            sti->avctx_inited = 1;

        codec = find_probe_decoder(ic, st, st->codecpar->codec_id);

        /* Force thread count to 1 since the H.264 decoder will not extract
         * SPS and PPS to extradata during multi-threaded decoding. */
        av_dict_set(options ? &options[i] : &thread_opt, "threads", "1", 0);
        /* Force lowres to 0. The decoder might reduce the video size by the
         * lowres factor, and we don't want that propagated to the stream's
         * codecpar */
        av_dict_set(options ? &options[i] : &thread_opt, "lowres", "0", 0);

        if (ic->codec_whitelist)
            av_dict_set(options ? &options[i] : &thread_opt, "codec_whitelist", ic->codec_whitelist, 0);

        // Try to just open decoders, in case this is enough to get parameters.
        // Also ensure that subtitle_header is properly set.
        if (!has_codec_parameters(st, NULL) && sti->request_probe <= 0 ||
            st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE) {
            if (codec && !avctx->codec)
                if (avcodec_open2(avctx, codec, options ? &options[i] : &thread_opt) < 0)
                    av_log(ic, AV_LOG_WARNING,
                           "Failed to open codec in %s\n", __func__);
        }
        if (!options)
            av_dict_free(&thread_opt);
    }

    read_size = 0;
    for (;;) {
        const AVPacket *pkt;
        AVStream *st;
        FFStream *sti;
        AVCodecContext *avctx;
        int analyzed_all_streams;
        unsigned i;
        if (ff_check_interrupt(&ic->interrupt_callback)) {
            ret = AVERROR_EXIT;
            av_log(ic, AV_LOG_DEBUG, "interrupted\n");
            break;
        }

        /* check if one codec still needs to be handled */
        for (i = 0; i < ic->nb_streams; i++) {
            AVStream *const st  = ic->streams[i];
            FFStream *const sti = ffstream(st);
            int fps_analyze_framecount = 20;
            int count;

            if (!has_codec_parameters(st, NULL))
                break;
            /* If the timebase is coarse (like the usual millisecond precision
             * of mkv), we need to analyze more frames to reliably arrive at
             * the correct fps. */
            if (av_q2d(st->time_base) > 0.0005)
                fps_analyze_framecount *= 2;
            if (!tb_unreliable(ic, st))
                fps_analyze_framecount = 0;
            if (ic->fps_probe_size >= 0)
                fps_analyze_framecount = ic->fps_probe_size;
            if (st->disposition & AV_DISPOSITION_ATTACHED_PIC)
                fps_analyze_framecount = 0;
            /* variable fps and no guess at the real fps */
            count = (ic->iformat->flags & AVFMT_NOTIMESTAMPS) ?
                       sti->info->codec_info_duration_fields/2 :
                       sti->info->duration_count;
            if (!(st->r_frame_rate.num && st->avg_frame_rate.num) &&
                st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
                if (count < fps_analyze_framecount)
                    break;
            }
            // Look at the first 3 frames if there is evidence of frame delay
            // but the decoder delay is not set.
            if (sti->info->frame_delay_evidence && count < 2 && sti->avctx->has_b_frames == 0)
                break;
            if (!sti->avctx->extradata &&
                (!sti->extract_extradata.inited || sti->extract_extradata.bsf) &&
                extract_extradata_check(st))
                break;
            if (sti->first_dts == AV_NOPTS_VALUE &&
                (!(ic->iformat->flags & AVFMT_NOTIMESTAMPS) || sti->need_parsing == AVSTREAM_PARSE_FULL_RAW) &&
                sti->codec_info_nb_frames < ((st->disposition & AV_DISPOSITION_ATTACHED_PIC) ? 1 : ic->max_ts_probe) &&
                (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO ||
                 st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO))
                break;
        }
        analyzed_all_streams = 0;
        if (!missing_streams || !*missing_streams)
            if (i == ic->nb_streams) {
                analyzed_all_streams = 1;
                /* NOTE: If the format has no header, then we need to read some
                 * packets to get most of the streams, so we cannot stop here. */
                if (!(ic->ctx_flags & AVFMTCTX_NOHEADER)) {
                    /* If we found the info for all the codecs, we can stop. */
                    ret = count;
                    av_log(ic, AV_LOG_DEBUG, "All info found\n");
                    flush_codecs = 0;
                    break;
                }
            }
        /* We did not get all the codec info, but we read too much data. */
        if (read_size >= probesize) {
            ret = count;
            av_log(ic, AV_LOG_DEBUG,
                   "Probe buffer size limit of %"PRId64" bytes reached\n", probesize);
            for (unsigned i = 0; i < ic->nb_streams; i++) {
                AVStream *const st  = ic->streams[i];
                FFStream *const sti = ffstream(st);
                if (!st->r_frame_rate.num &&
                    sti->info->duration_count <= 1 &&
                    st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&
                    strcmp(ic->iformat->name, "image2"))
                    av_log(ic, AV_LOG_WARNING,
                           "Stream #%d: not enough frames to estimate rate; "
                           "consider increasing probesize\n", i);
            }
            break;
        }

        /* NOTE: A new stream can be added there if no header in file
         * (AVFMTCTX_NOHEADER). */
        ret = read_frame_internal(ic, pkt1);
        if (ret == AVERROR(EAGAIN))
            continue;

        if (ret < 0) {
            /* EOF or error*/
            eof_reached = 1;
            break;
        }

        if (!(ic->flags & AVFMT_FLAG_NOBUFFER)) {
            ret = avpriv_packet_list_put(&si->packet_buffer,
                                         pkt1, NULL, 0);
            if (ret < 0)
                goto unref_then_goto_end;

            pkt = &si->packet_buffer.tail->pkt;
        } else {
            pkt = pkt1;
        }

        st  = ic->streams[pkt->stream_index];
        sti = ffstream(st);
        if (!(st->disposition & AV_DISPOSITION_ATTACHED_PIC))
            read_size += pkt->size;

        avctx = sti->avctx;
        if (!sti->avctx_inited) {
            ret = avcodec_parameters_to_context(avctx, st->codecpar);
            if (ret < 0)
                goto unref_then_goto_end;
            sti->avctx_inited = 1;
        }

        if (pkt->dts != AV_NOPTS_VALUE && sti->codec_info_nb_frames > 1) {
            /* check for non-increasing dts */
            if (sti->info->fps_last_dts != AV_NOPTS_VALUE &&
                sti->info->fps_last_dts >= pkt->dts) {
                av_log(ic, AV_LOG_DEBUG,
                       "Non-increasing DTS in stream %d: packet %d with DTS "
                       "%"PRId64", packet %d with DTS %"PRId64"\n",
                       st->index, sti->info->fps_last_dts_idx,
                       sti->info->fps_last_dts, sti->codec_info_nb_frames,
                       pkt->dts);
                sti->info->fps_first_dts =
                sti->info->fps_last_dts  = AV_NOPTS_VALUE;
            }
            /* Check for a discontinuity in dts. If the difference in dts
             * is more than 1000 times the average packet duration in the
             * sequence, we treat it as a discontinuity. */
            if (sti->info->fps_last_dts != AV_NOPTS_VALUE &&
                sti->info->fps_last_dts_idx > sti->info->fps_first_dts_idx &&
                (pkt->dts - (uint64_t)sti->info->fps_last_dts) / 1000 >
                (sti->info->fps_last_dts     - (uint64_t)sti->info->fps_first_dts) /
                (sti->info->fps_last_dts_idx - sti->info->fps_first_dts_idx)) {
                av_log(ic, AV_LOG_WARNING,
                       "DTS discontinuity in stream %d: packet %d with DTS "
                       "%"PRId64", packet %d with DTS %"PRId64"\n",
                       st->index, sti->info->fps_last_dts_idx,
                       sti->info->fps_last_dts, sti->codec_info_nb_frames,
                       pkt->dts);
                sti->info->fps_first_dts =
                sti->info->fps_last_dts  = AV_NOPTS_VALUE;
            }

            /* update stored dts values */
            if (sti->info->fps_first_dts == AV_NOPTS_VALUE) {
                sti->info->fps_first_dts     = pkt->dts;
                sti->info->fps_first_dts_idx = sti->codec_info_nb_frames;
            }
            sti->info->fps_last_dts     = pkt->dts;
            sti->info->fps_last_dts_idx = sti->codec_info_nb_frames;
        }
        if (sti->codec_info_nb_frames > 1) {
            int64_t t = 0;
            int64_t limit;

            if (st->time_base.den > 0)
                t = av_rescale_q(sti->info->codec_info_duration, st->time_base, AV_TIME_BASE_Q);
            if (st->avg_frame_rate.num > 0)
                t = FFMAX(t, av_rescale_q(sti->codec_info_nb_frames, av_inv_q(st->avg_frame_rate), AV_TIME_BASE_Q));

            if (   t == 0
                && sti->codec_info_nb_frames > 30
                && sti->info->fps_first_dts != AV_NOPTS_VALUE
                && sti->info->fps_last_dts  != AV_NOPTS_VALUE) {
                int64_t dur = av_sat_sub64(sti->info->fps_last_dts, sti->info->fps_first_dts);
                t = FFMAX(t, av_rescale_q(dur, st->time_base, AV_TIME_BASE_Q));
            }

            if (analyzed_all_streams)                                limit = max_analyze_duration;
            else if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE) limit = max_subtitle_analyze_duration;
            else                                                     limit = max_stream_analyze_duration;

            if (t >= limit) {
                av_log(ic, AV_LOG_VERBOSE, "max_analyze_duration %"PRId64" reached at %"PRId64" microseconds st:%d\n",
                       limit,
                       t, pkt->stream_index);
                if (ic->flags & AVFMT_FLAG_NOBUFFER)
                    av_packet_unref(pkt1);
                break;
            }
            if (pkt->duration > 0) {
                const int fields = sti->codec_desc && (sti->codec_desc->props & AV_CODEC_PROP_FIELDS);
                if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE && pkt->pts != AV_NOPTS_VALUE && st->start_time != AV_NOPTS_VALUE && pkt->pts >= st->start_time
                    && (uint64_t)pkt->pts - st->start_time < INT64_MAX
                ) {
                    sti->info->codec_info_duration = FFMIN(pkt->pts - st->start_time, sti->info->codec_info_duration + pkt->duration);
                } else
                    sti->info->codec_info_duration += pkt->duration;
                sti->info->codec_info_duration_fields += sti->parser && sti->need_parsing && fields
                                                         ? sti->parser->repeat_pict + 1 : 2;
            }
        }
        if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
#if FF_API_R_FRAME_RATE
            ff_rfps_add_frame(ic, st, pkt->dts);
#endif
            if (pkt->dts != pkt->pts && pkt->dts != AV_NOPTS_VALUE && pkt->pts != AV_NOPTS_VALUE)
                sti->info->frame_delay_evidence = 1;
        }
        if (!sti->avctx->extradata) {
            ret = extract_extradata(si, st, pkt);
            if (ret < 0)
                goto unref_then_goto_end;
        }

        /* If still no information, we try to open the codec and to
         * decompress the frame. We try to avoid that in most cases as
         * it takes longer and uses more memory. For MPEG-4, we need to
         * decompress for QuickTime.
         *
         * If AV_CODEC_CAP_CHANNEL_CONF is set this will force decoding of at
         * least one frame of codec data, this makes sure the codec initializes
         * the channel configuration and does not only trust the values from
         * the container. */
        try_decode_frame(ic, st, pkt,
                         (options && i < orig_nb_streams) ? &options[i] : NULL);

        if (ic->flags & AVFMT_FLAG_NOBUFFER)
            av_packet_unref(pkt1);

        sti->codec_info_nb_frames++;
        count++;
    }

    if (eof_reached) {
        for (unsigned stream_index = 0; stream_index < ic->nb_streams; stream_index++) {
            AVStream *const st = ic->streams[stream_index];
            AVCodecContext *const avctx = ffstream(st)->avctx;
            if (!has_codec_parameters(st, NULL)) {
                const AVCodec *codec = find_probe_decoder(ic, st, st->codecpar->codec_id);
                if (codec && !avctx->codec) {
                    AVDictionary *opts = NULL;
                    if (ic->codec_whitelist)
                        av_dict_set(&opts, "codec_whitelist", ic->codec_whitelist, 0);
                    if (avcodec_open2(avctx, codec, (options && stream_index < orig_nb_streams) ? &options[stream_index] : &opts) < 0)
                        av_log(ic, AV_LOG_WARNING,
                               "Failed to open codec in %s\n", __func__);
                    av_dict_free(&opts);
                }
            }

            // EOF already reached while reading the stream above.
            // So continue with reoordering DTS with whatever delay we have.
            if (si->packet_buffer.head && !has_decode_delay_been_guessed(st)) {
                update_dts_from_pts(ic, stream_index, si->packet_buffer.head);
            }
        }
    }

    if (flush_codecs) {
        AVPacket *empty_pkt = si->pkt;
        int err = 0;
        av_packet_unref(empty_pkt);

        for (unsigned i = 0; i < ic->nb_streams; i++) {
            AVStream *const st  = ic->streams[i];
            FFStream *const sti = ffstream(st);

            /* flush the decoders */
            if (sti->info->found_decoder == 1) {
                err = try_decode_frame(ic, st, empty_pkt,
                                        (options && i < orig_nb_streams)
                                        ? &options[i] : NULL);

                if (err < 0) {
                    av_log(ic, AV_LOG_INFO,
                        "decoding for stream %d failed\n", st->index);
                }
            }
        }
    }

    ff_rfps_calculate(ic);

    for (unsigned i = 0; i < ic->nb_streams; i++) {
        AVStream *const st  = ic->streams[i];
        FFStream *const sti = ffstream(st);
        AVCodecContext *const avctx = sti->avctx;

        if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
            if (avctx->codec_id == AV_CODEC_ID_RAWVIDEO && !avctx->codec_tag && !avctx->bits_per_coded_sample) {
                uint32_t tag= avcodec_pix_fmt_to_codec_tag(avctx->pix_fmt);
                if (avpriv_pix_fmt_find(PIX_FMT_LIST_RAW, tag) == avctx->pix_fmt)
                    avctx->codec_tag= tag;
            }

            /* estimate average framerate if not set by demuxer */
            if (sti->info->codec_info_duration_fields &&
                !st->avg_frame_rate.num &&
                sti->info->codec_info_duration) {
                int best_fps      = 0;
                double best_error = 0.01;
                AVRational codec_frame_rate = avctx->framerate;

                if (sti->info->codec_info_duration        >= INT64_MAX / st->time_base.num / 2||
                    sti->info->codec_info_duration_fields >= INT64_MAX / st->time_base.den ||
                    sti->info->codec_info_duration        < 0)
                    continue;
                av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                          sti->info->codec_info_duration_fields * (int64_t) st->time_base.den,
                          sti->info->codec_info_duration * 2 * (int64_t) st->time_base.num, 60000);

                /* Round guessed framerate to a "standard" framerate if it's
                 * within 1% of the original estimate. */
                for (int j = 0; j < MAX_STD_TIMEBASES; j++) {
                    AVRational std_fps = { get_std_framerate(j), 12 * 1001 };
                    double error       = fabs(av_q2d(st->avg_frame_rate) /
                                              av_q2d(std_fps) - 1);

                    if (error < best_error) {
                        best_error = error;
                        best_fps   = std_fps.num;
                    }

                    if (si->prefer_codec_framerate && codec_frame_rate.num > 0 && codec_frame_rate.den > 0) {
                        error       = fabs(av_q2d(codec_frame_rate) /
                                           av_q2d(std_fps) - 1);
                        if (error < best_error) {
                            best_error = error;
                            best_fps   = std_fps.num;
                        }
                    }
                }
                if (best_fps)
                    av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                              best_fps, 12 * 1001, INT_MAX);
            }
            if (!st->r_frame_rate.num) {
                const AVCodecDescriptor *desc = sti->codec_desc;
                AVRational mul = (AVRational){ desc && (desc->props & AV_CODEC_PROP_FIELDS) ? 2 : 1, 1 };
                AVRational  fr = av_mul_q(avctx->framerate, mul);

                if (fr.num && fr.den && av_cmp_q(st->time_base, av_inv_q(fr)) <= 0) {
                    st->r_frame_rate = fr;
                } else {
                    st->r_frame_rate.num = st->time_base.den;
                    st->r_frame_rate.den = st->time_base.num;
                }
            }
            st->codecpar->framerate = avctx->framerate;
            if (sti->display_aspect_ratio.num && sti->display_aspect_ratio.den) {
                AVRational hw_ratio = { avctx->height, avctx->width };
                st->sample_aspect_ratio = av_mul_q(sti->display_aspect_ratio,
                                                   hw_ratio);
            }
        } else if (avctx->codec_type == AVMEDIA_TYPE_AUDIO) {
            if (!avctx->bits_per_coded_sample)
                avctx->bits_per_coded_sample =
                    av_get_bits_per_sample(avctx->codec_id);
            // set stream disposition based on audio service type
            switch (avctx->audio_service_type) {
            case AV_AUDIO_SERVICE_TYPE_EFFECTS:
                st->disposition = AV_DISPOSITION_CLEAN_EFFECTS;
                break;
            case AV_AUDIO_SERVICE_TYPE_VISUALLY_IMPAIRED:
                st->disposition = AV_DISPOSITION_VISUAL_IMPAIRED;
                break;
            case AV_AUDIO_SERVICE_TYPE_HEARING_IMPAIRED:
                st->disposition = AV_DISPOSITION_HEARING_IMPAIRED;
                break;
            case AV_AUDIO_SERVICE_TYPE_COMMENTARY:
                st->disposition = AV_DISPOSITION_COMMENT;
                break;
            case AV_AUDIO_SERVICE_TYPE_KARAOKE:
                st->disposition = AV_DISPOSITION_KARAOKE;
                break;
            }
        }
    }

    if (probesize)
        estimate_timings(ic, old_offset);

    av_opt_set_int(ic, "skip_clear", 0, AV_OPT_SEARCH_CHILDREN);

    if (ret >= 0 && ic->nb_streams)
        /* We could not have all the codec parameters before EOF. */
        ret = -1;
    for (unsigned i = 0; i < ic->nb_streams; i++) {
        AVStream *const st  = ic->streams[i];
        FFStream *const sti = ffstream(st);
        const char *errmsg;

        /* if no packet was ever seen, update context now for has_codec_parameters */
        if (!sti->avctx_inited) {
            if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO &&
                st->codecpar->format == AV_SAMPLE_FMT_NONE)
                st->codecpar->format = sti->avctx->sample_fmt;
            ret = avcodec_parameters_to_context(sti->avctx, st->codecpar);
            if (ret < 0)
                goto find_stream_info_err;
        }
        if (!has_codec_parameters(st, &errmsg)) {
            char buf[256];
            avcodec_string(buf, sizeof(buf), sti->avctx, 0);
            av_log(ic, AV_LOG_WARNING,
                   "Could not find codec parameters for stream %d (%s): %s\n"
                   "Consider increasing the value for the 'analyzeduration' (%"PRId64") and 'probesize' (%"PRId64") options\n",
                   i, buf, errmsg, ic->max_analyze_duration, ic->probesize);
        } else {
            ret = 0;
        }
    }

    ret = compute_chapters_end(ic);
    if (ret < 0)
        goto find_stream_info_err;

    /* update the stream parameters from the internal codec contexts */
    for (unsigned i = 0; i < ic->nb_streams; i++) {
        AVStream *const st  = ic->streams[i];
        FFStream *const sti = ffstream(st);

        if (sti->avctx_inited) {
            ret = avcodec_parameters_from_context(st->codecpar, sti->avctx);
            if (ret < 0)
                goto find_stream_info_err;

            if (sti->avctx->rc_buffer_size > 0 || sti->avctx->rc_max_rate > 0 ||
                sti->avctx->rc_min_rate) {
                size_t cpb_size;
                AVCPBProperties *props = av_cpb_properties_alloc(&cpb_size);
                if (props) {
                    if (sti->avctx->rc_buffer_size > 0)
                        props->buffer_size = sti->avctx->rc_buffer_size;
                    if (sti->avctx->rc_min_rate > 0)
                        props->min_bitrate = sti->avctx->rc_min_rate;
                    if (sti->avctx->rc_max_rate > 0)
                        props->max_bitrate = sti->avctx->rc_max_rate;
                    if (!av_packet_side_data_add(&st->codecpar->coded_side_data,
                                                 &st->codecpar->nb_coded_side_data,
                                                 AV_PKT_DATA_CPB_PROPERTIES,
                                                 (uint8_t *)props, cpb_size, 0))
                        av_free(props);
                }
            }
        }

        sti->avctx_inited = 0;
#if FF_API_AVSTREAM_SIDE_DATA
FF_DISABLE_DEPRECATION_WARNINGS
        if (st->codecpar->nb_coded_side_data > 0) {
            av_assert0(!st->side_data && !st->nb_side_data);
            st->side_data = av_calloc(st->codecpar->nb_coded_side_data, sizeof(*st->side_data));
            if (!st->side_data) {
                ret = AVERROR(ENOMEM);
                goto find_stream_info_err;
            }

            for (int j = 0; j < st->codecpar->nb_coded_side_data; j++) {
                uint8_t *data = av_memdup(st->codecpar->coded_side_data[j].data,
                                          st->codecpar->coded_side_data[j].size);
                if (!data) {
                    ret = AVERROR(ENOMEM);
                    goto find_stream_info_err;
                }
                st->side_data[j].type = st->codecpar->coded_side_data[j].type;
                st->side_data[j].size = st->codecpar->coded_side_data[j].size;
                st->side_data[j].data = data;
                st->nb_side_data++;
            }
        }
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    }

find_stream_info_err:
    for (unsigned i = 0; i < ic->nb_streams; i++) {
        AVStream *const st  = ic->streams[i];
        FFStream *const sti = ffstream(st);
        if (sti->info) {
            av_freep(&sti->info->duration_error);
            av_freep(&sti->info);
        }
        avcodec_close(sti->avctx);
        // FIXME: avcodec_close() frees AVOption settable fields which includes ch_layout,
        //        so we need to restore it.
        av_channel_layout_copy(&sti->avctx->ch_layout, &st->codecpar->ch_layout);
        av_bsf_free(&sti->extract_extradata.bsf);
    }
    if (ic->pb) {
        FFIOContext *const ctx = ffiocontext(ic->pb);
        av_log(ic, AV_LOG_DEBUG, "After avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d frames:%d\n",
               avio_tell(ic->pb), ctx->bytes_read, ctx->seek_count, count);
    }
    return ret;

unref_then_goto_end:
    av_packet_unref(pkt1);
    goto find_stream_info_err;
}

  代码调用流程图：

  代码详细分析：
    在进行探测前下面这部分代码用来设置探测的时长，可以看到部分时长和格式强相关的，应该和文件本身关系比较大。

max_stream_analyze_duration = max_analyze_duration;
max_subtitle_analyze_duration = max_analyze_duration;
if (!max_analyze_duration) {
    max_stream_analyze_duration =
    max_analyze_duration        = 5*AV_TIME_BASE;
    max_subtitle_analyze_duration = 30*AV_TIME_BASE;
    if (!strcmp(ic->iformat->name, "flv"))
        max_stream_analyze_duration = 90*AV_TIME_BASE;
    if (!strcmp(ic->iformat->name, "mpeg") || !strcmp(ic->iformat->name, "mpegts"))
        max_stream_analyze_duration = 7*AV_TIME_BASE;
}

  然后就是循环遍历打开每个流的解码器，为后续解码做准备，这部分就是调用avcodec_find_decoder和avcodec_open2查找解码器和打开解码器，具体实现等到讲解码时再说。
  之后便是不断循环检测，循环内部会不断判断当前是否已经成功获取完整的流信息，成功或者超出探测时长上限的话就结束；否则就会从读取AVPacket和解码帧AVFrame，读取AVPacket和解码AVFrame分别是调用ff_read_packet和avcodec_send_packet,avcodec_receive_frame进行码流的读取和解码。
  estimate_timings用于估算当前媒体文件的时长，根据不同的格式会采取不同的方式：
    1. mpeg和mpegts会采用pts的方式估算，即调用estimate_timings_from_pts估算。
    2. 如果非情况1且流本身已经探测到一部分时长信息，则调用fill_all_stream_timings根据已有的时长相关信息进行统一。
    3. 其他情况调用estimate_timings_from_bit_rate，利用文件码流大小和码率估算。
timate_timings_from_pts核心就是累加AVPacket的时长，如下：

do {
    is_end = found_duration;
    offset = filesize - (DURATION_MAX_READ_SIZE << retry);
    if (offset < 0)
        offset = 0;

    avio_seek(ic->pb, offset, SEEK_SET);
    read_size = 0;
    for (;;) {
        AVStream *st;
        FFStream *sti;
        if (read_size >= DURATION_MAX_READ_SIZE << (FFMAX(retry - 1, 0)))
            break;

        do {
            ret = ff_read_packet(ic, pkt);
        } while (ret == AVERROR(EAGAIN));
        if (ret != 0)
            break;
        read_size += pkt->size;
        st         = ic->streams[pkt->stream_index];
        sti        = ffstream(st);
        if (pkt->pts != AV_NOPTS_VALUE &&
            (st->start_time != AV_NOPTS_VALUE ||
             sti->first_dts != AV_NOPTS_VALUE)) {
            if (pkt->duration == 0) {
                compute_frame_duration(ic, &num, &den, st, sti->parser, pkt);
                if (den && num) {
                    pkt->duration = av_rescale_rnd(1,
                                       num * (int64_t) st->time_base.den,
                                       den * (int64_t) st->time_base.num,
                                       AV_ROUND_DOWN);
                }
            }
            duration = pkt->pts + pkt->duration;
            found_duration = 1;
            if (st->start_time != AV_NOPTS_VALUE)
                duration -= st->start_time;
            else
                duration -= sti->first_dts;
            if (duration > 0) {
                if (st->duration == AV_NOPTS_VALUE || sti->info->last_duration<= 0 ||
                    (st->duration < duration && FFABS(duration - sti->info->last_duration) < 60LL*st->time_base.den / st->time_base.num))
                    st->duration = duration;
                sti->info->last_duration = duration;
            }
        }
        av_packet_unref(pkt);
    }

    /* check if all audio/video streams have valid duration */
    if (!is_end) {
        is_end = 1;
        for (unsigned i = 0; i < ic->nb_streams; i++) {
            const AVStream *const st = ic->streams[i];
            switch (st->codecpar->codec_type) {
                case AVMEDIA_TYPE_VIDEO:
                case AVMEDIA_TYPE_AUDIO:
                    if (st->duration == AV_NOPTS_VALUE)
                        is_end = 0;
            }
        }
    }
} while (!is_end &&
         offset &&
         ++retry <= DURATION_MAX_RETRY);

4.4、av_read_frame

  FFmpeg使用av_read_frame()方法读取音频流、视频流、字幕流，得到AVPacket数据包。FFmpeg官方提供的samples有使用示例，或者在ffplay.c代码中：打开文件/网络流后，while循环调用av_read_frame()读取帧数据，也就是解封装demux过程，直到文件末尾EOF。
  av_read_frame()的定义位于libavformat\utils.c，如下所示：

int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{
    FFFormatContext *const si = ffformatcontext(s);
    const int genpts = s->flags & AVFMT_FLAG_GENPTS;
    int eof = 0;
    int ret;
    AVStream *st;

    if (!genpts) {
        ret = si->packet_buffer.head
              ? avpriv_packet_list_get(&si->packet_buffer, pkt)
              : read_frame_internal(s, pkt);
        if (ret < 0)
            return ret;
        goto return_packet;
    }

    for (;;) {
        PacketListEntry *pktl = si->packet_buffer.head;

        if (pktl) {
            AVPacket *next_pkt = &pktl->pkt;

            if (next_pkt->dts != AV_NOPTS_VALUE) {
                int wrap_bits = s->streams[next_pkt->stream_index]->pts_wrap_bits;
                // last dts seen for this stream. if any of packets following
                // current one had no dts, we will set this to AV_NOPTS_VALUE.
                int64_t last_dts = next_pkt->dts;
                av_assert2(wrap_bits <= 64);
                while (pktl && next_pkt->pts == AV_NOPTS_VALUE) {
                    if (pktl->pkt.stream_index == next_pkt->stream_index &&
                        av_compare_mod(next_pkt->dts, pktl->pkt.dts, 2ULL << (wrap_bits - 1)) < 0) {
                        if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2ULL << (wrap_bits - 1))) {
                            // not B-frame
                            next_pkt->pts = pktl->pkt.dts;
                        }
                        if (last_dts != AV_NOPTS_VALUE) {
                            // Once last dts was set to AV_NOPTS_VALUE, we don't change it.
                            last_dts = pktl->pkt.dts;
                        }
                    }
                    pktl = pktl->next;
                }
                if (eof && next_pkt->pts == AV_NOPTS_VALUE && last_dts != AV_NOPTS_VALUE) {
                    // Fixing the last reference frame had none pts issue (For MXF etc).
                    // We only do this when
                    // 1. eof.
                    // 2. we are not able to resolve a pts value for current packet.
                    // 3. the packets for this stream at the end of the files had valid dts.
                    next_pkt->pts = last_dts + next_pkt->duration;
                }
                pktl = si->packet_buffer.head;
            }

            /* read packet from packet buffer, if there is data */
            st = s->streams[next_pkt->stream_index];
            if (!(next_pkt->pts == AV_NOPTS_VALUE && st->discard < AVDISCARD_ALL &&
                  next_pkt->dts != AV_NOPTS_VALUE && !eof)) {
                ret = avpriv_packet_list_get(&si->packet_buffer, pkt);
                goto return_packet;
            }
        }

        ret = read_frame_internal(s, pkt);
        if (ret < 0) {
            if (pktl && ret != AVERROR(EAGAIN)) {
                eof = 1;
                continue;
            } else
                return ret;
        }

        ret = avpriv_packet_list_put(&si->packet_buffer,
                                     pkt, NULL, 0);
        if (ret < 0) {
            av_packet_unref(pkt);
            return ret;
        }
    }

return_packet:
    st = s->streams[pkt->stream_index];
    if ((s->iformat->flags & AVFMT_GENERIC_INDEX) && pkt->flags & AV_PKT_FLAG_KEY) {
        ff_reduce_index(s, st->index);
        av_add_index_entry(st, pkt->pos, pkt->dts, 0, 0, AVINDEX_KEYFRAME);
    }

    if (is_relative(pkt->dts))
        pkt->dts -= RELATIVE_TS_BASE;
    if (is_relative(pkt->pts))
        pkt->pts -= RELATIVE_TS_BASE;

    return ret;
}

  av_read_frame()的调用链如下图所示：

  可以从源代码中看出，av_read_frame()调用了read_frame_internal()。

4.4.1、read_frame_internal

  read_frame_internal 在ffmpeg中实现了将format格式的packet,最终转换成一帧帧的es流packet，并解析填充了packet的pts,dts等信息，为最终解码提供了重要的数据，read_frame_internal，调用ff_read_packet，每次只读取一个包，然后直到parser完这个包的所有数据，才开始读取下一个包，parser完的数据被保存在parser结构的数据缓冲中，这样即使ff_read_packet读取的下一包和前一包的流不一样，由于parser也不一样，所以实现了read_frame_internal这个函数调用，可以解析出不同流的es流,而read_frame_internal函数除非出错否则必须解析出一帧数据才能返回。

static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{
    FFFormatContext *const si = ffformatcontext(s);
    int ret, got_packet = 0;
    AVDictionary *metadata = NULL;

    while (!got_packet && !si->parse_queue.head) {
        AVStream *st;
        FFStream *sti;

        // 读取下一个数据包
        ret = ff_read_packet(s, pkt);
        if (ret < 0) {
            if (ret == AVERROR(EAGAIN))
                return ret;
            /* flush the parsers */
            for (unsigned i = 0; i < s->nb_streams; i++) {
                AVStream *const st  = s->streams[i];
                FFStream *const sti = ffstream(st);
                if (sti->parser && sti->need_parsing)
                    parse_packet(s, pkt, st->index, 1);
            }
            /* all remaining packets are now in parse_queue =>
             * really terminate parsing */
            break;
        }
        ret = 0;
        st  = s->streams[pkt->stream_index];
        sti = ffstream(st);

        st->event_flags |= AVSTREAM_EVENT_FLAG_NEW_PACKETS;

        /* update context if required */
        if (sti->need_context_update) {
            if (avcodec_is_open(sti->avctx)) {
                av_log(s, AV_LOG_DEBUG, "Demuxer context update while decoder is open, closing and trying to re-open\n");
                avcodec_close(sti->avctx);
                sti->info->found_decoder = 0;
            }

            /* close parser, because it depends on the codec */
            if (sti->parser && sti->avctx->codec_id != st->codecpar->codec_id) {
                av_parser_close(sti->parser);
                sti->parser = NULL;
            }

            ret = avcodec_parameters_to_context(sti->avctx, st->codecpar);
            if (ret < 0) {
                av_packet_unref(pkt);
                return ret;
            }

            sti->codec_desc = avcodec_descriptor_get(sti->avctx->codec_id);

            sti->need_context_update = 0;
        }

        if (pkt->pts != AV_NOPTS_VALUE &&
            pkt->dts != AV_NOPTS_VALUE &&
            pkt->pts < pkt->dts) {
            av_log(s, AV_LOG_WARNING,
                   "Invalid timestamps stream=%d, pts=%s, dts=%s, size=%d\n",
                   pkt->stream_index,
                   av_ts2str(pkt->pts),
                   av_ts2str(pkt->dts),
                   pkt->size);
        }
        if (s->debug & FF_FDEBUG_TS)
            av_log(s, AV_LOG_DEBUG,
                   "ff_read_packet stream=%d, pts=%s, dts=%s, size=%d, duration=%"PRId64", flags=%d\n",
                   pkt->stream_index,
                   av_ts2str(pkt->pts),
                   av_ts2str(pkt->dts),
                   pkt->size, pkt->duration, pkt->flags);

        if (sti->need_parsing && !sti->parser && !(s->flags & AVFMT_FLAG_NOPARSE)) {
            sti->parser = av_parser_init(st->codecpar->codec_id);
            if (!sti->parser) {
                av_log(s, AV_LOG_VERBOSE, "parser not found for codec "
                       "%s, packets or times may be invalid.\n",
                       avcodec_get_name(st->codecpar->codec_id));
                /* no parser available: just output the raw packets */
                sti->need_parsing = AVSTREAM_PARSE_NONE;
            } else if (sti->need_parsing == AVSTREAM_PARSE_HEADERS)
                sti->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
            else if (sti->need_parsing == AVSTREAM_PARSE_FULL_ONCE)
                sti->parser->flags |= PARSER_FLAG_ONCE;
            else if (sti->need_parsing == AVSTREAM_PARSE_FULL_RAW)
                sti->parser->flags |= PARSER_FLAG_USE_CODEC_TS;
        }

        if (!sti->need_parsing || !sti->parser) {
            /* no parsing needed: we just output the packet as is */
            compute_pkt_fields(s, st, NULL, pkt, AV_NOPTS_VALUE, AV_NOPTS_VALUE);
            if ((s->iformat->flags & AVFMT_GENERIC_INDEX) &&
                (pkt->flags & AV_PKT_FLAG_KEY) && pkt->dts != AV_NOPTS_VALUE) {
                ff_reduce_index(s, st->index);
                av_add_index_entry(st, pkt->pos, pkt->dts,
                                   0, 0, AVINDEX_KEYFRAME);
            }
            got_packet = 1;
        } else if (st->discard < AVDISCARD_ALL) {
            if ((ret = parse_packet(s, pkt, pkt->stream_index, 0)) < 0)
                return ret;
            st->codecpar->sample_rate = sti->avctx->sample_rate;
            st->codecpar->bit_rate = sti->avctx->bit_rate;
#if FF_API_OLD_CHANNEL_LAYOUT
FF_DISABLE_DEPRECATION_WARNINGS
            st->codecpar->channels = sti->avctx->ch_layout.nb_channels;
            st->codecpar->channel_layout = sti->avctx->ch_layout.order == AV_CHANNEL_ORDER_NATIVE ?
                                           sti->avctx->ch_layout.u.mask : 0;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
            ret = av_channel_layout_copy(&st->codecpar->ch_layout, &sti->avctx->ch_layout);
            if (ret < 0)
                return ret;
            st->codecpar->codec_id = sti->avctx->codec_id;
        } else {
            /* free packet */
            av_packet_unref(pkt);
        }
        if (pkt->flags & AV_PKT_FLAG_KEY)
            sti->skip_to_keyframe = 0;
        if (sti->skip_to_keyframe) {
            av_packet_unref(pkt);
            got_packet = 0;
        }
    }

    if (!got_packet && si->parse_queue.head)
        ret = avpriv_packet_list_get(&si->parse_queue, pkt);

    if (ret >= 0) {
        AVStream *const st  = s->streams[pkt->stream_index];
        FFStream *const sti = ffstream(st);
        int discard_padding = 0;
        if (sti->first_discard_sample && pkt->pts != AV_NOPTS_VALUE) {
            int64_t pts = pkt->pts - (is_relative(pkt->pts) ? RELATIVE_TS_BASE : 0);
            int64_t sample = ts_to_samples(st, pts);
            int64_t duration = ts_to_samples(st, pkt->duration);
            int64_t end_sample = sample + duration;
            if (duration > 0 && end_sample >= sti->first_discard_sample &&
                sample < sti->last_discard_sample)
                discard_padding = FFMIN(end_sample - sti->first_discard_sample, duration);
        }
        if (sti->start_skip_samples && (pkt->pts == 0 || pkt->pts == RELATIVE_TS_BASE))
            sti->skip_samples = sti->start_skip_samples;
        sti->skip_samples = FFMAX(0, sti->skip_samples);
        if (sti->skip_samples || discard_padding) {
            uint8_t *p = av_packet_new_side_data(pkt, AV_PKT_DATA_SKIP_SAMPLES, 10);
            if (p) {
                AV_WL32(p, sti->skip_samples);
                AV_WL32(p + 4, discard_padding);
                av_log(s, AV_LOG_DEBUG, "demuxer injecting skip %u / discard %u\n",
                       (unsigned)sti->skip_samples, (unsigned)discard_padding);
            }
            sti->skip_samples = 0;
        }

#if FF_API_AVSTREAM_SIDE_DATA
        if (sti->inject_global_side_data) {
            for (int i = 0; i < st->codecpar->nb_coded_side_data; i++) {
                const AVPacketSideData *const src_sd = &st->codecpar->coded_side_data[i];
                uint8_t *dst_data;

                if (av_packet_get_side_data(pkt, src_sd->type, NULL))
                    continue;

                dst_data = av_packet_new_side_data(pkt, src_sd->type, src_sd->size);
                if (!dst_data) {
                    av_log(s, AV_LOG_WARNING, "Could not inject global side data\n");
                    continue;
                }

                memcpy(dst_data, src_sd->data, src_sd->size);
            }
            sti->inject_global_side_data = 0;
        }
#endif
    }

    if (!si->metafree) {
        int metaret = av_opt_get_dict_val(s, "metadata", AV_OPT_SEARCH_CHILDREN, &metadata);
        if (metadata) {
            s->event_flags |= AVFMT_EVENT_FLAG_METADATA_UPDATED;
            av_dict_copy(&s->metadata, metadata, 0);
            av_dict_free(&metadata);
            av_opt_set_dict_val(s, "metadata", NULL, AV_OPT_SEARCH_CHILDREN);
        }
        si->metafree = metaret == AVERROR_OPTION_NOT_FOUND;
    }

    if (s->debug & FF_FDEBUG_TS)
        av_log(s, AV_LOG_DEBUG,
               "read_frame_internal stream=%d, pts=%s, dts=%s, "
               "size=%d, duration=%"PRId64", flags=%d\n",
               pkt->stream_index,
               av_ts2str(pkt->pts),
               av_ts2str(pkt->dts),
               pkt->size, pkt->duration, pkt->flags);

    /* A demuxer might have returned EOF because of an IO error, let's
     * propagate this back to the user. */
    if (ret == AVERROR_EOF && s->pb && s->pb->error < 0 && s->pb->error != AVERROR(EAGAIN))
        ret = s->pb->error;

    return ret;
}

  read_frame_internal()代码比较长，这里只简单看一下它前面的部分。它前面部分有2步是十分关键的：
    1. 调用了ff_read_packet()从相应的AVInputFormat读取数据。
    2. 如果媒体频流需要使用AVCodecParser，则调用parse_packet()解析相应的AVPacket。
  下面我们分成分别看一下ff_read_packet()和parse_packet()的源代码。

4.4.2、ff_read_packet

  ff_read_packet函数也是会判断缓存是否有数据，若有则从缓存中取出，若没有，则调用demuxer的read_packet来读取数据。ff_read_packet()中最关键的地方就是调用了AVInputFormat的read_packet()方法。AVInputFormat的read_packet()是一个函数指针，指向当前的AVInputFormat的读取数据的函数，比如flv的flv_read_packet，flv_read_packet()的代码比较长，但是逻辑比较简单。它的主要功能就是根据FLV文件格式的规范，逐层解析Tag以及TagData，获取Tag以及TagData中的信息。
  ff_read_packet的缓存和av_read_frame函数里的缓存是不一样的，ff_read_packet的缓存为AVFormatInternal::raw_packet_buffer，在av_read_frame函数里缓存则为：
  AVFormatInternal::packet_buffer，简单的理解为在codec被识别之前用raw_packet_buffer缓存，codec识别后用packet_buffer。

int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    FFFormatContext *const si = ffformatcontext(s);
    int err;

#if FF_API_INIT_PACKET
FF_DISABLE_DEPRECATION_WARNINGS
    pkt->data = NULL;
    pkt->size = 0;
    av_init_packet(pkt);
FF_ENABLE_DEPRECATION_WARNINGS
#else
    av_packet_unref(pkt);
#endif
    /* 这里是不断从队列中取出一个pkt（raw buffer只要有数据）返回给上层和不断的读取数据加入到raw buffer */
    for (;;) {
        PacketListEntry *pktl = si->raw_packet_buffer.head;
        AVStream *st;
        FFStream *sti;
        const AVPacket *pkt1;

        if (pktl) {
            /* 得到AVPacket包 */
            AVStream *const st = s->streams[pktl->pkt.stream_index];
            if (si->raw_packet_buffer_size >= s->probesize)
                if ((err = probe_codec(s, st, NULL)) < 0)
                    return err;
            /* -1是probe完成，0是不要probe */
            if (ffstream(st)->request_probe <= 0) {
                avpriv_packet_list_get(&si->raw_packet_buffer, pkt);
                /* 归还队列空间 */
                si->raw_packet_buffer_size -= pkt->size;
                return 0;
            }
        }
        
        /*  0表示成功，小于0表示错误，pkt就在这里获取到。*/
        err = s->iformat->read_packet(s, pkt);
        if (err < 0) {
            av_packet_unref(pkt);

            /* Some demuxers return FFERROR_REDO when they consume
               data and discard it (ignored streams, junk, extradata).
               We must re-call the demuxer to get the real packet. */
            if (err == FFERROR_REDO)
                continue;
            if (!pktl || err == AVERROR(EAGAIN))
                return err;
            for (unsigned i = 0; i < s->nb_streams; i++) {
                AVStream *const st  = s->streams[i];
                FFStream *const sti = ffstream(st);
                if (sti->probe_packets || sti->request_probe > 0)
                    if ((err = probe_codec(s, st, NULL)) < 0)
                        return err;
                av_assert0(sti->request_probe <= 0);
            }
            continue;
        }

        err = av_packet_make_refcounted(pkt);
        if (err < 0) {
            av_packet_unref(pkt);
            return err;
        }
        
        /* 读取成功，判断是否需要丢弃 */
        if (pkt->flags & AV_PKT_FLAG_CORRUPT) {
            av_log(s, AV_LOG_WARNING,
                   "Packet corrupt (stream = %d, dts = %s)",
                   pkt->stream_index, av_ts2str(pkt->dts));
            if (s->flags & AVFMT_FLAG_DISCARD_CORRUPT) {
                av_log(s, AV_LOG_WARNING, ", dropping it.\n");
                av_packet_unref(pkt);
                continue;
            }
            av_log(s, AV_LOG_WARNING, ".\n");
        }

        av_assert0(pkt->stream_index < (unsigned)s->nb_streams &&
                   "Invalid stream index.\n");

        /* 得到读取到的这个包所在的stream，以为pkt获取时间戳 */
        st  = s->streams[pkt->stream_index];
        sti = ffstream(st);

        if (update_wrap_reference(s, st, pkt->stream_index, pkt) && sti->pts_wrap_behavior == AV_PTS_WRAP_SUB_OFFSET) {
            // correct first time stamps to negative values
            if (!is_relative(sti->first_dts))
                sti->first_dts = wrap_timestamp(st, sti->first_dts);
            if (!is_relative(st->start_time))
                st->start_time = wrap_timestamp(st, st->start_time);
            if (!is_relative(sti->cur_dts))
                sti->cur_dts = wrap_timestamp(st, sti->cur_dts);
        }

        pkt->dts = wrap_timestamp(st, pkt->dts);
        pkt->pts = wrap_timestamp(st, pkt->pts);

        force_codec_ids(s, st);

        /* TODO: audio: time filter; video: frame reordering (pts != dts) */
        if (s->use_wallclock_as_timestamps)
            pkt->dts = pkt->pts = av_rescale_q(av_gettime(), AV_TIME_BASE_Q, st->time_base);

        if (!pktl && sti->request_probe <= 0)
            return 0;

        err = avpriv_packet_list_put(&si->raw_packet_buffer,
                                     pkt, NULL, 0);
        if (err < 0) {
            av_packet_unref(pkt);
            return err;
        }
        pkt1 = &si->raw_packet_buffer.tail->pkt;
        si->raw_packet_buffer_size += pkt1->size;

        if ((err = probe_codec(s, st, pkt1)) < 0)
            return err;
    }
}

4.4.3、parse_packet

  用于将输入流中的音视频信息按帧封装为packet对象，供进一步处理。需传入输入流上下文和用于装载数据的packet对象

static int parse_packet(AVFormatContext *s, AVPacket *pkt,
                        int stream_index, int flush)
{
    FFFormatContext *const si = ffformatcontext(s);
    AVPacket *out_pkt = si->parse_pkt;
    AVStream *st = s->streams[stream_index];
    FFStream *const sti = ffstream(st);
    const uint8_t *data = pkt->data;
    int size = pkt->size;
    int ret = 0, got_output = flush;

    if (!size && !flush && sti->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) {
        // preserve 0-size sync packets
        compute_pkt_fields(s, st, sti->parser, pkt, AV_NOPTS_VALUE, AV_NOPTS_VALUE);
    }

    while (size > 0 || (flush && got_output)) {
        int64_t next_pts = pkt->pts;
        int64_t next_dts = pkt->dts;
        int len;
        /* 真正去解析数据包 */
        len = av_parser_parse2(sti->parser, sti->avctx,
                               &out_pkt->data, &out_pkt->size, data, size,
                               pkt->pts, pkt->dts, pkt->pos);

        pkt->pts = pkt->dts = AV_NOPTS_VALUE;
        pkt->pos = -1;
        /* increment read pointer */
        av_assert1(data || !len);
        data  = len ? data + len : data;
        size -= len;

        got_output = !!out_pkt->size;

        if (!out_pkt->size)
            continue;

        if (pkt->buf && out_pkt->data == pkt->data) {
            /* reference pkt->buf only when out_pkt->data is guaranteed to point
             * to data in it and not in the parser's internal buffer. */
            /* XXX: Ensure this is the case with all parsers when sti->parser->flags
             * is PARSER_FLAG_COMPLETE_FRAMES and check for that instead? */
            out_pkt->buf = av_buffer_ref(pkt->buf);
            if (!out_pkt->buf) {
                ret = AVERROR(ENOMEM);
                goto fail;
            }
        } else {
            ret = av_packet_make_refcounted(out_pkt);
            if (ret < 0)
                goto fail;
        }

        if (pkt->side_data) {
            out_pkt->side_data       = pkt->side_data;
            out_pkt->side_data_elems = pkt->side_data_elems;
            pkt->side_data          = NULL;
            pkt->side_data_elems    = 0;
        }

        /* set the duration */
        out_pkt->duration = (sti->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) ? pkt->duration : 0;
        if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
            if (sti->avctx->sample_rate > 0) {
                out_pkt->duration =
                    av_rescale_q_rnd(sti->parser->duration,
                                     (AVRational) { 1, sti->avctx->sample_rate },
                                     st->time_base,
                                     AV_ROUND_DOWN);
            }
        } else if (st->codecpar->codec_id == AV_CODEC_ID_GIF) {
            if (st->time_base.num > 0 && st->time_base.den > 0 &&
                sti->parser->duration) {
                out_pkt->duration = sti->parser->duration;
            }
        }

        out_pkt->stream_index = st->index;
        out_pkt->pts          = sti->parser->pts;
        out_pkt->dts          = sti->parser->dts;
        out_pkt->pos          = sti->parser->pos;
        out_pkt->flags       |= pkt->flags & (AV_PKT_FLAG_DISCARD | AV_PKT_FLAG_CORRUPT);

        if (sti->need_parsing == AVSTREAM_PARSE_FULL_RAW)
            out_pkt->pos = sti->parser->frame_offset;

        if (sti->parser->key_frame == 1 ||
            (sti->parser->key_frame == -1 &&
             sti->parser->pict_type == AV_PICTURE_TYPE_I))
            out_pkt->flags |= AV_PKT_FLAG_KEY;

        if (sti->parser->key_frame == -1 && sti->parser->pict_type ==AV_PICTURE_TYPE_NONE && (pkt->flags&AV_PKT_FLAG_KEY))
            out_pkt->flags |= AV_PKT_FLAG_KEY;

        compute_pkt_fields(s, st, sti->parser, out_pkt, next_dts, next_pts);

        ret = avpriv_packet_list_put(&si->parse_queue,
                                     out_pkt, NULL, 0);
        if (ret < 0)
            goto fail;
    }

    /* end of the stream => close and free the parser */
    if (flush) {
        av_parser_close(sti->parser);
        sti->parser = NULL;
    }

fail:
    if (ret < 0)
        av_packet_unref(out_pkt);
    av_packet_unref(pkt);
    return ret;
}

  最终是调用libavcodec/parser.c的av_parser_parse2()方法去解析数据包。

4.5、avformat_seek_file

  在做音视频数据分析的时候，经常会遇到这样的需求，每隔5分钟抽取一帧数据进行分析。
  在做播放器开发的时候，也会遇到这种情况，就是拖动进度条跳转到某个位置进行播放。
  如果直接用 av_read_frame() 不断读数据，读到第 5 分钟的 AVPacket 才开始处理，其他读出来的 AVPacket 丢弃，这样做会带来非常大的磁盘IO。
  其实上面两种场景，都可以用同一个函数解决，那就是 avformat_seek_file()，这个函数类似于 Linux 的 lseek() ，设置文件的读取位置。
  只不过 avformat_seek_file() 是用于音视频文件的。

int avformat_seek_file(AVFormatContext *s, int stream_index, int64_t min_ts,
                       int64_t ts, int64_t max_ts, int flags)
{
    if (min_ts > ts || max_ts < ts)
        return -1;
    if (stream_index < -1 || stream_index >= (int)s->nb_streams)
        return AVERROR(EINVAL);

    if (s->seek2any > 0)
        flags |= AVSEEK_FLAG_ANY;
    flags &= ~AVSEEK_FLAG_BACKWARD;

    if (s->iformat->read_seek2) {
        int ret;
        ff_read_frame_flush(s);

        if (stream_index == -1 && s->nb_streams == 1) {
            AVRational time_base = s->streams[0]->time_base;
            ts = av_rescale_q(ts, AV_TIME_BASE_Q, time_base);
            min_ts = av_rescale_rnd(min_ts, time_base.den,
                                    time_base.num * (int64_t)AV_TIME_BASE,
                                    AV_ROUND_UP   | AV_ROUND_PASS_MINMAX);
            max_ts = av_rescale_rnd(max_ts, time_base.den,
                                    time_base.num * (int64_t)AV_TIME_BASE,
                                    AV_ROUND_DOWN | AV_ROUND_PASS_MINMAX);
            stream_index = 0;
        }

        ret = s->iformat->read_seek2(s, stream_index, min_ts,
                                     ts, max_ts, flags);

        if (ret >= 0)
            ret = avformat_queue_attached_pictures(s);
        return ret;
    }

    if (s->iformat->read_timestamp) {
        // try to seek via read_timestamp()
    }

    // Fall back on old API if new is not implemented but old is.
    // Note the old API has somewhat different semantics.
    if (s->iformat->read_seek || 1) {
        int dir = (ts - (uint64_t)min_ts > (uint64_t)max_ts - ts ? AVSEEK_FLAG_BACKWARD : 0);
        int ret = av_seek_frame(s, stream_index, ts, flags | dir);
        if (ret < 0 && ts != min_ts && max_ts != ts) {
            ret = av_seek_frame(s, stream_index, dir ? max_ts : min_ts, flags | dir);
            if (ret >= 0)
                ret = av_seek_frame(s, stream_index, ts, flags | (dir^AVSEEK_FLAG_BACKWARD));
        }
        return ret;
    }

    // try some generic seek like seek_frame_generic() but with new ts semantics
    return -1; //unreachable
}

参数解释如下：
    1. AVFormatContext *s，已经打开的容器示例。
    2. int stream_index，流索引，但是只有在 flags 包含 AVSEEK_FLAG_FRAME 的时候才是 设置某个流的读取位置。其他情况都只是把这个流的 time_base （时间基）作为参考。
    3. int64_t min_ts，跳转到的最小的时间，但是这个变量不一定是时间单位，也有可能是字节单位，也可能是帧数单位（第几帧）。
    4. int64_t ts，要跳转到的读取位置，单位同上。
    5. int64_t max_ts，跳转到的最大的时间，单位同上，通常填 INT64_MAX 即可。
    6. int flags，跳转的方式，有 4 个 flags，如下：
      1. AVSEEK_FLAG_BYTE，按字节大小进行跳转。
      2. AVSEEK_FLAG_FRAME，按帧数大小进行跳转。
      3. AVSEEK_FLAG_ANY，可以跳转到非关键帧的读取位置，但是解码会出现马赛克。
      4. AVSEEK_FLAG_BACKWARD，往 ts 的后面找关键帧，默认是往 ts 的前面找关键帧。
  通过代码可知avformat_seek_file函数最终还是调用av_seek_frame函数来实现跳转操作的。
  avformat_seek_file调用流程如下：

4.6、av_seek_frame

int av_seek_frame(AVFormatContext *s, int stream_index,
                  int64_t timestamp, int flags)
{
    int ret;

    if (s->iformat->read_seek2 && !s->iformat->read_seek) {
        int64_t min_ts = INT64_MIN, max_ts = INT64_MAX;
        if ((flags & AVSEEK_FLAG_BACKWARD))
            max_ts = timestamp;
        else
            min_ts = timestamp;
        return avformat_seek_file(s, stream_index, min_ts, timestamp, max_ts,
                                  flags & ~AVSEEK_FLAG_BACKWARD);
    }

    ret = seek_frame_internal(s, stream_index, timestamp, flags);

    if (ret >= 0)
        ret = avformat_queue_attached_pictures(s);

    return ret;
}

  av_seek_frame函数首先判断是否存在iformat->read_seek2，如果存在就调用对应read_seek2()，如果不存在则调用seek_frame_internal()去执行seek操作：

4.6.1、seek_frame_internal

static int seek_frame_internal(AVFormatContext *s, int stream_index,
                               int64_t timestamp, int flags)
{
    AVStream *st;
    int ret;
    
    /* 以字节数方式寻帧 */
    if (flags & AVSEEK_FLAG_BYTE) {
        if (s->iformat->flags & AVFMT_NO_BYTE_SEEK)
            return -1;
        ff_read_frame_flush(s);
        return seek_frame_byte(s, stream_index, timestamp, flags);
    }

    if (stream_index < 0) {
        stream_index = av_find_default_stream_index(s);
        if (stream_index < 0)
            return -1;

        st = s->streams[stream_index];
        /* timestamp for default must be expressed in AV_TIME_BASE units */
        timestamp = av_rescale(timestamp, st->time_base.den,
                               AV_TIME_BASE * (int64_t) st->time_base.num);
    }

    /* first, we try the format specific seek */
    if (s->iformat->read_seek) {
        ff_read_frame_flush(s);
        ret = s->iformat->read_seek(s, stream_index, timestamp, flags);
    } else
        ret = -1;
    if (ret >= 0)
        return 0;

    if (s->iformat->read_timestamp &&
        !(s->iformat->flags & AVFMT_NOBINSEARCH)) {
        /* 以二分查找方式寻帧 */
        ff_read_frame_flush(s);
        return ff_seek_frame_binary(s, stream_index, timestamp, flags);
    } else if (!(s->iformat->flags & AVFMT_NOGENSEARCH)) {
        /* 以通用方式寻帧 */
        ff_read_frame_flush(s);
        return seek_frame_generic(s, stream_index, timestamp, flags);
    } else
        return -1;
}

  由此可见，该函数按照顺序有4个执行步骤：
    1. 以字节数方式寻帧；
    2. 以指定方式read_seek寻帧；
    3. 以二分查找方式寻帧；
    4. 以通用方式寻帧；

4.6.2、seek_frame_byte

  我们看看byte方式的seek操作，直接调用avio_seek移动到指定位置：

static int seek_frame_byte(AVFormatContext *s, int stream_index,
                           int64_t pos, int flags)
{
    FFFormatContext *const si = ffformatcontext(s);
    int64_t pos_min, pos_max;

    pos_min = si->data_offset;
    pos_max = avio_size(s->pb) - 1;

    if (pos < pos_min)
        pos = pos_min;
    else if (pos > pos_max)
        pos = pos_max;

    avio_seek(s->pb, pos, SEEK_SET);

    s->io_repositioned = 1;

    return 0;
}

4.7、avformat_close_input

  该函数用于关闭一个AVFormatContext，一般情况下是和avformat_open_input()成对使用的。avformat_close_input()的声明位于libavformat\avformat.h，如下所示。

void avformat_close_input(AVFormatContext **ps)
{
    AVFormatContext *s;
    AVIOContext *pb;

    if (!ps || !*ps)
        return;

    s  = *ps;
    pb = s->pb;

    if ((s->iformat && strcmp(s->iformat->name, "image2") && s->iformat->flags & AVFMT_NOFILE) ||
        (s->flags & AVFMT_FLAG_CUSTOM_IO))
        pb = NULL;

    if (s->iformat)
        if (s->iformat->read_close)
            s->iformat->read_close(s);

    avformat_free_context(s);

    *ps = NULL;

    avio_close(pb);
}

  函数调用关系图

  从源代码中可以看出，avformat_close_input()主要做了以下几步工作：
    1. 调用AVInputFormat的read_close()方法关闭输入流。
    2. 调用avformat_free_context()释放AVFormatContext。
    3. 调用avio_close()关闭并且释放AVIOContext。

  AVInputFormat-> read_close()
    1. AVInputFormat的read_close()是一个函数指针，指向关闭输入流的函数。
    2. 不同的AVInputFormat包含有不同的read_close()方法。
    3. 例如，FLV格式对应的AVInputFormat的定义如下。

五、实例

#include <stdio.h>
#include <libavformat/avformat.h>
 
int main(int argc, char **argv)
{
    // 1. 打开文件
    const char *ifilename = "believe.flv";
    printf("in_filename = %s\n", ifilename);
    // AVFormatContext是描述一个媒体文件或媒体流的构成和基本信息的结构体
    AVFormatContext *ifmt_ctx = NULL;           // 输入文件的demux
    // 打开文件，主要是探测协议类型，如果是网络文件则创建网络链接
    int ret = avformat_open_input(&ifmt_ctx, ifilename, NULL, NULL);
    if (ret < 0) {
        char buf[1024] = {0};
        av_strerror(ret, buf, sizeof (buf) - 1);
        printf("open %s failed: %s\n", ifilename, buf);
        return -1;
    }
 
    // 2. 读取码流信息
    ret = avformat_find_stream_info(ifmt_ctx, NULL);
    if (ret < 0)  //如果打开媒体文件失败，打印失败原因
    {
        char buf[1024] = { 0 };
        av_strerror(ret, buf, sizeof(buf) - 1);
        printf("avformat_find_stream_info %s failed:%s\n", ifilename, buf);
        avformat_close_input(&ifmt_ctx);
        return -1;
    }
 
    // 3.打印总体信息
    printf_s("\n==== av_dump_format in_filename:%s ===\n", ifilename);
    av_dump_format(ifmt_ctx, 0, ifilename, 0);
    printf_s("\n==== av_dump_format finish =======\n\n");
    printf("media name:%s\n", ifmt_ctx->url);
    printf("stream number:%d\n", ifmt_ctx->nb_streams); //  nb_streams媒体流数量
    printf("media average ratio:%lldkbps\n",(int64_t)(ifmt_ctx->bit_rate/1024)); //  媒体文件的码率,单位为bps
    // duration: 媒体文件时长，单位微妙
    int total_seconds = (ifmt_ctx->duration) / AV_TIME_BASE;  // 1000us = 1ms, 1000ms = 1秒
    printf("audio duration: %02d:%02d:%02d\n",
           total_seconds / 3600, (total_seconds % 3600) / 60, (total_seconds % 60));
    printf("\n");
 
    // 4.读取码流信息
    // 音频
    int audioindex = av_find_best_stream(ifmt_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
    if (audioindex < 0) {
        printf("av_find_best_stream %s eror.", av_get_media_type_string(AVMEDIA_TYPE_AUDIO));
        return -1;
    }
    AVStream *audio_stream = ifmt_ctx->streams[audioindex];
    printf("----- Audio info:\n");
    printf("index: %d\n", audio_stream->index); // 序列号
    printf("samplarate: %d Hz\n", audio_stream->codecpar->sample_rate); // 采样率
    printf("sampleformat: %d\n", audio_stream->codecpar->format); // 采样格式 AV_SAMPLE_FMT_FLTP:8
    printf("audio codec: %d\n", audio_stream->codecpar->codec_id); // 编码格式 AV_CODEC_ID_MP3:86017 AV_CODEC_ID_AAC:86018
    if (audio_stream->duration != AV_NOPTS_VALUE) {
        int audio_duration = audio_stream->duration * av_q2d(audio_stream->time_base);
        printf("audio duration: %02d:%02d:%02d\n",
               audio_duration / 3600, (audio_duration % 3600) / 60, (audio_duration % 60));
    }
 
    // 视频
    int videoindex = av_find_best_stream(ifmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
    if (videoindex < 0) {
        printf("av_find_best_stream %s eror.", av_get_media_type_string(AVMEDIA_TYPE_VIDEO));
        return -1;
    }
    AVStream *video_stream = ifmt_ctx->streams[videoindex];
    printf("----- Video info:\n");
    printf("index: %d\n", video_stream->index); // 序列号
    printf("fps: %lf\n", av_q2d(video_stream->avg_frame_rate)); // 帧率
    printf("width: %d, height:%d \n", video_stream->codecpar->width, video_stream->codecpar->height);
    printf("video codec: %d\n", video_stream->codecpar->codec_id); // 编码格式 AV_CODEC_ID_H264: 27
    if (video_stream->duration != AV_NOPTS_VALUE) {
        int video_duration = video_stream->duration * av_q2d(video_stream->time_base);
        printf("audio duration: %02d:%02d:%02d\n",
               video_duration / 3600, (video_duration % 3600) / 60, (video_duration % 60));
    }
 
    // 5.提取码流
    AVPacket *pkt = av_packet_alloc();
    int pkt_count = 0;
    int print_max_count = 10;
    printf("\n-----av_read_frame start\n");
    while (1)
    {
        ret = av_read_frame(ifmt_ctx, pkt);
        if (ret < 0) {
            printf("av_read_frame end\n");
            break;
        }
 
        if(pkt_count++ < print_max_count)
        {
            if (pkt->stream_index == audioindex)
            {
                printf("audio pts: %lld\n", pkt->pts);
                printf("audio dts: %lld\n", pkt->dts);
                printf("audio size: %d\n", pkt->size);
                printf("audio pos: %lld\n", pkt->pos);
                printf("audio duration: %lf\n\n",
                       pkt->duration * av_q2d(ifmt_ctx->streams[audioindex]->time_base));
            }
            else if (pkt->stream_index == videoindex)
            {
                printf("video pts: %lld\n", pkt->pts);
                printf("video dts: %lld\n", pkt->dts);
                printf("video size: %d\n", pkt->size);
                printf("video pos: %lld\n", pkt->pos);
                printf("video duration: %lf\n\n",
                       pkt->duration * av_q2d(ifmt_ctx->streams[videoindex]->time_base));
            }
            else
            {
                printf("unknown stream_index:\n", pkt->stream_index);
            }
        }
        av_packet_unref(pkt);
    }
 
    // 6.结束
    if(pkt)
        av_packet_free(&pkt);
    if(ifmt_ctx)
        avformat_close_input(&ifmt_ctx);
 
    getchar(); //加上这一句，防止程序打印完信息马上退出
    return 0;
}

  输出：

in_filename = believe.flv
 
==== av_dump_format in_filename:believe.flv ===
Input #0, flv, from 'believe.flv':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    comment         : www.ieway.cn
    encoder         : Lavf58.29.100
  Duration: 00:03:42.53, start: 0.000000, bitrate: 286 kb/s
    Stream #0:0: Video: h264 (Constrained Baseline), yuv420p(progressive), 1920x1080, 150 kb/s, 14.46 fps, 15 tbr, 1k tbn, 30 tbc
    Stream #0:1: Audio: aac (LC), 48000 Hz, stereo, fltp, 128 kb/s
 
==== av_dump_format finish =======
 
media name:believe.flv
stream number:2
media average ratio:279kbps
audio duration: 00:03:42
 
----- Audio info:
index: 1
samplarate: 48000 Hz
sampleformat: 8
audio codec: 86018
----- Video info:
index: 0
fps: 14.464286
width: 1920, height:1080
video codec: 27
 
-----av_read_frame start
audio pts: 0
audio dts: 0
audio size: 341
audio pos: 502
audio duration: 0.021000
 
video pts: 14
video dts: 14
video size: 66736
video pos: 860
video duration: 0.066000
 
audio pts: 21
audio dts: 21
audio size: 341
audio pos: 67616
audio duration: 0.021000
 
audio pts: 43
audio dts: 43
audio size: 342
audio pos: 67974
audio duration: 0.021000
 
audio pts: 64
audio dts: 64
audio size: 341
audio pos: 68333
audio duration: 0.021000
 
video pts: 81
video dts: 81
video size: 580
video pos: 68691
video duration: 0.066000
 
audio pts: 85
audio dts: 85
audio size: 341
audio pos: 69291
audio duration: 0.021000
 
audio pts: 107
audio dts: 107
audio size: 342
audio pos: 69649
audio duration: 0.021000
 
audio pts: 128
audio dts: 128
audio size: 341
audio pos: 70008
audio duration: 0.021000
 
video pts: 147
video dts: 147
video size: 11289
video pos: 70366
video duration: 0.066000
 
av_read_frame end