AVFormatContext结构体源码解析

/** * 利用avformat_alloc_context()来create一个 AVFormatContext. */typedef struct AVFormatContext {    /**     * AVClass最主要的作用就是给结构体（如AVFormatContext等）增加AVOption功能的     *支持。换句话说就是AVClass是AVOption和目标结构体之间的桥梁，AVClass要求必须     *为目标结构体声明的第一个变量。     *AVOption是用来设置ffmpeg中变量的结构体。后续会具体介绍     */    const AVClass *av_class;    /**     * 输入文件的格式     * 在解复用是出现，由avformat_open_input()设置.     */    struct AVInputFormat *iformat;    /**     * 输出文件的格式     *     * 复用的时候出现, 必须在avformat_write_header()之前由用户设置.     */    struct AVOutputFormat *oformat;    /**     * 指向具体某个文件格式的私有数据. This is an AVOptions-enabled struct     * if and only if iformat/oformat.priv_class is not NULL.     *     * - 复用时由avformat_write_header()设置     * - 解复用时由avformat_open_input()设置     */    void *priv_data;    /**     * 管理输入输出数据的结构体     * 通过对该变量的赋值可以改变输入源或输出目的地     * - 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;    /* 码流信息 */    /**     * 标志位又来表示码流的性能. A combination of AVFMTCTX_*.     * Set by libavformat.     */    int ctx_flags;    /**     * AVFormatContext.streams的个数.     *     * Set by avformat_new_stream(),不能被随意修改.     */    unsigned int nb_streams;    /**     * 输入文件的所有码流信息.新的码流信息可由     * avformat_new_stream()生成。     *     * - 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().     *     * 通过avformat_free_context()释放内存.     */    AVStream **streams;    /**     * 输入输出文件名     *     * - demuxing: set by avformat_open_input()     * - muxing: may be set by the caller before avformat_write_header()     */    char filename[1024];    /**     * 定位第一帧的位置,利用     * AV_TIME_BASE fractional seconds. 不能直接设定:     * 可以从 AVStream values中推导得出.     *     * Demuxing only, set by libavformat.     */    int64_t start_time;    /**     * 码流的时长, 也是利用 AV_TIME_BASE fractional     * seconds表征. 一般情况下由     * AVStream values 推导得出.     *     * Demuxing only, set by libavformat.     */    int64_t duration;    /**     * 这个stream的码率, 0 if not     * available. ffmpeg可以根据file_size和duration自动计算出来，所以无需指定.     */    int64_t bit_rate;    unsigned int packet_size;    int max_delay;    /**     * 标志位用来修改解/复用行为. 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_MP4A_LATM    0x8000 ///< Enable RTP MP4A-LATM payload#define AVFMT_FLAG_SORT_DTS    0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)#define AVFMT_FLAG_PRIV_OPT    0x20000 ///< Enable use of private options by delaying codec open (this could be made default once all code is converted)#define AVFMT_FLAG_KEEP_SIDE_DATA 0x40000 ///< Don't merge side data but keep it separate.#define AVFMT_FLAG_FAST_SEEK   0x80000 ///< Enable fast, but inaccurate seeks for some formats    /**     * 从输入文件中读取数据的最大size， for determining     * the input container format.     * Demuxing only, set by the caller before avformat_open_input().     */    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（字幕） codec_id.     * Demuxing: Set by user.     */    enum AVCodecID subtitle_codec_id;    /**     * 每个stream的index所需要的内存字节数.     * 如果index尺寸超过了这个值, 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;    /**     * 帧率 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    /**     * 交叉存取的最大buffer.     *     * 为了保证所有streams交叉存取正确,     * 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（稀疏）" (i.e. there are large gaps between     * successive packets), this can result in excessive（过大的） buffering.     *     * This field specifies the maximum difference between the timestamps（时间戳） 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 for the user to detect events happening on the file. Flags must     * be cleared by the user once the event has been handled.     * A combination of AVFMT_EVENT_FLAG_*.     */    int event_flags;#define AVFMT_EVENT_FLAG_METADATA_UPDATED 0x0001 ///< The call resulted in updated metadata.    /**     * Maximum number of packets to read while waiting for the first timestamp.     * Decoding only.     */    int max_ts_probe;    /**     * 复用时为了避免出现负的时间戳     * Any value of the AVFMT_AVOID_NEG_TS_* constants.     * Note, this only works when using av_interleaved_write_frame. (interleave_packet_per_dts is in use)     * - 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_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（预加载） in microseconds.     * Note, not all formats support this and unpredictable things may happen if it is used when not supported.     * - encoding: Set by user via AVOptions (NO direct access)     * - decoding: unused     */    int audio_preload;    /**     * Max chunk time in microseconds.     * Note, not all formats support this and unpredictable things may happen if it is used when not supported.     * - encoding: Set by user via AVOptions (NO direct access)     * - decoding: unused     */    int max_chunk_duration;    /**     * Max chunk size in bytes     * Note, not all formats support this and unpredictable things may happen if it is used when not supported.     * - encoding: Set by user via AVOptions (NO direct access)     * - decoding: unused     */    int max_chunk_size;    /**     * forces the use of wallclock timestamps as pts/dts of packets     * This has undefined results in the presence of B frames.     * - encoding: unused     * - decoding: Set by user via AVOptions (NO direct access)     */    int use_wallclock_as_timestamps;    /**     * avio flags, used to force AVIO_FLAG_DIRECT.     * - encoding: unused     * - decoding: Set by user via AVOptions (NO direct access)     */    int avio_flags;    /**     * The duration field can be estimated through various ways, and this field can be used     * to know how the duration was estimated.     * - encoding: unused     * - decoding: Read by user via AVOptions (NO direct access)     */    enum AVDurationEstimationMethod duration_estimation_method;    /**     * Skip initial bytes when opening stream     * - encoding: unused     * - decoding: Set by user via AVOptions (NO direct access)     */    int64_t skip_initial_bytes;    /**     * Correct single timestamp overflows     * - encoding: unused     * - decoding: Set by user via AVOptions (NO direct access)     */    unsigned int correct_ts_overflow;    /**     * Force seeking to any (also non key) frames.     * - encoding: unused     * - decoding: Set by user via AVOptions (NO direct access)     */    int seek2any;    /**     * Flush the I/O context after each packet.     * - encoding: Set by user via AVOptions (NO direct access)     * - 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 via av_format_get_probe_score() (NO direct access)     */    int probe_score;    /**     * number of bytes to read maximally to identify format.     * - encoding: unused     * - decoding: set by user through AVOPtions (NO direct access)     */    int format_probesize;    /**     * ',' separated list of allowed decoders.     * If NULL then all are allowed     * - encoding: unused     * - decoding: set by user through AVOptions (NO direct access)     */    char *codec_whitelist;    /**     * ',' separated list of allowed demuxers.     * If NULL then all are allowed     * - encoding: unused     * - decoding: set by user through AVOptions (NO direct access)     */    char *format_whitelist;    /**     * An opaque（不透明的） field for libavformat internal usage.     * Must not be accessed in any way by callers.     */    AVFormatInternal *internal;    /**     * IO repositioned flag.     * This is set by avformat when the underlaying IO context read pointer     * is repositioned, for example when doing byte based seeking.     * Demuxers can use the flag to detect such changes.     */    int io_repositioned;    /**     * Forced video codec.     * This allows forcing a specific decoder, even when there are multiple with     * the same codec_id.     * Demuxing: Set by user via av_format_set_video_codec (NO direct access).     */    AVCodec *video_codec;    /**     * Forced audio codec.     * This allows forcing a specific decoder, even when there are multiple with     * the same codec_id.     * Demuxing: Set by user via av_format_set_audio_codec (NO direct access).     */    AVCodec *audio_codec;    /**     * Forced subtitle codec.     * This allows forcing a specific decoder, even when there are multiple with     * the same codec_id.     * Demuxing: Set by user via av_format_set_subtitle_codec (NO direct access).     */    AVCodec *subtitle_codec;    /**     * Forced data codec.     * This allows forcing a specific decoder, even when there are multiple with     * the same codec_id.     * Demuxing: Set by user via av_format_set_data_codec (NO direct access).     */    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 via AVOptions (NO direct access)     */    int64_t output_ts_offset;    /**     * dump（转储） format separator（分隔符）.     * can be ", " or "\n      " or anything else     * Code outside libavformat should access this field using AVOptions     * (NO direct access).     * - 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;#if FF_API_OLD_OPEN_CALLBACKS    /**     * Called to open further IO contexts when needed for demuxing.     *     * This can be set by the user application to perform security checks on     * the URLs before opening them.     * The function should behave like avio_open2(), AVFormatContext is provided     * as contextual information and to reach AVFormatContext.opaque.     *     * If NULL then some simple checks are used together with avio_open2().     *     * Must not be accessed directly from outside avformat.     * @See av_format_set_open_cb()     *     * Demuxing: Set by user.     *     * @deprecated Use io_open and io_close.     */    attribute_deprecated    int (*open_cb)(struct AVFormatContext *s, AVIOContext **p, const char *url, int flags, const AVIOInterruptCB *int_cb, AVDictionary **options);#endif    /**     * ',' separated list of allowed protocols（协议）.     * - encoding: unused     * - decoding: set by user through AVOptions (NO direct access)     */    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);    /**     * A callback for closing the streams opened with AVFormatContext.io_open().     */    void (*io_close)(struct AVFormatContext *s, AVIOContext *pb);    /**     * ',' separated list of disallowed protocols.     * - encoding: unused     * - decoding: set by user through AVOptions (NO direct access)     */    char *protocol_blacklist;} AVFormatContext;