AVFormatContext描述一个媒体文件或媒体流的构成和基本信息。
这是FFMpeg中最为基本的一个结构，是其他所有结构的根，是一个多媒体文件或流的根本抽象。其中nb_streams和streams所表示的AVStream结构指针数组包含了所有内嵌媒体流的描述；iformat和oformat指向对应的demuxer和muxer指针；pb则指向一个控制底层数据读写的ByteIOContext结构。
/** * Format I/O context. * New fields can be added to the end with minor version bumps. * Removal, reordering and changes to existing fields require a major * version bump. * sizeof(AVFormatContext) must not be used outside libav*, use * avformat_alloc_context() to create an AVFormatContext. * * Fields can be accessed through AVOptions (av_opt*), * the name string used matches the associated command line parameter name and * can be found in libavformat/options_table.h. * The AVOption/command line parameter names differ in some cases from the C * structure field names for historic reasons or brevity. */ 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(). */ struct AVInputFormat *iformat; /** * The output container format. * * Muxing only, must be set by the caller before avformat_write_header(). */ 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 filename * * - demuxing: set by avformat_open_input() * - muxing: may be set by the caller before avformat_write_header() */ char filename[1024]; /** * 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_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) #if FF_API_LAVF_KEEPSIDE_FLAG #define AVFMT_FLAG_KEEP_SIDE_DATA 0x40000 ///< Don't merge side data but keep it separate. Deprecated, will be the default. #endif #define AVFMT_FLAG_FAST_SEEK 0x80000 ///< Enable fast, but inaccurate seeks for some formats #define AVFMT_FLAG_SHORTEST 0x100000 ///< Stop muxing when the shortest stream stops. #define AVFMT_FLAG_AUTO_BSF 0x200000 ///< Wait for packet data before writing a header, and add bitstream filters as requested by the muxer /** * Maximum size of the data read from input 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 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 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; /** * Avoid negative timestamps during muxing. * 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 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; /** * number of bytes to read maximally to identify format. * - encoding: unused * - decoding: set by user */ 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; /** * An opaque field for libavformat internal usage. * Must not be accessed in any way by callers. */ AVFormatInternal *internal; /** * IO repositioned flag. * This is set by avformat when the underlaying IO context read pointer * is repositioned, for example when doing byte based seeking. * Demuxers can use the flag to detect such changes. */ int io_repositioned; /** * Forced video codec. * This allows forcing a specific decoder, even when there are multiple with * the same codec_id. * Demuxing: Set by user */ 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 */ 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 */ 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 */ 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 " " 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; #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. * - 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); /** * 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 */ char *protocol_blacklist; /** * The maximum number of streams. * - encoding: unused * - decoding: set by user */ int max_streams; } AVFormatContext;