FFMPEG结构体分析之AVFormatContext

它是FFMPEG解封装（flv，mp4，rmvb，avi）功能的结构体。

接口定义文件路径ffmpeg\libavformat\avformat.h, 看完该结构体的定义，就一句感慨，尼玛好长


/*** 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().*/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)
#if FF_API_LAVF_PRIV_OPT
#define AVFMT_FLAG_PRIV_OPT    0x20000 ///< Enable use of private options by delaying codec open (deprecated, does nothing)
#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 ///< 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 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;/*** 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;/*** 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*/const 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 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 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 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);/*** 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;/*** 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;
} AVFormatContext;

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