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[]; /**
* 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 "\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; #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;