FFmpeg的AVInputFormat

通过上面的分析，基本可以看到ffmpeg的套路了，首先一个context上下文，上下文里面一个priv_data 指针，然后再插件结构体中有一个priv_data_size，然后回调函数。

结构体定义

首先肯定是AVFormatContext，这个就是上下文，这个结构体太庞大了。特别要注意里面的AVIOContext，这两个回调可以从自己的内存中读取数据。另外还有一个AVStream的结构体，专门用来存储流。

/**
 * 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;

    /**
     * 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;

    /**
     * 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;

下面的结构体就是需要插件实现的回调函数。

/**
 * @addtogroup lavf_encoding
 * @{
 */
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_ALLOW_FLUSH,
     * 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

    /*****************************************************************
     * 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.
     *****************************************************************
     */
    /**
     * 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;

    int (*write_header)(struct AVFormatContext *);
    /**
     * Write a packet. If AVFMT_ALLOW_FLUSH is set in flags,
     * pkt can be NULL in order to flush data buffered in the muxer.
     * When flushing, return 0 if there still is more data to flush,
     * or 1 if everything was flushed and there is no more buffered
     * data.
     */
    int (*write_packet)(struct AVFormatContext *, AVPacket *pkt);
    int (*write_trailer)(struct AVFormatContext *);
    /**
     * A format-specific function for interleavement.
     * If unset, packets will be interleaved by dts.
     *
     * @param s           An AVFormatContext for output. pkt will be added to
     *                    resp. taken from its packet buffer.
     * @param[in,out] pkt A packet to be interleaved if has_packet is set;
     *                    also used to return packets. If no packet is returned
     *                    (e.g. on error), pkt is blank on return.
     * @param flush       1 if no further packets are available as input and
     *                    all remaining packets should be output.
     * @param has_packet  If set, pkt contains a packet to be interleaved
     *                    on input; otherwise pkt is blank on input.
     * @return 1 if a packet was output, 0 if no packet could be output,
     *         < 0 if an error occurred
     */
    int (*interleave_packet)(struct AVFormatContext *s, AVPacket *pkt,
                             int flush, int has_packet);
    /**
     * Test if the given codec can be stored in this container.
     *
     * @return 1 if the codec is supported, 0 if it is not.
     *         A negative number if unknown.
     *         MKTAG('A', 'P', 'I', 'C') if the codec is only supported as AV_DISPOSITION_ATTACHED_PIC
     */
    int (*query_codec)(enum AVCodecID id, int std_compliance);

    void (*get_output_timestamp)(struct AVFormatContext *s, int stream,
                                 int64_t *dts, int64_t *wall);
    /**
     * Allows sending messages from application to device.
     */
    int (*control_message)(struct AVFormatContext *s, int type,
                           void *data, size_t data_size);

    /**
     * Write an uncoded AVFrame.
     *
     * See av_write_uncoded_frame() for details.
     *
     * The library will free *frame afterwards, but the muxer can prevent it
     * by setting the pointer to NULL.
     */
    int (*write_uncoded_frame)(struct AVFormatContext *, int stream_index,
                               AVFrame **frame, unsigned 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);
    enum AVCodecID data_codec; /**< default data codec */
    /**
     * Initialize format. May allocate data here, and set any AVFormatContext or
     * AVStream parameters that need to be set before packets are sent.
     * This method must not write output.
     *
     * Return 0 if streams were fully configured, 1 if not, negative AVERROR on failure
     *
     * Any allocations made here must be freed in deinit().
     */
    int (*init)(struct AVFormatContext *);
    /**
     * Deinitialize format. If present, this is called whenever the muxer is being
     * destroyed, regardless of whether or not the header has been written.
     *
     * If a trailer is being written, this is called after write_trailer().
     *
     * This is called if init() fails as well.
     */
    void (*deinit)(struct AVFormatContext *);
    /**
     * Set up any necessary bitstream filtering and extract any extra data needed
     * for the global header.
     *
     * @note pkt might have been directly forwarded by a meta-muxer; therefore
     *       pkt->stream_index as well as the pkt's timebase might be invalid.
     * Return 0 if more packets from this stream must be checked; 1 if not.
     */
    int (*check_bitstream)(struct AVFormatContext *s, struct AVStream *st,
                           const AVPacket *pkt);
} AVOutputFormat;

操作函数

/**
 * Allocate an AVFormatContext.
 * avformat_free_context() can be used to free the context and everything
 * allocated by the framework within it.
 */
AVFormatContext *avformat_alloc_context(void);

/**
 * Free an AVFormatContext and all its streams.
 * @param s context to free
 */
void avformat_free_context(AVFormatContext *s);

/**
 * Add a new stream to a media file.
 *
 * When demuxing, it is called by the demuxer in read_header(). If the
 * flag AVFMTCTX_NOHEADER is set in s.ctx_flags, then it may also
 * be called in read_packet().
 *
 * When muxing, should be called by the user before avformat_write_header().
 *
 * User is required to call avformat_free_context() to clean up the allocation
 * by avformat_new_stream().
 *
 * @param s media file handle
 * @param c unused, does nothing
 *
 * @return newly created stream or NULL on error.
 */
AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c);


/**
 * Wrap an existing array as stream side data.
 *
 * @param st stream
 * @param type side information type
 * @param data the side data array. It must be allocated with the av_malloc()
 *             family of functions. The ownership of the data is transferred to
 *             st.
 * @param size side information size
 * @return zero on success, a negative AVERROR code on failure. On failure,
 *         the stream is unchanged and the data remains owned by the caller.
 */
int av_stream_add_side_data(AVStream *st, enum AVPacketSideDataType type,
                            uint8_t *data, size_t size);

/**
 * Allocate new information from stream.
 *
 * @param stream stream
 * @param type desired side information type
 * @param size side information size
 * @return pointer to fresh allocated data or NULL otherwise
 */
uint8_t *av_stream_new_side_data(AVStream *stream,
                                 enum AVPacketSideDataType type, size_t size);
/**
 * Get side information from stream.
 *
 * @param stream stream
 * @param type desired side information type
 * @param size If supplied, *size will be set to the size of the side data
 *             or to zero if the desired side data is not present.
 * @return pointer to data if present or NULL otherwise
 */
uint8_t *av_stream_get_side_data(const AVStream *stream,
                                 enum AVPacketSideDataType type, size_t *size);


/**
 * Allocate an AVFormatContext for an output format.
 * avformat_free_context() can be used to free the context and
 * everything allocated by the framework within it.
 *
 * @param *ctx is set to the created format context, or to NULL in
 * case of failure
 * @param oformat format to use for allocating the context, if NULL
 * format_name and filename are used instead
 * @param format_name the name of output format to use for allocating the
 * context, if NULL filename is used instead
 * @param filename the name of the filename to use for allocating the
 * context, may be NULL
 * @return >= 0 in case of success, a negative AVERROR code in case of
 * failure
 */
int avformat_alloc_output_context2(AVFormatContext **ctx, const AVOutputFormat *oformat,
                                   const char *format_name, const char *filename);

/**
 * @addtogroup lavf_decoding
 * @{
 */

/**
 * Find AVInputFormat based on the short name of the input format.
 */
const AVInputFormat *av_find_input_format(const char *short_name);

/**
 * Guess the file format.
 *
 * @param pd        data to be probed
 * @param is_opened Whether the file is already opened; determines whether
 *                  demuxers with or without AVFMT_NOFILE are probed.
 */
const AVInputFormat *av_probe_input_format(const AVProbeData *pd, int is_opened);

/**
 * Guess the file format.
 *
 * @param pd        data to be probed
 * @param is_opened Whether the file is already opened; determines whether
 *                  demuxers with or without AVFMT_NOFILE are probed.
 * @param score_max A probe score larger that this is required to accept a
 *                  detection, the variable is set to the actual detection
 *                  score afterwards.
 *                  If the score is <= AVPROBE_SCORE_MAX / 4 it is recommended
 *                  to retry with a larger probe buffer.
 */
const AVInputFormat *av_probe_input_format2(const AVProbeData *pd,
                                            int is_opened, int *score_max);

/**
 * Guess the file format.
 *
 * @param is_opened Whether the file is already opened; determines whether
 *                  demuxers with or without AVFMT_NOFILE are probed.
 * @param score_ret The score of the best detection.
 */
const AVInputFormat *av_probe_input_format3(const AVProbeData *pd,
                                            int is_opened, int *score_ret);

/**
 * Probe a bytestream to determine the input format. Each time a probe returns
 * with a score that is too low, the probe buffer size is increased and another
 * attempt is made. When the maximum probe size is reached, the input format
 * with the highest score is returned.
 *
 * @param pb the bytestream to probe
 * @param fmt the input format is put here
 * @param url the url of the stream
 * @param logctx the log context
 * @param offset the offset within the bytestream to probe from
 * @param max_probe_size the maximum probe buffer size (zero for default)
 * @return the score in case of success, a negative value corresponding to an
 *         the maximal score is AVPROBE_SCORE_MAX
 * AVERROR code otherwise
 */
int av_probe_input_buffer2(AVIOContext *pb, const AVInputFormat **fmt,
                           const char *url, void *logctx,
                           unsigned int offset, unsigned int max_probe_size);

/**
 * Like av_probe_input_buffer2() but returns 0 on success
 */
int av_probe_input_buffer(AVIOContext *pb, const AVInputFormat **fmt,
                          const char *url, void *logctx,
                          unsigned int offset, unsigned int max_probe_size);

/**
 * Open an input stream and read the header. The codecs are not opened.
 * The stream must be closed with avformat_close_input().
 *
 * @param ps Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context).
 *           May be a pointer to NULL, in which case an AVFormatContext is allocated by this
 *           function and written into ps.
 *           Note that a user-supplied AVFormatContext will be freed on failure.
 * @param url URL of the stream to open.
 * @param fmt If non-NULL, this parameter forces a specific input format.
 *            Otherwise the format is autodetected.
 * @param options  A dictionary filled with AVFormatContext and demuxer-private options.
 *                 On return this parameter will be destroyed and replaced with a dict containing
 *                 options that were not found. May be NULL.
 *
 * @return 0 on success, a negative AVERROR on failure.
 *
 * @note If you want to use custom IO, preallocate the format context and set its pb field.
 */
int avformat_open_input(AVFormatContext **ps, const char *url,
                        const AVInputFormat *fmt, AVDictionary **options);

/**
 * Read packets of a media file to get stream information. This
 * is useful for file formats with no headers such as MPEG. This
 * function also computes the real framerate in case of MPEG-2 repeat
 * frame mode.
 * The logical file position is not changed by this function;
 * examined packets may be buffered for later processing.
 *
 * @param ic media file handle
 * @param options  If non-NULL, an ic.nb_streams long array of pointers to
 *                 dictionaries, where i-th member contains options for
 *                 codec corresponding to i-th stream.
 *                 On return each dictionary will be filled with options that were not found.
 * @return >=0 if OK, AVERROR_xxx on error
 *
 * @note this function isn't guaranteed to open all the codecs, so
 *       options being non-empty at return is a perfectly normal behavior.
 *
 * @todo Let the user decide somehow what information is needed so that
 *       we do not waste time getting stuff the user does not need.
 */
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);

/**
 * Find the programs which belong to a given stream.
 *
 * @param ic    media file handle
 * @param last  the last found program, the search will start after this
 *              program, or from the beginning if it is NULL
 * @param s     stream index
 * @return the next program which belongs to s, NULL if no program is found or
 *         the last program is not among the programs of ic.
 */
AVProgram *av_find_program_from_stream(AVFormatContext *ic, AVProgram *last, int s);

void av_program_add_stream_index(AVFormatContext *ac, int progid, unsigned int idx);

/**
 * Find the "best" stream in the file.
 * The best stream is determined according to various heuristics as the most
 * likely to be what the user expects.
 * If the decoder parameter is non-NULL, av_find_best_stream will find the
 * default decoder for the stream's codec; streams for which no decoder can
 * be found are ignored.
 *
 * @param ic                media file handle
 * @param type              stream type: video, audio, subtitles, etc.
 * @param wanted_stream_nb  user-requested stream number,
 *                          or -1 for automatic selection
 * @param related_stream    try to find a stream related (eg. in the same
 *                          program) to this one, or -1 if none
 * @param decoder_ret       if non-NULL, returns the decoder for the
 *                          selected stream
 * @param flags             flags; none are currently defined
 * @return  the non-negative stream number in case of success,
 *          AVERROR_STREAM_NOT_FOUND if no stream with the requested type
 *          could be found,
 *          AVERROR_DECODER_NOT_FOUND if streams were found but no decoder
 * @note  If av_find_best_stream returns successfully and decoder_ret is not
 *        NULL, then *decoder_ret is guaranteed to be set to a valid AVCodec.
 */
int av_find_best_stream(AVFormatContext *ic,
                        enum AVMediaType type,
                        int wanted_stream_nb,
                        int related_stream,
                        const AVCodec **decoder_ret,
                        int flags);

其实想要说明的一些函数是下面的，这些函数可以用来控制输入流的播放，暂停，跳转，等，特别是在rtsp等流媒体服务器上，非常有用。
就这些吧，等到后面分析rtsp和rtmp的客户端的时候，我们再来详细的讲一下这些函数的调用流程，今天就先列出来吧。

/**
 * Return the next frame of a stream.
 * This function returns what is stored in the file, and does not validate
 * that what is there are valid frames for the decoder. It will split what is
 * stored in the file into frames and return one for each call. It will not
 * omit invalid data between valid frames so as to give the decoder the maximum
 * information possible for decoding.
 *
 * On success, the returned packet is reference-counted (pkt->buf is set) and
 * valid indefinitely. The packet must be freed with av_packet_unref() when
 * it is no longer needed. For video, the packet contains exactly one frame.
 * For audio, it contains an integer number of frames if each frame has
 * a known fixed size (e.g. PCM or ADPCM data). If the audio frames have
 * a variable size (e.g. MPEG audio), then it contains one frame.
 *
 * pkt->pts, pkt->dts and pkt->duration are always set to correct
 * values in AVStream.time_base units (and guessed if the format cannot
 * provide them). pkt->pts can be AV_NOPTS_VALUE if the video format
 * has B-frames, so it is better to rely on pkt->dts if you do not
 * decompress the payload.
 *
 * @return 0 if OK, < 0 on error or end of file. On error, pkt will be blank
 *         (as if it came from av_packet_alloc()).
 *
 * @note pkt will be initialized, so it may be uninitialized, but it must not
 *       contain data that needs to be freed.
 */
int av_read_frame(AVFormatContext *s, AVPacket *pkt);

/**
 * Seek to the keyframe at timestamp.
 * 'timestamp' in 'stream_index'.
 *
 * @param s media file handle
 * @param stream_index If stream_index is (-1), a default
 * stream is selected, and timestamp is automatically converted
 * from AV_TIME_BASE units to the stream specific time_base.
 * @param timestamp Timestamp in AVStream.time_base units
 *        or, if no stream is specified, in AV_TIME_BASE units.
 * @param flags flags which select direction and seeking mode
 * @return >= 0 on success
 */
int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp,
                  int flags);

/**
 * 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.
 *
 * If flags contain AVSEEK_FLAG_BYTE, then all timestamps are in bytes and
 * are the file position (this may not be supported by all demuxers).
 * If flags contain AVSEEK_FLAG_FRAME, then all timestamps are in frames
 * in the stream with stream_index (this may not be supported by all demuxers).
 * Otherwise all timestamps are in units of the stream selected by stream_index
 * or if stream_index is -1, in AV_TIME_BASE units.
 * If flags contain AVSEEK_FLAG_ANY, then non-keyframes are treated as
 * keyframes (this may not be supported by all demuxers).
 * If flags contain AVSEEK_FLAG_BACKWARD, it is ignored.
 *
 * @param s media file handle
 * @param stream_index index of the stream which is used as time base reference
 * @param min_ts smallest acceptable timestamp
 * @param ts target timestamp
 * @param max_ts largest acceptable timestamp
 * @param flags flags
 * @return >=0 on success, error code otherwise
 *
 * @note This is part of the new seek API which is still under construction.
 */
int avformat_seek_file(AVFormatContext *s, int stream_index, int64_t min_ts, int64_t ts, int64_t max_ts, int flags);

/**
 * Discard all internally buffered data. This can be useful when dealing with
 * discontinuities in the byte stream. Generally works only with formats that
 * can resync. This includes headerless formats like MPEG-TS/TS but should also
 * work with NUT, Ogg and in a limited way AVI for example.
 *
 * The set of streams, the detected duration, stream parameters and codecs do
 * not change when calling this function. If you want a complete reset, it's
 * better to open a new AVFormatContext.
 *
 * This does not flush the AVIOContext (s->pb). If necessary, call
 * avio_flush(s->pb) before calling this function.
 *
 * @param s media file handle
 * @return >=0 on success, error code otherwise
 */
int avformat_flush(AVFormatContext *s);

/**
 * Start playing a network-based stream (e.g. RTSP stream) at the
 * current position.
 */
int av_read_play(AVFormatContext *s);

/**
 * Pause a network-based stream (e.g. RTSP stream).
 *
 * Use av_read_play() to resume it.
 */
int av_read_pause(AVFormatContext *s);

/**
 * Close an opened input AVFormatContext. Free it and all its contents
 * and set *s to NULL.
 */
void avformat_close_input(AVFormatContext **s);
/**
 * @}
 */

支持的AVOutputFormat