1.描述

AVOutpufFormat與AVInputFormat類似，是類似COM 接口的數據結構，表示輸出文件容器格式，著重于功能函數，位于Avoformat.h文件中。
ffmpeg支持各種各樣的輸出文件格式，MP4，FLV，3GP等等。而 AVOutputFormat 結構體則保存了這些格式的信息和一些常規設置。

每一種封裝對應一個 AVOutputFormat 結構，ffmpeg將AVOutputFormat 按照鏈表存儲：

2.結構體定義

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.
     *****************************************************************
     */
    struct AVOutputFormat *next;
    /**
     * size of private data so that it can be allocated in the wrapper
     */
    int priv_data_size;

    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 *);
    /**
     * Currently only used to set pixel format if not YUV420P.
     */
    int (*interleave_packet)(struct AVFormatContext *, AVPacket *out,
                             AVPacket *in, int flush);
    /**
     * 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);
    /**
     * Initialize device capabilities submodule.
     * @see avdevice_capabilities_create() for more details.
     */
    int (*create_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);
    /**
     * Free device capabilities submodule.
     * @see avdevice_capabilities_free() for more details.
     */
    int (*free_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);
    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.
     *
     * 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.
     * Return 0 if more packets from this stream must be checked; 1 if not.
     */
    int (*check_bitstream)(struct AVFormatContext *, const AVPacket *pkt);
} AVOutputFormat;

3.常見變量及其作用

const char *name;
const char *long_name;//格式的描述性名稱，易于閱讀。
enum AVCodecID audio_codec; //默認的音頻編解碼器
enum AVCodecID video_codec; //默認的視頻編解碼器
enum AVCodecID subtitle_codec; //默認的字幕編解碼器
struct AVOutputFormat *next;
int (*write_header)(struct AVFormatContext *);
int (*write_packet)(struct AVFormatContext *, AVPacket *pkt);//寫一個數據包。 如果在標志中設置AVFMT_ALLOW_FLUSH，則pkt可以為NULL。
int (*write_trailer)(struct AVFormatContext *);
int (*interleave_packet)(struct AVFormatContext *, AVPacket *out, AVPacket *in, int flush);
int (*control_message)(struct AVFormatContext *s, int type, void *data, size_t data_size);//允許從應用程序向設備發送消息。
int (*write_uncoded_frame)(struct AVFormatContext *, int stream_index,   AVFrame **frame, unsigned flags);//寫一個未編碼的AVFrame。
int (*init)(struct AVFormatContext *);//初始化格式。 可以在此處分配數據，并設置在發送數據包之前需要設置的任何AVFormatContext或AVStream參數。
void (*deinit)(struct AVFormatContext *);//取消初始化格式。
int (*check_bitstream)(struct AVFormatContext *, const AVPacket *pkt);//設置任何必要的比特流過濾，并提取全局頭部所需的任何額外數據。