ffmpeg 基本数据结构和对象（二）： muxer/demuxer 和 encoder/decoder

一、

muxer（封装器） 和 demuxer（解封装器）存放在 libavformat 目录下的mux.c 文件中；

1、封装器 muxer 

muxer 提供的接口有：

int avformat_write_header(AVFormatContext *s, AVDictionary **options)

int av_write_frame(AVFormatContext *s, AVPacket *pkt)

int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt)

static int write_packet(AVFormatContext *s, AVPacket *pkt)
int av_write_trailer(AVFormatContext *s)







从API也可以看出封装器的工作主要就是写数据文件的头和尾以及数据帧
2、封装器实现对应的数据结构是： AVIputFormat
typedef struct AVInputFormat {
     /**
      * A comma separated list of short names for the format. New names
      * may be appended with a minor bump.
      */
     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;
 
     int flags;
 
     /**
      * If extensions are defined, then no probe is done. You should
      * usually not use extension format guessing because it is not
      * reliable enough
      */
     const char *extensions;
 
     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 AVInputFormat *next;
 
     /**
      * Raw demuxers store their codec ID here.
      */
     int raw_codec_id;
 
     /**
      * Size of private data so that it can be allocated in the wrapper.
      */
     int priv_data_size;
 
     /**
      * Tell if a given file has a chance of being parsed as this format.
      * The buffer provided is guaranteed to be AVPROBE_PADDING_SIZE bytes
      * big so you do not have to check for that unless you need more.
      */
     int (*read_probe)(AVProbeData *);
 
     /**
      * Read the format header and initialize the AVFormatContext
      * structure. Return 0 if OK. Only used in raw format right
      * now. 'avformat_new_stream' should be called to create new streams.
      */
     int (*read_header)(struct AVFormatContext *);
 
     /**
      * Read one packet and put it in 'pkt'. pts and flags are also
      * set. 'avformat_new_stream' can be called only if the flag
      * AVFMTCTX_NOHEADER is used and only in the calling thread (not in a
      * background thread).
      * @return 0 on success, < 0 on error.
      *         When returning an error, pkt must not have been allocated
      *         or must be freed before returning
      */
     int (*read_packet)(struct AVFormatContext *, AVPacket *pkt);
 
     /**
      * Close the stream. The AVFormatContext and AVStreams are not
      * freed by this function
      */
     int (*read_close)(struct AVFormatContext *);
 
     /**
      * Seek to a given timestamp relative to the frames in
      * stream component stream_index.
      * @param stream_index Must not be -1.
      * @param flags Selects which direction should be preferred if no exact
      *              match is available.
      * @return >= 0 on success (but not necessarily the new offset)
      */
     int (*read_seek)(struct AVFormatContext *,
                      int stream_index, int64_t timestamp, int flags);
 
     /**
      * Get the next timestamp in stream[stream_index].time_base units.
      * @return the timestamp or AV_NOPTS_VALUE if an error occurred
      */
     int64_t (*read_timestamp)(struct AVFormatContext *s, int stream_index,
                               int64_t *pos, int64_t pos_limit);
 
     /**
      * Start/resume playing - only meaningful if using a network-based format
      * (RTSP).
      */
     int (*read_play)(struct AVFormatContext *);
 
     /**
      * Pause playing - only meaningful if using a network-based format
      * (RTSP).
      */
     int (*read_pause)(struct AVFormatContext *);
 
     /**
      * 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.
      */
     int (*read_seek2)(struct AVFormatContext *s, int stream_index, int64_t min_ts, int64_t ts, int64_t max_ts, int flags);
 } AVInputFormat;




不同格式的视频文件会对应不同的封装器，这就需要有封装器的多个实现。而在ffmpeg中，所有的封装器都继承自AVInputFormat结构，并实现AVInputFormat中定义的函数指针（虚函数）：具体整理如下
int (*read_probe)(AVProbeData *);
 int (*read_header)(struct AVFormatContext *, AVFormatParameters *ap);
 int (*read_packet)(struct AVFormatContext *, AVPacket *pkt);
 int (*read_close)(struct AVFormatContext *);
 int (*read_seek)(struct AVFormatContext *, int stream_index, int64_t timestamp, int flags);





二、解封装器 demuxer
解封装器存在于文件 libavformat/dxa.c 文件中
在这里面主要定义了如下接口：