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