这个函数会打开输入媒体文件，读取文件头，将文件格式信息存储在第一个参数AVFormatContext中。
2.2 avformat_find_stream_info() /** * 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);这个函数会读取一段视频文件数据并尝试解码，将取到的流信息填入AVFormatContext.streams中。AVFormatContext.streams是一个指针数组，数组大小是AVFormatContext.nb_streams
2.3 av_read_frame() /** * 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. * * If pkt->buf is NULL, then the packet is valid until the next * av_read_frame() or until avformat_close_input(). Otherwise the packet * is valid indefinitely. In both cases 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 */ int av_read_frame(AVFormatContext *s, AVPacket *pkt);本函数用于解复用过程。
本函数将存储在输入文件中的数据分割为多个packet，每次调用将得到一个packet。packet可能是视频帧、音频帧或其他数据，解码器只会解码视频帧或音频帧，非音视频数据并不会被扔掉、从而能向解码器提供尽可能多的信息。
对于视频来说，一个packet只包含一个frame；对于音频来说，若是帧长固定的格式则一个packet可包含整数个frame，若是帧长可变的格式则一个packet只包含一个frame。
读取到的packet每次使用完之后应调用av_packet_unref(AVPacket *pkt)清空packet。否则会千万内存泄露。
2.4 av_write_frame() /** * Write a packet to an output media file. * * This function passes the packet directly to the muxer, without any buffering * or reordering. The caller is responsible for correctly interleaving the * packets if the format requires it. Callers that want libavformat to handle * the interleaving should call av_interleaved_write_frame() instead of this * function. * * @param s media file handle * @param pkt The packet containing the data to be written. Note that unlike * av_interleaved_write_frame(), this function does not take * ownership of the packet passed to it (though some muxers may make * an internal reference to the input packet). * <br> * This parameter can be NULL (at any time, not just at the end), in * order to immediately flush data buffered within the muxer, for * muxers that buffer up data internally before writing it to the * output. * <br> * Packet's @ref AVPacket.stream_index "stream_index" field must be * set to the index of the corresponding stream in @ref * AVFormatContext.streams "s->streams". * <br> * The timestamps (@ref AVPacket.pts "pts", @ref AVPacket.dts "dts") * must be set to correct values in the stream's timebase (unless the * output format is flagged with the AVFMT_NOTIMESTAMPS flag, then * they can be set to AV_NOPTS_VALUE). * The dts for subsequent packets passed to this function must be strictly * increasing when compared in their respective timebases (unless the * output format is flagged with the AVFMT_TS_NONSTRICT, then they * merely have to be nondecreasing). @ref AVPacket.duration * "duration") should also be set if known. * @return < 0 on error, = 0 if OK, 1 if flushed and there is no more data to flush * * @see av_interleaved_write_frame() */ int av_write_frame(AVFormatContext *s, AVPacket *pkt);本函数用于复用过程，将packet写入输出媒体。
packet交织是指：不同流的packet在输出媒体文件中应严格按照packet中dts递增的顺序交错存放。
本函数直接将packet写入复用器(muxer)，不会缓存或记录任何packet。本函数不负责不同流的packet交织问题。由调用者负责。