如果调用者不愿处理packet交织问题，应调用av_interleaved_write_frame()替代本函数。
2.5 av_interleaved_write_frame() /** * Write a packet to an output media file ensuring correct interleaving. * * This function will buffer the packets internally as needed to make sure the * packets in the output file are properly interleaved in the order of * increasing dts. Callers doing their own interleaving should call * av_write_frame() instead of this function. * * Using this function instead of av_write_frame() can give muxers advance * knowledge of future packets, improving e.g. the behaviour of the mp4 * muxer for VFR content in fragmenting mode. * * @param s media file handle * @param pkt The packet containing the data to be written. * <br> * If the packet is reference-counted, this function will take * ownership of this reference and unreference it later when it sees * fit. * The caller must not access the data through this reference after * this function returns. If the packet is not reference-counted, * libavformat will make a copy. * <br> * This parameter can be NULL (at any time, not just at the end), to * flush the interleaving queues. * <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 in one stream must be strictly * increasing (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 success, a negative AVERROR on error. Libavformat will always * take care of freeing the packet, even if this function fails. * * @see av_write_frame(), AVFormatContext.max_interleave_delta */ int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt);本函数用于复用过程，将packet写入输出媒体。
本函数将按需在内部缓存packet，从而确保输出媒体中不同流的packet能按照dts增长的顺序正确交织。
2.6 avio_open() /** * Create and initialize a AVIOContext for accessing the * resource indicated by url. * @note When the resource indicated by url has been opened in * read+write mode, the AVIOContext can be used only for writing. * * @param s Used to return the pointer to the created AVIOContext. * In case of failure the pointed to value is set to NULL. * @param url resource to access * @param flags flags which control how the resource indicated by url * is to be opened * @return >= 0 in case of success, a negative value corresponding to an * AVERROR code in case of failure */ int avio_open(AVIOContext **s, const char *url, int flags);创建并初始化一个AVIOContext，用于访问输出媒体文件。
2.7 avformat_write_header() /** * Allocate the stream private data and write the stream header to * an output media file. * * @param s Media file handle, must be allocated with avformat_alloc_context(). * Its oformat field must be set to the desired output format; * Its pb field must be set to an already opened AVIOContext. * @param options An AVDictionary filled with AVFormatContext and muxer-private options. * On return this parameter will be destroyed and replaced with a dict containing * options that were not found. May be NULL. * * @return AVSTREAM_INIT_IN_WRITE_HEADER on success if the codec had not already been fully initialized in avformat_init, * AVSTREAM_INIT_IN_INIT_OUTPUT on success if the codec had already been fully initialized in avformat_init, * negative AVERROR on failure. * * @see av_opt_find, av_dict_set, avio_open, av_oformat_next, avformat_init_output. */ av_warn_unused_result int avformat_write_header(AVFormatContext *s, AVDictionary **options);