使用说明

    当前为了避免在调用init_input函数的时候，读取缓存区的数据，从而设置了该标志位,但是最终在avformat_open_input的其他地方还是读取了缓冲区的数据

        pAVInputFormat = av_find_input_format("h264");

        pAVInputFormat->flags |= AVFMT_NOFILE;宏定义/// Demuxer will use avio_open, no opened file should be provided by the caller.//解复用器将调用avio_open函数，调用者提供一个没有打开的文件，估计是打开的文件会被占用#define AVFMT_NOFILE        0x0001#define AVFMT_NEEDNUMBER    0x0002 /**< Needs '%d' in filename. */#define AVFMT_SHOW_IDS      0x0008 /**< Show format stream IDs numbers. */AVFMT_NOFILE formats will not have a AVIOContext当设置了AVFMT_NOFILE标志，将不会携带AVIOContext/* Open input file and probe the format if necessary. */static int init_input(AVFormatContext *s, const char *filename,                      AVDictionary **options){     int ret;    AVProbeData pd = { filename, NULL, 0 };    int score = AVPROBE_SCORE_RETRY;    //这里探测码流的方式，企图通过AVIOContext结构体中的read_packet函数    //如果码流格式已经指定并且指定了标志位，直接返回    if (s->pb) {         s->flags |= AVFMT_FLAG_CUSTOM_IO;    //如果没有指定输入格式，开始探测码流格式        if (!s->iformat)            return av_probe_input_buffer2(s->pb, &s->iformat, filename,                                         s, 0, s->format_probesize);        else if (s->iformat->flags & AVFMT_NOFILE)            av_log(s, AV_LOG_WARNING, "Custom AVIOContext makes no sense and "                                      "will be ignored with AVFMT_NOFILE format.\n");        return 0;    }   //这里探测码流的方式，企图通过通过进来的文件名称   //如果码流格式已经指定并且指定了标志位，直接返回   //这里非常明显网络RTSP流，肯定是不会走到这里    if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||        (!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))        return score;    //如果指定了iformat结构体，并且没有设置标志位，肯定执行下面的语句，该语句会调用read_packet函数    //进行分析码流    if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)        return ret;    if (s->iformat)        return 0;    return av_probe_input_buffer2(s->pb, &s->iformat, filename,                                 s, 0, s->format_probesize);}从下面的说明可以得知，当添加了AVFMT_NOFILE标志位，AVIOContext *pb会设置为空    /**     * I/O context.     *     * - demuxing: either set by the user before avformat_open_input() (then     *             the user must close it manually) or set by avformat_open_input().     * - muxing: set by the user before avformat_write_header(). The caller must     *           take care of closing / freeing the IO context.     *     * Do NOT set this field if AVFMT_NOFILE flag is set in     * iformat/oformat.flags. In such a case, the (de)muxer will handle     * I/O in some other way and this field will be NULL.     */    AVIOContext *pb;    /**     * Custom interrupt callbacks for the I/O layer.     *     * demuxing: set by the user before avformat_open_input().     * muxing: set by the user before avformat_write_header()     * (mainly useful for AVFMT_NOFILE formats). The callback     * should also be passed to avio_open2() if it's used to     * open the file.     */    AVIOInterruptCB interrupt_callback;/** * Guess the file format. * * @param pd        data to be probed * @param is_opened Whether the file is already opened; determines whether *                  demuxers with or without AVFMT_NOFILE are probed. */AVInputFormat *av_probe_input_format(AVProbeData *pd, int is_opened);AVInputFormat *av_probe_input_format2(AVProbeData *pd, int is_opened, int *score_max);AVInputFormat *av_probe_input_format3(AVProbeData *pd, int is_opened, int *score_ret);