void demux( const char* filepath )
{
    // AVFormatContext是描述一个媒体文件或媒体流的构成和基本信息的结构体
    AVFormatContext* fmt_context = NULL;  
    int video_index = -1; // 存储视频索引
    int audio_index = -1; // 存储音频索引
    // 打开文件，主要是探测协议类型，如果是网络文件则创建网络链接，使用open必须使用close来关闭
    int ret = avformat_open_input(&fmt_context, filepath, NULL, NULL); 
    if (ret < 0) { char buf[1024] = { 0 }; av_strerror(ret, buf, sizeof(buf)); printf("open %s failed: %s\n", filepath, buf); return; }
    ret = avformat_find_stream_info(fmt_context, NULL);
    if (ret < 0) { char buf[1024] = { 0 }; av_strerror(ret, buf, sizeof(buf)); printf("avformat_find_stream_info %s failed: %s\n", filepath, buf); return; }
    // dump媒体文件信息
    av_dump_format(fmt_context, 0, filepath, 0);
    // 打印context已经得到的信息
    {
        printf("media->url: %s\n", fmt_context->url);
        printf("media->bit_rate: %lld\n", fmt_context->bit_rate);
        printf("media->nb_streams: %d\n", fmt_context->nb_streams);
        // 时间计算 总秒数 * AV_TIME_BASE = duration
        int total_seconds = (fmt_context->duration) / AV_TIME_BASE;
        int hours = total_seconds / 3600; int minutes = total_seconds % 3600 / 60; int seconds = total_seconds % 3600 % 60;
        printf("media->duration: %d-%d-%d\n", hours, minutes, seconds); printf("\n");
    }
    // 使用遍历来找出媒体流
    for (auto i = 0; i < fmt_context->nb_streams; i++)
    {
        AVStream* is = fmt_context->streams[i]; // 音频流、视频流、字幕流
        if (AVMEDIA_TYPE_AUDIO == is->codecpar->codec_type) // 音频流
        {
            audio_index = i;
            printf("stream index:%d\n", is->index); // 流唯一索引
            printf("audio sample_rate:%d Hz\n", is->codecpar->sample_rate); // 音频采样率
            switch (is->codecpar->format){ // 音频采样格式
                case AV_SAMPLE_FMT_FLTP: printf("sampleformat:AV_SAMPLE_FMT_FLTP\n"); break;
                case AV_SAMPLE_FMT_S16P: printf("sampleformat:AV_SAMPLE_FMT_S16P\n"); break;
                default:break;}
            printf("audio channels: %d\n", is->codecpar->channels); // 音频信道数
            switch (is->codecpar->codec_id){ // 音频编码格式
                case AV_CODEC_ID_AAC:printf("audio codec:AAC\n"); break;
                case AV_CODEC_ID_MP3:printf("audio codec:MP3\n"); break;
                default:printf("audio codec_id:%d\n", is->codecpar->codec_id);break;}
            if (is->duration != AV_NOPTS_VALUE) // flv格式这里读不出时长
            {
                int duration_audio_seconds = is->duration * av_q2d(is->time_base); // 音频时长
                printf("audio duration: %02d:%02d:%02d\n", duration_audio_seconds / 3600, (duration_audio_seconds % 3600) / 60, (duration_audio_seconds % 60));
            }
            else { printf("duration unknown\n"); };
        }
        else if (AVMEDIA_TYPE_VIDEO == is->codecpar->codec_type) // 视频流
        {
            video_index = i;
            printf("stream index:%d\n", is->index); // 流唯一索引
            printf("video fps: %llf / s\n", av_q2d(is->avg_frame_rate)); // 视频帧率
            switch (is->codecpar->codec_id) { // 视频编码格式
                case AV_CODEC_ID_MPEG4: printf("video codec:MPEG4\n"); break;
                case AV_CODEC_ID_H264: printf("video codec:H264\n"); break;
                default:printf("video codec_id:%d\n", is->codecpar->codec_id);break;}
            printf("video width:%d px height:%d px \n", is->codecpar->width, is->codecpar->height); // 视频帧宽度和帧高度
            if (is->duration != AV_NOPTS_VALUE) // flv格式这里读不出时长
            {
                int duration_video_seconds = is->duration * av_q2d(is->time_base); // 视频时长
                printf("video duration: %02d:%02d:%02d\n", duration_video_seconds / 3600, (duration_video_seconds % 3600) / 60, (duration_video_seconds % 60));
            }
            else { printf("duration unknown\n"); };
         }
    }
    printf("\n");
    AVPacket* pkt = av_packet_alloc(); // 使用alloc必须进行free
    int frame_max = 10; // 最多读取十帧数据
    do
    {
        int ret = av_read_frame(fmt_context, pkt); // ！！实际得到的是packet
        if (ret < 0){printf("av_read_frame end\n");break;}
        if (pkt->stream_index == video_index) { // 视频包
            printf("A audio packet\n");
        }
        else if (pkt->stream_index == audio_index) // 音频包
        {
            printf("A video packet\n");
        }
        printf("pts:%lld\n", pkt->pts);
        printf("dts:%lld\n", pkt->dts);
        printf("size:%d\n", pkt->size);
        printf("pos:%lld\n", pkt->pos);
        printf("duration:%llf\n", pkt->duration * av_q2d(fmt_context->streams[pkt->stream_index]->time_base));
        // 每次循环结束释放掉上一帧的数据
        av_packet_unref(pkt); printf("\n");
        frame_max--;
    } while (frame_max > 0);

    // 使用alloc必须进行free
    if (pkt){av_packet_free(&pkt);}

    // 使用open必须使用close来关闭
    if (fmt_context){avformat_close_input(&fmt_context);}
}