windows下ffmpeg一直打不开摄像头

windows下ffmpeg一直打不开摄像头：

av_register_all();
avcodec_register_all();
avdevice_register_all();
avformat_network_init();

show_dshow_device();
show_dshow_device_option();
show_vfw_device();

pFormatCtx = avformat_alloc_context();
pInputFormat = av_find_input_format("dshow");

AVDictionary *format_opts = nullptr;
av_dict_set_int(&format_opts, "video_size", 2073600, 0);

ret = avformat_open_input(&pFormatCtx, "video=GENERAL_WEBCAM", pInputFormat, &format_opts);
if (ret != 0) {
    return;
}