How to Use "libvlc_vlm_add_broadcast" from dshow to file.

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I have a problem. I want Save a file which from a dshow device.
but my file always only small size ( under 1KB). Does anyone can help me? Thanks.

my code is:

char* arguments = {"-I", "dummy", "--ignore-config", "--no-video-title", plugin_arg };
libvlc_instance = libvlc_new(arguments.Length, arguments);
char *sout = "#transcode{vcodec=mp4v,acodec=mpga,vb=800,ab=128,deinterlace}:duplicate{dst=file{dst=D:\\VidetTest2.mp4}}";
char *sin = "dshow:// :dshow-vdev=AVerMedia USB Polaris Analog Capture :dshow-adev=none";
char* opt= { "--sout-keep", "--live-caching=100"};
libvlc_vlm_add_broadcast(libvlc_instance, "mytest", sin , sout , 2, opt, 1, 0);
libvlc_vlm_play_media(libvlc_instance, "mytest");