Hi, I have written a USB filter which can be installed successfully as the lower filter of the standard USB audio driver. I need to send IOCTL to my filter driver to control my hardware so I create a symbolic link with a specific GUID in my filter driver. In my application, I got the device path (by calling SetupDi... functions) and call CreateFile() to open it. However, it fails with device not found error. If I installed my filter driver under a non-standard USB driver, then IOCTL works fine. Does anyone know if there's any restriction to use symbolic link in a USB audio filter driver? Thanks! Mike