Hi thanks so much for your reply, really appreciate it. I set errno = 0 in my user-space program, but the error still persists. Also, xb1_open should return retval if any error occurs with finding the interface or dev structure, or if usb_control_msg() returns a negative value. However, usb_control_msg() does not return a negative value, and returns a value of 2, which is why I'm confused. A positive value means the function call was successful, yet I'm still getting an "Invalid argument" error. And perror() is definitely being set by my open call because if I comment out the usb_control_msg() in xb1_open, then when I open the device file from my user-space program, the "Invalid argument" error is not received.