How to set Nikon camera to the UIUnlock state like Canon,i want get picture by press physical button.

[faxiangui]

Hi,
I want take photo by press physical button when previewing with usb cable(I do not want to control by gp_camera_trigger_capture) .But now there are no effect when i press the button ,because the screen show this message:（PC SD ....................）. I can do this with Canon camera by using EDSDK(kEdsCamerastatusCommand_UIUnLock).How can i do the same thing with libgphoto2?I use C++.