Help on OpenGL selection ???

I have a simple OpenGL application to display a lot of 3D points in 3D view. Here I want to draw 2D circle in the view to select some 3D points, which are included in this 2D circle. It seems that “gluPickMatrix” can realize the rectangle selection by setting its parameters.

Does anyone have some idea about how to implement the 2D circle selection in OpenGL ??? Thanks in advance!!!