how to get view volume vertex?

i am beginer to opengl. but i know opengl multiply various matrix with vertex to show it. my problem is how i can get vertex value of view volume(i mean object after clipping and projecting).
thanks in advance.

i think gluProject is what you need.

thanks a lot.
but i think it will give only one point at a time.
my problem is i have a object made up of lots of polygon. when i clip it with additional clipping plane, opengl display it very well. but i want the vertex value of whole object that opengl is displaying. suppose i draw a sphere and i clip half with verticle plane. now i want vertex of clipped sphere. thanks again.