I will draw the circle.I know how can ý do that but there is a problem .I don’t know painting the inside of the circle.I want to see that the inside of the circle color is different from the window color.

If you don’t know the fill-area algorithm, just try glBegin(GL_POLYGON)

I dont’t know your exact requirement.

If you are trying to have a painted circle,
you can go for gluDisk which would draw you a (disk)/circle with filled color.