gluNurbsCallbackData - set a user data pointer.
void gluNurbsCallbackData (GLUnurbs *nurb, GLvoid *userData);
gluNurbsCallbackData is used to pass a pointer to the application's data to NURBS tessellator. A copy of this pointer will be passed by the tessellator in the NURBS callback functions (set by gluNurbsCallback).
gluNurbsCallbackData is available only if the GLU version is 1.3 or greater.