[SDL] Question about Pointer to Display Surface

[Adrian Hey]

Hello,

Some functions in the SDL api only seem to make sense for the display
surface, but to take a pointer to a surface as argument (usually
called "screen"). I'm thinking of functions like SDL_UpdateRect(s) and
SDL_Flip,SDL_WM_ToggleFullScreen 

Am I right in thinking that..

 There is only one display surface.

 The SDL_GetVideoSurface returns a pointer to that surface.

 This pointer is always the same as that returned by
 the last SDL_SetVideoMode.

 This pointer (and only this pointer) is what should be
 supplied as argument to SDL_UpdateRect(s), SDL_Flip, etc..

The only reason I have doubts about this is that if the above
statements are true then SDL should already know what the current
display surface is, so the screen argument seems superfluous. But
I guess it's there for a reason, so one or more of the above
statements might be false :-(

Also, is the pointer always the same (so I can reuse it again and
again), or might it change (in which case I guess I have to call
SDL_GetVideoSurface each time I need it)?

Thanks
--
Adrian Hey