SDL_Flip man page
SDL_Flip — Swaps screen buffers
int SDL_Flip(SDL_Surface *screen);
On hardware that supports double-buffering, this function sets up a flip and returns. The hardware will wait for vertical retrace, and then swap video buffers before the next video surface blit or lock will return. On hardware that doesn't support double-buffering, this is equivalent to calling SDL_UpdateRect(screen, 0, 0, 0, 0)
The SDL_DOUBLEBUF flag must have been passed to SDL_SetVideoMode, when setting the video mode for this function to perform hardware flipping.
This function returns 0 if successful, or -1 if there was an error.
SDL_SetVideoMode, SDL_UpdateRect, SDL_Surface