SDL_UnlockSurface (3) - Linux Manuals
SDL_UnlockSurface: Unlocks a previously locked surface.
NAME
SDL_UnlockSurface - Unlocks a previously locked surface.SYNOPSIS
#include "SDL.h"
void SDL_UnlockSurface(SDL_Surface *surface);
DESCRIPTION
Surfaces that were previously locked using SDL_LockSurface must be unlocked with SDL_UnlockSurface. Surfaces should be unlocked as soon as possible.
It should be noted that since 1.1.8, surface locks are recursive. See SDL_LockSurface.
SEE ALSO
SDL_LockSurface