SDL_GetKeyState man page
SDL_GetKeyState — Get a snapshot of the current keyboard state
Uint8 *SDL_GetKeyState(int *numkeys);
Gets a snapshot of the current keyboard state. The current state is return as a pointer to an array, the size of this array is stored in numkeys. The array is indexed by the SDLK_* symbols. A value of 1 means the key is pressed and a value of 0 means its not. The pointer returned is a pointer to an internal SDL array and should not be freed by the caller.
Use SDL_PumpEvents to update the state array.
Uint8 *keystate = SDL_GetKeyState(NULL); if ( keystate[SDLK_RETURN] ) printf("Return Key Pressed. ");
SDL Key Symbols, SDL_PumpEvents