Gfx.c

#include "SDL.h"
#include <stdbool.h>
#include <unistd.h>

SDL_Surface *screen;
SDL_Event event;

void Do(void) {
    screen = SDL_SetVideoMode(640, 480, 16, SDL_SWSURFACE);

    SDL_WM_SetCaption("Simple Window", "Simple Window");

    bool done=false;

    while(!done) {
        while(SDL_PollEvent(&event)) {
            if (event.type == SDL_QUIT) {
                done=true;
            }
        }

        // fill the screen with black color
        SDL_FillRect(screen, &screen->clip_rect, SDL_MapRGB(screen->format, 0, 0, 0));

        // update the screen buffer
        SDL_Flip(screen);
        //usleep(1000 * 1000);
    }
}

Gfx.h

void Do(void);

rect2.c

// gcc -framework Foundation -o rect2 rect2.c Gfx.c `sdl-config --cflags` `sdl-config --libs` && ./rect2

#include "SDL.h"
#include "Gfx.h"

int main(int argc, char *argv[]) {
    SDL_Init(SDL_INIT_VIDEO);

    Do();

    SDL_Quit();

    return 0;
}