SDL problemer!

Kode 

#include <stdio.h>
#include <stdlib.h>

#include <SDL/SDL.h>

void DrawPixel(SDL_Surface *screen, int x, int y,
                                    Uint8 R, Uint8 G, Uint8 B);
void Slock(SDL_Surface *screen);
void Sulock(SDL_Surface *screen);

void DrawPixel(SDL_Surface *screen, int x, int y, Uint8 R, Uint8 G, Uint8 B)                                    
{
  Uint32 color = SDL_MapRGB(screen->format, R, G, B);
  switch (screen->format->BytesPerPixel)
  {
    case 1: // Assuming 8-bpp
      {
        Uint8 *bufp;
        bufp = (Uint8 *)screen->pixels + y*screen->pitch + x;
        *bufp = color;
      }
      break;
    case 2: // Probably 15-bpp or 16-bpp
      {
        Uint16 *bufp;
        bufp = (Uint16 *)screen->pixels + y*screen->pitch/2 + x;
        *bufp = color;
      }
      break;
    case 3: // Slow 24-bpp mode, usually not used
      {
        Uint8 *bufp;
        bufp = (Uint8 *)screen->pixels + y*screen->pitch + x * 3;
        if(SDL_BYTEORDER == SDL_LIL_ENDIAN)
        {
          bufp[0] = color;
          bufp[1] = color >> 8;
          bufp[2] = color >> 16;
        } else {
          bufp[2] = color;
          bufp[1] = color >> 8;
          bufp[0] = color >> 16;
        }
      }
      break;
    case 4: // Probably 32-bpp
      {
        Uint32 *bufp;
        bufp = (Uint32 *)screen->pixels + y*screen->pitch/4 + x;
        *bufp = color;
      }
      break;
  }
}


void Slock(SDL_Surface *screen)
{
  if ( SDL_MUSTLOCK(screen) )
  {
    if ( SDL_LockSurface(screen) < 0 )
    {
      return;
    }
  }
}

void Sulock(SDL_Surface *screen)
{
  if ( SDL_MUSTLOCK(screen) )
  {
    SDL_UnlockSurface(screen);
  }
}

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

  if ( SDL_Init(SDL_INIT_AUDIO|SDL_INIT_VIDEO) < 0 )
  {
    printf("Unable to init SDL: %s\\n", SDL_GetError());
    exit(1);
  }
  atexit(SDL_Quit);

  SDL_Surface *screen;
  screen=SDL_SetVideoMode(640,480,32,SDL_HWSURFACE|SDL_DOUBLEBUF);
  if ( screen == NULL )
  {
    printf("Unable to set 640x480 video: %s\\n", SDL_GetError());
    exit(1);
  }

  // DRAWING GOES HERE

  return 0;
}