Why Error?

// "Graphics with SDL" turorial series Lesson 2
// by Marius Andra (http;//cone3d.gamedev.net/)
//
// NOTE: don't be alarmed if running this from
// Dev-C++ turns up a black screen. Compile it and then
// run it from the sources folder.

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

#include <SDL/SDL.h>

SDL_Surface *back;  <<< ERROR
SDL_Surface *image;
SDL_Surface *screen;

int xpos=0,ypos=0;

int InitImages()
{
  back = SDL_LoadBMP("bg.bmp");
  image = SDL_LoadBMP("image.bmp");
  return 0;
}

void DrawIMG(SDL_Surface *img, int x, int y)
{
  SDL_Rect dest;
  dest.x = x;
  dest.y = y;
  SDL_BlitSurface(img, NULL, screen, &dest);
}

void DrawIMG(SDL_Surface *img, int x, int y, int w, int h, int x2, int y2)
{
  SDL_Rect dest;
  dest.x = x;
  dest.y = y;
  SDL_Rect dest2;
  dest2.x = x2;
  dest2.y = y2;
  dest2.w = w;
  dest2.h = h;
  SDL_BlitSurface(img, &dest2, screen, &dest);
}

void DrawBG()
{
  DrawIMG(back, 0, 0);
}

void DrawScene()
{
  DrawIMG(back, xpos-2, ypos-2, 132, 132, xpos-2, ypos-2);
  DrawIMG(image, xpos, ypos);

  SDL_Flip(screen);
}

int main(int argc, char *argv[])
{
  Uint8* keys;

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

  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);
  }

  InitImages();
  DrawBG();

  int done=0;

  while(done == 0)
  {
    SDL_Event event;

    while ( SDL_PollEvent(&event) )
    {
      if ( event.type == SDL_QUIT )  {  done = 1;  }

      if ( event.type == SDL_KEYDOWN )
      {
        if ( event.key.keysym.sym == SDLK_ESCAPE ) { done = 1; }
      }
    }
    keys = SDL_GetKeyState(NULL);
    if ( keys[SDLK_UP] ) { ypos -= 1; }
    if ( keys[SDLK_DOWN] ) { ypos += 1; }
    if ( keys[SDLK_LEFT] ) { xpos -= 1; }
    if ( keys[SDLK_RIGHT] ) { xpos += 1; }

    DrawScene();
  }

  return 0;
}