为什么运行时出错呢 问题是什么（win7系统上）

#include <iostream>
#include <cmath>
#include <cstdlib>
#define DIM 1024
#define DM1 (DIM-1)
#define _sq(x) ((x)*(x)) // square
#define _cb(x) abs((x)*(x)*(x)) // absolute value of cube
#define _cr(x) (unsigned char)(pow((x),1.0/3.0)) // cube root
unsigned char GR(int,int);
unsigned char BL(int,int);
unsigned char RD(int i,int j){
static double k;k+=rand()/1./RAND_MAX;int l=k;l%=512;return l>255?511-l:l;
}
unsigned char GR(int i,int j){
static double k;k+=rand()/1./RAND_MAX;int l=k;l%=512;return l>255?511-l:l;
}
unsigned char BL(int i,int j){
static double k;k+=rand()/1./RAND_MAX;int l=k;l%=512;return l>255?511-l:l;
}
void pixel_write(int,int);
FILE *fp;
int main(){
fp = fopen("MathPic.ppm","wb");
fprintf(fp, "P6\n%d %d\n255\n", DIM, DIM);
for(int j=0;j<DIM;j++)
for(int i=0;i<DIM;i++)
pixel_write(i,j);
fclose(fp);
return 0;
}
void pixel_write(int i, int j){
static unsigned char color[3];
color[0] = RD(i,j)&255;
color[1] = GR(i,j)&255;
color[2] = BL(i,j)&255;
fwrite(color, 1, 3, fp);
}