C/Visual C - Porque me falla el bucle???

double ang;

double i;

double x,y;

double x1,y1;

double x2,y2;

double n_segments=12.0;

for (i=0.0; i<n_segments; i++) {

    ang = 2 *(M_PI)*i/n_segments; // Ángulo actual

    x = radius * cos(ang);

    y = radius * sin(ang);

    x1 = center_x + x;

    y1 = center_y + y;

    x2 = x1-1.0;

    y2 = y1-1.0;

    glVertex2d(x1,y1);

    glVertex2d(x2,y2);

}

printf("%d",i);