Render line

I got a code from online tutorial which could drawn program. But I madea slight modification which ask the user to input the coordinate for the startinng and end points of line to be drawn. However, I face some problem which always get trapped at the dos prompt, but never get the program to render the line. Please let me know what is wrong. Thanks.

  
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <GL/glut.h>


#define ESC 27
int Height=400, Width=400;


void swapd(int *a, int *b)
{
    int tmp;

       tmp=*a;*a=*b;*b=tmp;
}


void writepixel(int x, int y, int flag)
{ 
	
	glBegin(GL_POINTS);
		if (flag==0) glVertex2i (x,y);
		else if (flag==1) glVertex2i (y,x);
		else if (flag==10) glVertex2i (x,-y);
		else if (flag==11) glVertex2i (y,-x);
	glEnd();
}


void line(int x0,int y0,int xn,int yn)
{ /* Bresenham's midpoint line algorithm */
    int dx, dy, incrE, incrNE, d, x, y, flag = 0;

    if (xn<x0) { swapd(&x0,&xn); swapd(&y0,&yn); }
    if (yn<y0) { y0 = -y0; yn = -yn; flag=10; }

    dy=yn-y0; dx=xn-x0;

    if (dx<dy) { swapd(&x0,&y0); swapd(&xn,&yn); swapd(&dy,&dx); flag++;}

    x=x0; y=y0;
    d=2*dy-dx; incrE=2*dy; incrNE=2*(dy-dx);

    while (x<xn+1) { 
        writepixel(x,y,flag); /* write framebuffer */

		x++; /* consider next pixel */
        if (d<=0) { d+=incrE; }
        else { y++; d+=incrNE; };
    }
}


void display(void)
{
	int x0, y0, xn, yn;

	printf(" Please enter coordinate of x0 and y0
");
	scanf("%d %d", &x0,&y0);

	
	printf(" Please enter coordinate of x1 and y1
");
	scanf("%d %d", &xn,&yn);

	x0 = (x0 % Width) - Width/2; 
	y0 = (y0 % Height) - Height/2; 
	xn = (xn % Width) - Width/2; 
	yn = (yn % Height) - Height/2; 

    glClear(GL_COLOR_BUFFER_BIT);
	glColor3f(1, 1, 1);
	line(x0, y0, xn, yn);
	glFlush();
}



static void Reshape(int w, int h)
{
	//1. clear the framebuffer to black color
    glClearColor (0.0, 0.0, 0.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT);

	//2. coordinate adjusted if window reshaped
	Width = w; Height = h; 

	//3. adjust the size of the drawing area 
    glViewport (0, 0, Width, Height);

    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    glOrtho(-Width/2, Width/2, -Height/2, Height/2, -1.0, 1.0); 
}


static void Key(unsigned char key, int x, int y)
{ 
    switch (key) {
		case ESC:
			exit(0);
	
		default: display();
    }
}


int main(int argc, char **argv)
{
    glutInit(&argc, argv);
 	glutInitDisplayMode(GLUT_SINGLE);
    glutInitWindowSize(Width, Height);
	
    glutCreateWindow("Bresenham Line");
	
    
	glutKeyboardFunc(Key);
    glutReshapeFunc(Reshape); 
    glutDisplayFunc(display);

    glutMainLoop();
}