draw line

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
 
#include <GL/glut.h>
 
 
  int ViewportXFirst  , ViewportYFirst ;
  int ViewportXSecond , ViewportYSecond;
 
  int flag =0; // count 2 clicks at a time.
               // flag = 0 : first time. flag =1 : second time
 
 
void init (void)
{
    glClearColor (1.0,1.0,1.0,0.0);
 
    glMatrixMode(GL_PROJECTION);
    gluOrtho2D(0.0,200.0,0.0,150.0);
 
    glMatrixMode(GL_MODELVIEW);
}
 
void lineSegment (void)
{
     glClear( GL_COLOR_BUFFER_BIT);
 
    glLoadIdentity();
    ViewportXFirst = 0; ViewportYFirst = 0;
    ViewportXSecond = 200; ViewportYSecond = 150;
 
      glColor3f(1.0,0.0,0.0);
      glBegin(GL_LINES);
            glVertex2i(ViewportXFirst,ViewportYFirst);
            glVertex2i(ViewportXSecond,ViewportYSecond);
      glEnd();
      
      glFlush();
}
 
void processMouse(int button, int state, int x, int y) {
 
      if (state == GLUT_DOWN)
      {      
            if(flag ==0)
            {
                  printf("flag is %d\n",flag);
                  ViewportXFirst = x;
                  ViewportYFirst = 512- y;
 
                  flag ++;
            }
            else
            {
                                if(flag ==1)
                                {
                  printf("flag is %d\n",flag);
 
                  ViewportXSecond = x;
                  ViewportYSecond = 512- y;
                                }
                                else
                                {
                                        ViewportXFirst = ViewportXSecond;  // swap second point to first point 
                                        ViewportYFirst = ViewportYSecond;  //before obtain new point to second point
 
                                        ViewportXSecond = x;
                                        ViewportYSecond = 512- y;
 
                                }
                                flag++;
                  
            }
 
                
                                printf("ViewportXFirst is %d   ",ViewportXFirst);
                                printf("ViewportYFirst is %d",ViewportYFirst);
                                printf("\n");
 
                                printf("ViewportXSecond is %d   ",ViewportXSecond);
                                printf("ViewportYSecond is %d",ViewportYSecond);
                                printf("\n"); 
      }
        
          if(flag >=2)
          {
                glutDisplayFunc(lineSegment);
 
          }
 
}
 
void main(int argc, char ** argv)
{
      glutInit(&argc, argv);
      glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
      glutInitWindowPosition(50,100);
      glutInitWindowSize(512,512);
      glutCreateWindow("An example openGL sample");
 
      init();
     
       //adding here the mouse processing callbacks
      glutMouseFunc(processMouse);
 
          glutDisplayFunc(lineSegment);
          
      
      //glutMotionFunc(processMouseActiveMotion);
      //glutPassiveMotionFunc(processMousePassiveMotion);
      //glutEntryFunc(processMouseEntry);
 
      glutMainLoop();
}
                                              

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