gluunproject with glut c++

#include <cstdlib>
#include <iostream>
#include <gl/gl.h>
#include <gl/glu.h>
#include <gl/glut.h>
 
using namespace std;
 
double objx, objy, objz;
 
void processMouse(int button, int state, int x, int y)
{
    double modelview[16], projection[16];
    int viewport[4];
    float z;
    
        //get the modelview matrix              
    glGetDoublev( GL_MODELVIEW_MATRIX, modelview );
    //get the projection matrix
    glGetDoublev( GL_PROJECTION_MATRIX, projection );
        //get the viewport              
    glGetIntegerv( GL_VIEWPORT, viewport );
        
    //Read the window z co-ordinate 
    //(the z value on that point in unit cube)          
    glReadPixels( x, viewport[3]-y, 1, 1,
                 GL_DEPTH_COMPONENT, GL_FLOAT, &z );
 
    //Unproject the window co-ordinates to 
    //find the world co-ordinates.
    gluUnProject( x, viewport[3]-y, z, modelview, 
                projection, viewport, &objx, &objy, &objz );
 
    cout << objx << " , " << objy << " , " << objz << "\n";
}
 
void processNormalKeys(unsigned char key, int x, int y) {
 
        if (key == 27)//escape key ascii value 
                exit(0);
}
 
void processSpecialKeys(int key, int x, int y) {
 
}
 
void changeSize(int w, int h) {
 
        // Prevent a divide by zero, when window is too short
        // (you cant make a window of zero width).
        if(h == 0)
                h = 1;
 
        float ratio = 1.0* w / h;
 
        // Reset the coordinate system before modifying
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        
        // Set the viewport to be the entire window
        glViewport(0, 0, w, h);
 
        // Set the correct perspective.
        gluPerspective(45,ratio,1,1000);
        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();
}
 
void renderScene(void) {
        // changed, clear depth buffer too
        glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
        glLoadIdentity();
        // changed, render think lines
        glLineWidth(10.0f);
        glTranslatef(0,0,-300);
        glBegin(GL_LINES);
                glVertex3f(-100.0,0.0,0.0);
                glVertex3f(100.0,0.0,0.0);
                glVertex3f(0.0,-100.0,0.0);
                glVertex3f(0.0,100.0,0.0);
                glVertex3f(0.0,0.0,-100.0);
                glVertex3f(0.0,0.0,100.0);
        glEnd();
        glFlush();
}
 
int main(int argc, char **argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DEPTH | GLUT_SINGLE | GLUT_RGBA);
    glutInitWindowPosition(100,100);
    glutInitWindowSize(320,320);
    glutCreateWindow("glut");
 
    // changed (moved after glutCreateWindow)
    glClearColor(0,0,0,0);
    glEnable(GL_DEPTH_TEST);//Enable depth testing
        
    glutDisplayFunc(renderScene);
    glutReshapeFunc(changeSize);
    glutKeyboardFunc(processNormalKeys);
    glutSpecialFunc(processSpecialKeys);
    glutMouseFunc(processMouse);
    glutMainLoop();
}
                                              

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