OpenGl code to make self rotating Triangle

This source code is for making a simple triangle which rotates itself.

OpenGL program:-

#include<iostream>
#include<stdlib.h>

#ifdef __APPLE__
#include<openGL/openGL.h>
#include<GLUT/glut.h>
#else 
#include<GL/glut.h>
#endif

using namespace std;
void keyPress(unsigned char key,int x,int y)
{

    switch(key)
    {
       case 27:
            exit(0);     
    }
}

void initRendering()
{
    glEnable(GL_DEPTH_TEST);
}


//Called when the window is resized
void handleResize(int w, int h) {
    //Tell OpenGL how to convert from coordinates to pixel values
    glViewport(0, 0, w, h);
    
    glMatrixMode(GL_PROJECTION); //Switch to setting the camera perspective
    
    //Set the camera perspective
    glLoadIdentity(); //Reset the camera
    gluPerspective(45.0,                  //The camera angle
                   (double)w / (double)h, //The width-to-height ratio
                   1.0,                   //The near z clipping coordinate
                   200.0);                //The far z clipping coordinate
}



float _angle = 30.0f;
float _cameraAngle = 0.0f;

void drawScene()
{
    glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
   
    glMatrixMode(GL_MODELVIEW);
   
    glLoadIdentity();

    glPushMatrix(); //Save the transformations performed thus far

    glRotatef(_angle, -1.5f, 0.5f, -5.0f); //Rotate about the z-axis

    glBegin(GL_TRIANGLES);
   
        glVertex3f(-0.5f, 0.5f, -5.0f);
        glVertex3f(-1.0f, 1.5f, -5.0f);
        glVertex3f(-1.5f, 0.5f, -5.0f);

    glEnd();
    glPopMatrix(); //Undo the move to the center of the triangle

    glutSwapBuffers();
}

void update(int value) {
    _angle += 2.0f;
    if (_angle > 360) {
        _angle -= 360;
    }
   
    glutPostRedisplay(); //Tell GLUT that the display has changed
   
    //Tell GLUT to call update again in 25 milliseconds
    glutTimerFunc(25, update, 0);
}

int main(int argc,char** argv)
{
    glutInit(&argc,argv);
   
    glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB|GLUT_DEPTH);
   
    glutInitWindowSize(400,400);
   
    glutCreateWindow("My Triangle");
   
    initRendering();
   
    glutDisplayFunc(drawScene);
   
    glutKeyboardFunc(keyPress);

    glutReshapeFunc(handleResize);
   
    glutTimerFunc(25, update, 0); //Add a timer
   
    glutMainLoop();
   
    return(0);
}

//Output Of the above program:-

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