Shows how to use GLUT. Has minimal structure: only main() and a display callback. Draws only using glColor and glVertex within glBegin and glEnd in the display callback. Uses only the GL_POLYGON drawing mode. Illustrates glClear and glFlush.

// A simple introductory program; its main window contains a static picture
// of a triangle, whose three vertices are red, green and blue.  The program

#include <GL/glut.h>


// Clears the current window and draws a triangle.
void display() {

  // Set every pixel in the frame buffer to the current clear color.
  glClear(GL_COLOR_BUFFER_BIT);

  // Drawing is done by specifying a sequence of vertices.  The way these
  // vertices are connected (or not connected) depends on the argument to
  // glBegin.  GL_POLYGON constructs a filled polygon.
  glBegin(GL_POLYGON);
    glColor3f(1, 0, 0); glVertex3f(-0.6, -0.75, 0.5);
    glColor3f(0, 1, 0); glVertex3f(0.6, -0.75, 0);
    glColor3f(0, 0, 1); glVertex3f(0, 0.75, 0);
  glEnd();

  // Flush drawing command buffer to make drawing happen as soon as possible.
  glFlush();
}

// Initializes GLUT, the display mode, and main window; registers callbacks;
// enters the main event loop.
int main(int argc, char** argv) {

  // Use a single buffered window in RGB mode (as opposed to a double-buffered
  // window or color-index mode).
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);

  // Position window at (80,80)-(480,380) and give it a title.
  glutInitWindowPosition(80, 80);
  glutInitWindowSize(400, 300);
  glutCreateWindow("A Simple Triangle");

  // Tell GLUT that whenever the main window needs to be repainted that it
  // should call the function display().
  glutDisplayFunc(display);

  // Tell GLUT to start reading and processing events.  This function
  // never returns; the program only exits when the user closes the main
  // window or kills the process.
  glutMainLoop();
}

How to run glut/ OpenGL in g++ (Ubuntu)

g++ MyProg.cpp -lGL -lGLU -lglut

Output