How to do it...

The following are the steps for drawing a circle:

1. Initialize GLUT, define the size of a top-level window, and create it. Also, set the initial position of the window for displaying our circle.
2. Define a callback function that is auto-invoked after creating the window.
3. In the callback function, color buffers are cleared and the color for displaying the circle is set.
4. The statements for drawing a circle are grouped within a pair of glBegin and glEnd functions along with the GL_LINE_LOOP keyword.
5. Use a for loop to draw small lines from 0 to 360 to give the shape of a circle.

The program for drawing a circle is as follows:

//opengldrawshapes2.c

#include <GL/glut.h> 
#include<math.h> 
#define pi 3.142857 

void drawshapes() { 
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f); 
    glClear(GL_COLOR_BUFFER_BIT);  
    glColor3f(0.0f, 1.0f, 0.0f); 
    glBegin(GL_LINE_LOOP); 
        for (int i=0; i <360; i++) 
        { 
            float angle = i*pi/180; 
            glVertex2f(cos(angle)*0.5,sin(angle)*0.5);                                              }
    glEnd();
    glFlush(); 
} 

int main(int argc, char** argv) {
    glutInit(&argc, argv);    
    glutCreateWindow("Drawing some shapes"); 
    glutInitWindowSize(1500, 1500);  
    glutInitWindowPosition(0, 0);
    glutDisplayFunc(drawshapes); 
    glutMainLoop();     
    return 0; 
}

Now, let's go behind the scenes to understand the code better.