Use the GL_LINES function:

Use the GL_LINES function:

This primitive takes two vertices and draws a line segment between them. If the vertices are more than two, OpenGL draws a vertex for each pair of vertices of two vertices. If the application specifies n vertices, OpenGL renders n/2 line segments. If n is odd, OpenGL ignores the final vertex.

Example:

Main function:

the following code are write in main function

int main(int argc, char **argv)

{

glutInit(&argc, argv);                                                      // initializes the toolkit

glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);     //set the display mode

glutInitWindowSize(640,480);                                      //set the window size

glutInitWindowPosition(100,100);                       //set the window position on screen

glutCreateWindow(“draw the line”);                   //open the screen window

glutDisplayFunc(Displayline); 

myInit();                                                             // additional initializations, if necessary

glutMainLoop();                                             //go into a perpetual loop

}

Define the the myInit();  function:

void myInit(void)

{

glClearColor(1.0,1.0,1.0,0.0);

glColor3f(0.0f,0.0f,0.0f);

glPointSize(4.0);

glMatrixMode(GL_PROJECTION);

glLoadIdentity();

gluOrtho2D(0.0,640.0,0.0,480.0);

}

Define Displayline() function:

void Displayline()   {
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_LINES);
glVertex3f(0.25, 0.25, 0.0);
glVertex3f(0.75, 0.75, 0.0);
glEnd();
glFlush();

}

Note:
          Both myInit(); and Displayline() must be define above main()

glbegin():

                   Marks the beginning of a vertex-data list that describes a geometric primitive 

glEnd():

                  Marks the end of a vertex-data list.