Use the GL_TRIANGLES function:

GL_TRIANGLES function:

This primitive takes three vertices at a time and draws a triangle using them. If your application specifies n vertices, OpenGL renders n/3 triangles. If n isn't a multiple of 3, OpenGL ignores the excess vertices.

Example:

include header file

#include "stdafx.h"

#include<glut.h>

Defination of draw_trinagle function:

void Draw_triangle() {

glClear(GL_COLOR_BUFFER_BIT);

glColor3f(1.0, 0.0, 0.0);

glBegin(GL_TRIANGLES);

glVertex2f(0.20,0.95);

glVertex2f(0.15, 0.85);

glVertex2f(0.25,0.85);

glEnd();

glFlush();

}

int main(int iArgc, char** cppArgv) {

...

       ....

       /*basic initialization */

      ...

     ...

     ...

glutDisplayFunc(Draw_triangle);

glutMainLoop();

return 0;

}

output screen short:

 

Note:

        In /*basic initialization * area we create the window for drawing initialize the window and set the size of window u can see how to set and initialize the window click here