How to write openGl offscreen data in to JPG image

#include <stdlib.h> 
#include <iostream>
//Include OpenGL header files, so that we can use OpenGL
#ifdef __APPLE__
#include 
#include 
#else
#include 
#endif
#include "bitmap_image.hpp"
using namespace std;


//Draws the 3D scene
void drawScene()
{
	//Clear information from last draw
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glMatrixMode(GL_MODELVIEW); //Switch to the drawing perspective
	glLoadIdentity(); //Reset the drawing perspective	

	glEnable(GL_DEPTH_TEST);

	static unsigned char texture[3 * 400 * 400];
	static unsigned int texture_id;

	// Texture setting
	glEnable(GL_TEXTURE_2D);
	glGenTextures(1, &texture_id);
	glBindTexture(GL_TEXTURE_2D, texture_id);
	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 400, 400, 0, GL_RGB,	GL_UNSIGNED_BYTE, texture);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);

	glLoadIdentity();
	glTranslatef(0, 0, -10);	

	/* Define a view-port adapted to the texture */
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluPerspective(20, 1, 5, 15);
	glViewport(0, 0, 400, 400);
	glMatrixMode(GL_MODELVIEW);

	/* Render to buffer */
	glClearColor(1, 1, 1, 0);
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);	
	
	glColor3f(1.0,0.0,0.0);

	glBegin(GL_TRIANGLES);
	
	glVertex3f( 0.0f, 1.0f, 0.0f);             

	glVertex3f(-1.0f,-1.0f, 0.0f);             

	glVertex3f( 1.0f,-1.0f, 0.0f);             

	glEnd();  
	///glFlush();
	// Image Writing	
	unsigned char* imageData = (unsigned char *)malloc((int)(400*400*(3)));
	glReadPixels(0, 0, 400, 400, GL_RGB, GL_UNSIGNED_BYTE, imageData);
	//write 
	bitmap_image image(400,400);
	image_drawer draw(image);

	for (unsigned int i = 0; i < image.width(); ++i)
	{
		for (unsigned int j = 0; j < image.height(); ++j)
		{
			image.set_pixel(i,j,*(++imageData),*(++imageData),*(++imageData));		
		}
	}

	image.save_image("Trangle_image.bmp");
	///glutSwapBuffers(); 
}

int main(int argc, char** argv)
{
	//Initialize GLUT
	//glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
	glutInitWindowSize(400, 400); //Set the window size
	//Create the window
	glutCreateWindow("Test");	
	drawScene();
	return 0; //This line is never reached
}</iostream></stdlib.h>