#include <GL/glut.h>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <string.h>

#define WINDSIZEX 100
#define WINDSIZEY 100

void init(void)
{
   glClearColor (0,0,0,1);
   glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   glDisable (GL_DITHER);
}

//color data
static  GLfloat colbuf[] = {
   0.2, 0.4, 0.6, 0.8,
   0.4, 0.6, 0.8, 0.2,
   0.6, 0.8, 0.2, 0.4,
   0.8, 0.2, 0.4, 0.6
};

//vertex data
static  GLint vertexbuf[] = {
   0, 0,
   20, 0,
   20, 20,
   0, 20
};

#define BUF_OFFSET(i) ((char *)NULL+ (i))

static void test(void)
{   
   GLuint bufname[1];
   GLfloat *readBuf;
   GLubyte *allbuf, *bufptr, *p;
   int  allsize, elemoffset, offset;
   GLuint  elemArray[] = {0, 1, 2, 3};

   offset = 1;

   glGenBuffers(1, bufname);

   readBuf = (GLfloat *)malloc (4* sizeof (GLfloat) * WINDSIZEX * WINDSIZEY);
   //create an allbuf to store all data(vertex, color, element and extra space
   //to chnage offset values
   allsize = sizeof(colbuf) + sizeof(vertexbuf) + sizeof(elemArray) + offset;
   allbuf = malloc( allsize * sizeof(GLubyte) );

   glBindBuffer(GL_ARRAY_BUFFER, bufname);
   glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, bufname);
   glEnableClientState (GL_COLOR_ARRAY);
   glEnableClientState (GL_VERTEX_ARRAY);

   glColorPointer (4, GL_FLOAT, 0, BUF_OFFSET(offset));
   glVertexPointer(2, GL_INT, 0, BUF_OFFSET(offset + sizeof(colbuf)));
   //calculate the offset where indices are stored
   elemoffset = sizeof(colbuf) + sizeof(vertexbuf) + offset;

   glClear (GL_COLOR_BUFFER_BIT);
   bufptr = allbuf;

   p = memcpy(bufptr + offset, colbuf, sizeof(colbuf));
   p = memcpy(p + sizeof(colbuf),
               vertexbuf, sizeof(vertexbuf));

   glBufferData(GL_ARRAY_BUFFER, allsize,
                     bufptr, GL_STREAM_DRAW);
   glBufferSubData(GL_ELEMENT_ARRAY_BUFFER, elemoffset,
               sizeof(elemArray), elemArray);
   glDrawElements(GL_POLYGON, 4, GL_UNSIGNED_INT, BUF_OFFSET(elemoffset));
   glFlush();
   glReadPixels(0, 0, WINDSIZEX, WINDSIZEY, GL_RGBA,
               GL_FLOAT, readBuf);

   glDeleteBuffers(1, &bufname);
   glDisableClientState (GL_COLOR_ARRAY);
   glDisableClientState (GL_VERTEX_ARRAY);

   free(readBuf);
   free(allbuf);
}

void display(void)
{
   glViewport(0, 0, WINDSIZEX, WINDSIZEY);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   gluOrtho2D(0, WINDSIZEX, 0, WINDSIZEY);
   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();

   test();
}

int main(int argc, char** argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
   glutInitWindowSize(WINDSIZEX, WINDSIZEY);
   glutInitWindowPosition(100, 100);
   glutCreateWindow(argv[0]);
   init();
   glutDisplayFunc(display);
   glutMainLoop();
   return 0; 
}