doubt in frame buffer using with RHEL 6.0

[ShibuThomas]

Hi All,
I'm using an i5 core with builtin graphics accelerator GMA-HD running on 
an RHEL 6.0 OS.
I needed to test the graphics acceleration capabilities of the graphics 
driver(which I found out was i915 in my PC).
I used the following code(which I got from the internet ) for writing to 
framebuffer.



/*******************************************************/
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>

int main()
{
         int fbfd = 0;
         struct fb_var_screeninfo vinfo;
         struct fb_fix_screeninfo finfo;
         long int screensize = 0;
         //char *fbp = 0;
         char *fbp = 0;
         int x = 0, y = 0;
         long int location = 0;
         int count ;

         /* Open the file for reading and writing */
         fbfd = open("/dev/fb0", O_RDWR);
         if (!fbfd) {
                 printf("Error: cannot open framebuffer device.\n");
                 exit(1);
         }
         printf("The framebuffer device was opened successfully.\n");
      /* Get fixed screen information */
         if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
                printf("Error reading fixed information.\n");
                 exit(2);
         }

         /* Get variable screen information */
         if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
                 printf("Error reading variable information.\n");
                 exit(3);
         }

         /* Figure out the size of the screen in bytes */
         screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
         printf("\nScreen size is %d",screensize);
         printf("\nVinfo.bpp = %d",vinfo.bits_per_pixel);
//      printf("\nFinfo.bpp = %d",finfo.bits_per_pixel);


         /* Map the device to memory */
         fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, 
MAP_SHARED,fbfd, 0);
         if ((int)fbp == -1) {
                 printf("Error: failed to map framebuffer device to 
memory.\n");
                 exit(4);
         }
          printf("The framebuffer device was mapped to memory 
successfully.\n");


         x = 100; y = 100; /* Where we are going to put the pixel */

         /* Figure out where in memory to put the pixel */
         location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) + 
(y+vinfo.yoffset) * finfo.line_length;
         for(count = 1 ;count < 100 ;count++)
         {
                 *(fbp + location) = 255;    /* Some blue */
                 *(fbp + location + count) = 0; /* A little green */
                 *(fbp + location + count + 1) = 0; /* A lot of red */
                 *(fbp + location + count + 2) = 0; /* No transparency */
         }
         munmap(fbp, screensize);
         close(fbfd);
         return 0;
}
/*************************************************************************/

After running the above code no change was found on display but ' cat  
/dev/fb0 ' showed some data.
Could someone explain why nothing was seen on screen?
(I also found that fb0 correspond to the frame buffer 'inteldrmfb' .)

Regards,
ShibuThomas