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<h1 align="center">Testing the Linux Framebuffer<br /><small></small></h1>
<p>To test that the Linux framebuffer is set up correctly, and that the device permissions are correct, use the program below which opens the frame buffer and draws a gradient-filled red square:</p>
<pre> #include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
int main()
{
int fbfd = 0;
struct fb_var_screeninfo vinfo;
struct fb_fix_screeninfo finfo;
long int screensize = 0;
char *fbp = 0;
int x = 0, y = 0;
long int location = 0;
<span class="comment">// Open the file for reading and writing</span>
fbfd = open("/dev/fb0", O_RDWR);
if (fbfd == -1) {
perror("Error: cannot open framebuffer device");
exit(1);
}
printf("The framebuffer device was opened successfully.\n");
<span class="comment">// Get fixed screen information</span>
if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo) == -1) {
perror("Error reading fixed information");
exit(2);
}
<span class="comment">// Get variable screen information</span>
if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo) == -1) {
perror("Error reading variable information");
exit(3);
}
printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel);
<span class="comment">// Figure out the size of the screen in bytes</span>
screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
<span class="comment">// Map the device to memory</span>
fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,
fbfd, 0);
if ((int)fbp == -1) {
perror("Error: failed to map framebuffer device to memory");
exit(4);
}
printf("The framebuffer device was mapped to memory successfully.\n");
x = 100; y = 100; <span class="comment">// Where we are going to put the pixel</span>
<span class="comment">// Figure out where in memory to put the pixel</span>
for (y = 100; y < 300; y++)
for (x = 100; x < 300; x++) {
location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +
(y+vinfo.yoffset) * finfo.line_length;
if (vinfo.bits_per_pixel == 32) {
*(fbp + location) = 100; <span class="comment">// Some blue</span>
*(fbp + location + 1) = 15+(x-100)/2; <span class="comment">// A little green</span>
*(fbp + location + 2) = 200-(y-100)/5; <span class="comment">// A lot of red</span>
*(fbp + location + 3) = 0; <span class="comment">// No transparency</span>
} else { <span class="comment">//assume 16bpp</span>
int b = 10;
int g = (x-100)/6; <span class="comment">// A little green</span>
int r = 31-(y-100)/16; <span class="comment">// A lot of red</span>
unsigned short int t = r<<11 | g << 5 | b;
*((unsigned short int*)(fbp + location)) = t;
}
}
munmap(fbp, screensize);
close(fbfd);
return 0;
}</pre>
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