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# Testing the Linux Framebuffer

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:

        #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;
            int x = 0, y = 0;
            long int location = 0;

            // 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);
            }

            printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel);

            // Figure out the size of the screen in bytes
            screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;

            // 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
            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;        // Some blue
                        *(fbp + location + 1) = 15+(x-100)/2;     // A little green
                        *(fbp + location + 2) = 200-(y-100)/5;    // A lot of red
                        *(fbp + location + 3) = 0;      // No transparency
                    } else  { //assume 16bpp
                        int b = 10;
                        int g = (x-100)/6;     // A little green
                        int r = 31-(y-100)/16;    // A lot of red
                        unsigned short int t = r<<11 | g << 5 | b;
                        *((unsigned short int*)(fbp + location)) = t;
                    }

                }
            munmap(fbp, screensize);
            close(fbfd);
            return 0;
        }

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