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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 <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; // Open the file for reading and writing. 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"); // Get fixed screen information. if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo) == -1) { perror("Error reading fixed information"); exit(2); } // Get variable screen information. 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); // Figure out the size of the screen in bytes. screensize = finfo.smem_len; // Map the device to memory. 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; // 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 \starslash int b = 10; // Some blue 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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