Raspberry Pi direct (memory-mapped) framebuffer drawing PoC.

fb-flag.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#include <string.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>

#define RGB565(r, g, b) (uint16_t)(b|(g<<5)|(r<<11))

int main(int argc, char *argv[])
{
	int fbfd = 0;
	struct fb_var_screeninfo var_info;
	struct fb_fix_screeninfo fix_info;
	long int screensize = 0;
	long int i;
	uint16_t *fbp = 0;
	
	fbfd = open("/dev/fb0", O_RDWR);
	if (fbfd == -1) {
		fprintf(stderr, "Cannot open framebuffer!\n");
		return 1;
	}

	printf("Framebuffer opened\n");
	if (ioctl(fbfd, FBIOGET_VSCREENINFO, &var_info)) {
		fprintf(stderr, "Error reading screen info.\n");
		return 1;
	}
	printf("Display info: %dx %dy, %d bpp\n"
		, var_info.xres, var_info.yres
		, var_info.bits_per_pixel);

	if (ioctl(fbfd, FBIOGET_FSCREENINFO, &fix_info)) {
		fprintf(stderr, "Error reading fix screen info.\n");
		return 1;
	}

	screensize = fix_info.smem_len;
	fbp = (uint16_t *)mmap(0, screensize, PROT_READ|PROT_WRITE
				, MAP_SHARED, fbfd, 0);
	screensize /= 2;

	if (fbp == MMAP_FAILED) {
		fprintf(stderr, "Cannot map buffer!\n");
		return 1;
	}

        // Hungarian tricolor
	for (i = 0; i < screensize/3; i++) {
		fbp[i] = RGB565(240, 0, 0);
	}	
	for (; i < 2*(screensize/3); i++) {
		fbp[i] = RGB565(255, 255, 255);
	}	
	for (; i < screensize; i++) {
		fbp[i] = RGB565(0, 210, 0);
	}	
	sleep(3);
	munmap(fbp, screensize);
	close(fbfd);

	return 0;
}