init process that draws to framebuffer

main.c

#include <errno.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <linux/kd.h>
#include <linux/limits.h>
#include <stdarg.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/sysmacros.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

void exit_handler();
void mount_proc();
void mount_sys();
void mount_dev();
int open_dev_fb();
int open_dev_tty();
void run_terminal();
uint32_t pixel_color(uint8_t r, uint8_t g, uint8_t b, struct fb_var_screeninfo *vinfo);

void exit_handler() {
    #if 1
        run_terminal();
    #else
        exit(1);
    #endif
}

void mount_proc() {
    printf("Mounting /proc...");
    if (0 != mount("none", "/proc", "proc", 0, "")) {
        printf("\nFailed to mount /proc!\n");
        exit_handler();
    }
    printf("Done!\n");
}

void mount_sys() {
    printf("Mounting /sys...");
    if (0 != mount("none", "/sys", "sysfs", 0, "")) {
        printf("\nFailed to mount /proc!\n");
        exit_handler();
    }
    printf("Done!\n");
}

void mount_dev() {
    printf("Mounting /dev...");
    if (0 != mount("none", "/dev", "devtmpfs", 0, "")) {
        int errno2 = errno;
        // Error code 16 ("device or resource busy") is spurrious and doesn't really matter. Ignore it.
        if (errno2 != 16) {
            printf("\nFailed to mount /dev! errno=%d strerror=%s\n", errno, strerror(errno));
            exit_handler();
        }
    }
    printf("Done!\n");
}

int open_dev_fb() {
	int fb_fd = open("/dev/fb0",O_RDWR);
    if (fb_fd == -1) {
        int errno2 = errno;
        printf("Failed to open /dev/fb0! errno=%d strerror=%s\n", errno, strerror(errno));
        exit_handler();
    }
    return fb_fd;
}

void run_terminal() {
    execl("busybox-x86_64", "busybox-x86_64", "ash", NULL);
}

uint32_t pixel_color(uint8_t r, uint8_t g, uint8_t b, struct fb_var_screeninfo *vinfo) {
	return (r<<vinfo->red.offset) | (g<<vinfo->green.offset) | (b<<vinfo->blue.offset);
}

int main(int argc, char *argv[]) {
    printf("Hello from the init system\n");
    int parent_pid = getpid();

    mount_proc();
    mount_sys();
    mount_dev();

	struct fb_fix_screeninfo finfo;
	struct fb_var_screeninfo vinfo;

	int fb_fd = open_dev_fb();

	//Get variable screen information
	ioctl(fb_fd, FBIOGET_VSCREENINFO, &vinfo);
	vinfo.grayscale=0;
	vinfo.bits_per_pixel=32;
	ioctl(fb_fd, FBIOPUT_VSCREENINFO, &vinfo);
	ioctl(fb_fd, FBIOGET_VSCREENINFO, &vinfo);

	ioctl(fb_fd, FBIOGET_FSCREENINFO, &finfo);

	long screensize = vinfo.yres_virtual * finfo.line_length;

	uint8_t *fbp = mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED, fb_fd, (off_t)0);

	int x,y;

    printf("Should be rendering on monitor now...\n");
    int val = 0;
    for (y=0;y<vinfo.yres;y++) {
        for (x=0;x<vinfo.xres;x++){
            val++;
            if (val > 0xFF) {
                val = 0;
            }
            long location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) + (y+vinfo.yoffset) * finfo.line_length;
            *((uint32_t*)(fbp + location)) = pixel_color(
                (x > y) ? val : 0xFF - val,
                (y > x) ? val : 0xFF - val,
                (x == y) ? val : 0xFF - val,
                &vinfo);
        }
    }
    sleep(60);
    run_terminal();

	return 0;
}