Add main loop with framebuffer color change

src/main.c

@@ -1,7 +1,43 @@
 #include<stdio.h>
+#include<stdlib.h>
+#include<stdint.h>
+#include<stdbool.h>
+#include<fcntl.h>
+#include<linux/fb.h>
+#include<sys/ioctl.h>
+#include<sys/mman.h>
 
 int
 main(void) {
-	printf("hello world\n");
+	// Open frambuffer and get the size.
+	int fb = open("/dev/fb0", O_RDWR);
+	if (fb <= 0) {
+		fprintf(stderr, "couldn't open the framebuffer\n");
+		exit(EXIT_FAILURE);
+	}
+	struct fb_var_screeninfo info;
+	if (ioctl(fb, FBIOGET_VSCREENINFO, &info) != 0) {
+		fprintf(stderr, "couldn't get the framebuffer size\n");
+		exit(EXIT_FAILURE);
+	}
+
+	// Mmap the framebuffer to a buffer object.
+	size_t width = info.xres;
+	size_t height = info.yres;
+	size_t len = 4 * width * height;
+	uint32_t *buf = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fb, 0);
+	if (buf == MAP_FAILED) {
+		fprintf(stderr, "couldn't mmap the framebuffer\n");
+		exit(EXIT_FAILURE);
+	}
+	uint8_t shade = 0;
+	while (true) {
+		for (size_t j = 0; j < height; j++) {
+			for (size_t i = 0; i < width; i++) {
+				buf[j * width + i] = shade;
+			}
+		}
+		shade++;
+	}
 	return 0;
 }