Use exit define

Assignment9/src/main.c

@@ -20,7 +20,7 @@ int main(int argc, char *argv[]) {
 	char *png_buffer = load_file(get_fd(path));
 	if (!is_png(png_buffer)) {
 		printf("It's not a PNG file\n");
-		exit(1);
+		exit(EXIT_FAILURE);
 	} else {
 		printf("It's a PNG file\n");
 	}