init.c

#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <sys/reboot.h>
#include <linux/reboot.h>
#include <sys/syscall.h>

int main() {
  const char *msg =
    "Hey! I already woke up and I am saying hello to you... and I am shutting down now.\n";
  write(1, msg, 94);

  // Sync + poweroff (best effort)
  sync();

  // Try reboot syscall directly (works even without /sbin/poweroff)
  syscall(SYS_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
          LINUX_REBOOT_CMD_POWER_OFF, 0);

  // If poweroff didn't work for some reason, exit anyway.
  return 0;
}