Drop daemon privilegies

[Brilliantov Kirill Vladimirovich]

Hello!
I have use busybox-1.18.4 and I try run daemon on start system with 
minimal privilegies.

Below my code:
#define ERROR(fmt, args...)	syslog(LOG_ERR, fmt ": %m", ##args)
int8_t drop_root(const char *user)
{
	ERROR("Run %s, user %s", __func__, user);
	struct passwd *pwd = NULL;

	if (NULL == (pwd = getpwnam(user)))
		return -1;
	ERROR("After getpwnam, uid = %d, gid = %d", pwd->pw_uid, pwd->pw_gid);

	if (setgid(pwd->pw_gid)) {
		//ERROR("After setGID");
		return -1;
	}
	ERROR("After setGID");

	if (setuid(pwd->pw_uid)) {
		//ERROR("After setUID");
		return -1;
	}
	ERROR("After setUID");

	return 0;
}

/etc/init.d/monitor.sh:
PID=/var/run/monitd.pid
DAEMON=monitd
CONFIG=/etc/monitor
USER=monitor
start()
{
     start-stop-daemon --quiet --pidfile $PID --background \
         -S --exec $DAEMON -- --config $CONFIG --user $USER
}

~ # grep monitor /etc/passwd
monitor:x:100:100:Monitoring daemon:/home/monitor:/bin/false
# grep monitor /etc/group
monitor:x:100:monitor

If I start daemon manual all OK, but if starting daemon with system - 
daemon not running.
Below output in console:
monitor: Run drop_root, user monitor: Success
monitor: After getpwnam, uid = 100, gid = 100: Success
monitor: After setGID: Illegal seek

I try use seteuid and setegid, but it not solve problem.

How can I solve this problem - drop root privilegies for daemon on 
starting daemon with system?

Thank you and excuse me for my bad english.

-- 
Best regards,
Brilliantov Kirill Vladimirovich