https://imgur.com/a/y4mGB

sleepforever.c

#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <pwd.h>

#define EMPTYLINE() printf("\n");

void
sleep_forever()
{
	for (;;)
		sleep(1);
}

void
show_privileges()
{
	printf("UID   EUID  GID   EGID\n");
	printf("%-5d %-5d %-5d %-5d\n", getuid(), geteuid(), getgid(), getegid());
}

/*
 * returns 1 when some privileges may have been dropped, 0 on failure
 */
int
drop_privileges()
{
	struct passwd *pw;

	pw = getpwuid(getuid());
	if (getegid() != pw->pw_gid || geteuid() != pw->pw_uid) {
		if ((getegid() != pw->pw_gid && setgid(pw->pw_gid) < 0) ||
		    setuid(pw->pw_uid) < 0) {
			printf("Failed to drop privileges\n");
		} else {
			printf("Dropped privileges...\n");
		}
		return 1;
	} else {
		printf("Not running as different user, not dropping privileges.\n");
		return 0;
	}
}

int
main(int argc, char *argv[])
{
	printf("Starting sleepforever (PID %d)...\n", getpid());
	EMPTYLINE();
	show_privileges();
	EMPTYLINE();
	if (drop_privileges()) {
		EMPTYLINE();
		show_privileges();
	}

	sleep_forever();

	/* Not reached */
}