Example of how to use setuid() and setgid() under Linux in C.

linux-setuid.c

/*
	Created @ 03.02.2014
	Copyright (C) 2014 Christian Mayer <http://fox21.at>
	
	This program is free software: you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation, either version 3 of the License, or
	(at your option) any later version.
	
	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
	GNU General Public License for more details.
	
	You should have received a copy of the GNU General Public License
	along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/
	
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int main(){
	printf("start\n");
	
	printf("sleep\n"); usleep(10 * 1000 * 1000);
	
	uid_t uid = 1000; // user
	gid_t gid = 100;  // users
	
	printf("setuid: %d\n", uid);
	setuid(uid);
	
	printf("sleep\n"); usleep(10 * 1000 * 1000);
	
	printf("setgid: %d\n", gid);
	setgid(gid);
	
	printf("exit\n");
}