output

start: 0 0
uid: 0 0
uid: 0 0
ret: 1000 1000
uid: 1000 1000
uid: 1000 1000
uid: 1000 1000
uid: 1000 1000
uid: 1000 1000
uid: 1000 1000
uid: 1000 1000

test-setuid.c

#include <sys/types.h>
#include <pthread.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void* func(void* arg)
{
	while(1){
		printf("uid: %d %d\n", getuid(), geteuid());
		sleep(1);
	}

	return NULL;
}

int main()
{
	pthread_t thr;
	int err;

	printf("start: %d %d\n",getuid(), geteuid());

	err = pthread_create(&thr, NULL, func, NULL);

	sleep(1);
	err = setuid(1000);
	if (err != 0)
		perror("setuid"), exit(1);

	printf("ret: %d %d\n",getuid(), geteuid());

	pause();
	exit(0);
}