[root@antergos tmp]# ls -l test
-rwxr-xr-x 1 dolly users 16640 Aug 19 19:15 test
[root@antergos tmp]# chown root:root test
[root@antergos tmp]# chmod u+s test
[root@antergos tmp]# ls -l test
-rwsr-xr-x 1 root root 16640 Aug 19 19:15 test
[root@antergos tmp]# ./test
Real UID: 0
Effective UID: 0
[root@antergos tmp]# exit
logout
[dolly@antergos tmp]$ ./test
Real UID: 1000
Effective UID: 1000
[dolly@antergos tmp]$ cat test.c
#include <stdio.h>
#include <unistd.h>

int main() {
    printf("Real UID: %d\n", getuid());
    printf("Effective UID: %d\n", geteuid());
    return 0;
}