[icy@planetfall sir_lewk]$ ./test
UID: R: 500 E: 500 GID:R:500 E: 500
[icy@planetfall sir_lewk]$ ls -l test
-rwxrwxr-x 1 icy icy 7020 Sep  7 16:47 test
[icy@planetfall sir_lewk]$ chmod u+s test
[icy@planetfall sir_lewk]$ ls -l test
-rwsrwxr-x 1 icy icy 7020 Sep  7 16:47 test
[root@planetfall sir_lewk]# ./test
UID: R: 0 E: 500 GID:R:0 E: 0
[icy@planetfall sir_lewk]$ cat test.c
#include<unistd.h>
main(){printf("UID: R: %d E: %d GID:R:%d E: %d\n",getuid(),geteuid(),getgid(),getegid());}