setuid, setgid - set user / group ID
#include <unistd.h>
int setuid(uid_t);
int setgid(gid_t);Sets all of real, effective, and saved user or group ID to the given ID.
For non-superusers, the given ID has to be equal to the current real or effective ID for the call to succeed.
If the call was set successful, returns 0. Otherwise, returns -1 and sets errno to describe the error.
EPERM: The new ID is not equal to the real ID or effective ID, and the user is not superuser.