redhat man page for setegid

SETEUID(2)						     Linux Programmer's Manual							SETEUID(2)

NAME
       seteuid, setegid - set effective user or group ID

SYNOPSIS
       #include <sys/types.h>
       #include <unistd.h>

       int seteuid(uid_t euid);
       int setegid(gid_t egid);

DESCRIPTION
       seteuid sets the effective user ID of the current process.  Unprivileged user processes may only set the effective user ID to the real user
       ID, the effective user ID or the saved user ID.

       Precisely the same holds for setegid with "group" instead of "user".

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       EPERM  The current process is not the super-user and euid (resp.  egid) is not the real user (group) ID, the effective user (group)  ID	or
	      the saved user (group) ID.

NOTES
       Setting	the  effective	user  (group) ID to the saved user (group) ID is possible since Linux 1.1.37 (1.1.38).	On an arbitrary system one
       should check _POSIX_SAVED_IDS.

       Under libc4, libc5 and glibc2.0 seteuid(euid) is equivalent to setreuid(-1, euid) and hence may change the saved user ID.   Under  glibc2.1
       it is equivalent to setresuid(-1, euid,-1) and hence does not change the saved user ID.	Similar remarks hold for setegid.

CONFORMING TO
       BSD 4.3

SEE ALSO
       geteuid(2), setuid(2), setreuid(2), setresuid(2)

Linux 2.4							    2001-05-17								SETEUID(2)