SETEUID(2) Linux Programmer's Manual SETEUID(2)
seteuid, setegid - set effective user or group ID
int seteuid(uid_t euid);
int setegid(gid_t egid);
seteuid sets the effective user ID of the current process. Unprivi-
leged 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".
On success, zero is returned. On error, -1 is returned, and errno is
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.
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.
geteuid(2), setuid(2), setreuid(2), setresuid(2)
Linux 2.4 2001-05-17 SETEUID(2)