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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_BSD_SOURCE || _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
seteuid() sets the effective user ID of the calling process. Unprivi-
leged user processes may only set the effective user ID to the real
user ID, the effective user ID or the saved set-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
Note: there are cases where seteuid() can fail even when the caller is
UID 0; it is a grave security error to omit checking for a failure
return from seteuid().
EINVAL The target user or group ID is not valid in this user namespace.
EPERM The calling process is not privileged (Linux: does not have the
CAP_SETUID capability in the case of seteuid(), or the CAP_SET-
GID capability in the case of setegid()) and euid (respectively,
egid) is not the real user (group) ID, the effective user
(group) ID, or the saved set-user-ID (saved set-group-ID).
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
Setting the effective user (group) ID to the saved set-user-ID (saved
set-group-ID) is possible since Linux 1.1.37 (1.1.38). On an arbitrary
system one should check _POSIX_SAVED_IDS.
Under glibc 2.0 seteuid(euid) is equivalent to setreuid(-1, euid) and
hence may change the saved set-user-ID. Under glibc 2.1 and later it
is equivalent to setresuid(-1, euid, -1) and hence does not change the
saved set-user-ID. Analogous remarks hold for setegid(), with the dif-
ference that the change in implementation from setregid(-1, egid) to
setresgid(-1, egid, -1) occurred in glibc 2.2 or 2.3 (depending on the
According to POSIX.1, seteuid() (setegid()) need not permit euid (egid)
to be the same value as the current effective user (group) ID, and some
implementations do not permit this.
C library/kernel differences
On Linux, seteuid() and setegid() are implemented as library functions
that call, respectively, setreuid(2) and setregid(2).
geteuid(2), setresuid(2), setreuid(2), setuid(2), capabilities(7), cre-
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Linux 2015-07-23 SETEUID(2)
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