Section: Linux Programmer's Manual (2)
setreuid, setregid - set real and/or effective user or group ID
int setreuid(uid_t ruid, uid_t euid);
int setregid(gid_t rgid, gid_t egid);
Feature Test Macro Requirements for glibc (see
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE
sets real and effective user IDs of the calling process.
Supplying a value of -1 for either the real or effective user ID forces
the system to leave that ID unchanged.
Unprivileged processes may only set the effective user ID to the real user ID,
the effective user ID, or the saved set-user-ID.
Unprivileged users may only set the real user ID to
the real user ID or the effective user ID.
If the real user ID is set (i.e.,
is not -1) or the effective user ID is set to a value
not equal to the previous real user ID,
the saved set-user-ID will be set to the new effective user ID.
sets real and effective group ID's of the calling process,
and all of the above holds with "group" instead of "user".
On success, zero is returned.
On error, -1 is returned, and
is set appropriately.
there are cases where
can fail even when the caller is UID 0;
it is a grave security error to omit checking for a failure return from
The call would change the caller's real UID (i.e.,
does not match the caller's real UID),
but there was a temporary failure allocating the
necessary kernel data structures.
does not match the caller's real UID and this call would
bring the number of processes belonging to the real user ID
over the caller's
Since Linux 3.1, this error case no longer occurs
(but robust applications should check for this error);
see the description of
One or more of the target user or group IDs
is not valid in this user namespace.
The calling process is not privileged
(on Linux, does not have the necessary capability in its user namespace:
in the case of
in the case of
and a change other than (i)
swapping the effective user (group) ID with the real user (group) ID,
or (ii) setting one to the value of the other or (iii) setting the
effective user (group) ID to the value of the
saved set-user-ID (saved set-group-ID) was specified.
POSIX.1-2001, POSIX.1-2008, 4.3BSD
first appeared in 4.2BSD).
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).
POSIX.1 does not specify all of the UID changes that Linux permits
for an unprivileged process.
the effective user ID can be made the same as the
real user ID or the saved set-user-ID,
and it is unspecified whether unprivileged processes may set the
real user ID to the real user ID, the effective user ID, or the
the real group ID can be changed to the value of the saved set-group-ID,
and the effective group ID can be changed to the value of
the real group ID or the saved set-group-ID.
The precise details of what ID changes are permitted vary
POSIX.1 makes no specification about the effect of these calls
on the saved set-user-ID and saved set-group-ID.
The original Linux
system calls supported only 16-bit user and group IDs.
Subsequently, Linux 2.4 added
supporting 32-bit IDs.
wrapper functions transparently deal with the variations across kernel versions.
C library/kernel differences
At the kernel level, user IDs and group IDs are a per-thread attribute.
However, POSIX requires that all threads in a process
share the same credentials.
The NPTL threading implementation handles the POSIX requirements by
providing wrapper functions for
the various system calls that change process UIDs and GIDs.
These wrapper functions (including those for
employ a signal-based technique to ensure
that when one thread changes credentials,
all of the other threads in the process also change their credentials.
For details, see
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