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 feature_test_macros(7)
_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., ruid
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.
Completely analogously, setregid
() sets real and effective group ID's of
the calling process, and all of the above holds with "group" instead
On success, zero is returned. On error, -1 is returned, and errno
: there are cases where setreuid
() can fail even when the
caller is UID 0; it is a grave security error to omit checking for a failure
return from setreuid
- The call would change the caller's real UID (i.e.,
ruid does not match the caller's real UID), but there was a
temporary failure allocating the necessary kernel data structures.
- ruid does not match the caller's real UID and this
call would bring the number of processes belonging to the real user ID
ruid over the caller's RLIMIT_NPROC resource limit. Since
Linux 3.1, this error case no longer occurs (but robust applications
should check for this error); see the description of EAGAIN in
- 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: CAP_SETUID in
the case of setreuid(), or CAP_SETGID in the case of
setregid()) 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 (setreuid
() and setregid
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. For setreuid
(), 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 saved set-user-ID. For setregid
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 setreuid
() and setregid
() system calls
supported only 16-bit user and group IDs. Subsequently, Linux 2.4 added
() and setregid32
(), supporting 32-bit IDs. The glibc
() and setregid
() wrapper functions transparently deal
with the variations across kernel versions.
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 setreuid
()) 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 nptl(7)
This page is part of release 4.16 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at