Section: Linux Programmer's Manual (2)
setresuid, setresgid - set real, effective and saved user or group ID
/* See feature_test_macros
int setresuid(uid_t ruid, uid_t euid, uid_t suid);
int setresgid(gid_t rgid, gid_t egid, gid_t sgid);
sets the real user ID, the effective user ID, and the
saved set-user-ID of the calling process.
An unprivileged process may change its real UID,
effective UID, and saved set-user-ID, each to one of:
the current real UID, the current effective UID or the
current saved set-user-ID.
A privileged process (on Linux, one having the CAP_SETUID capability)
may set its real UID, effective UID, and
saved set-user-ID to arbitrary values.
If one of the arguments equals -1, the corresponding value is not changed.
Regardless of what changes are made to the real UID, effective UID,
and saved set-user-ID, the filesystem UID is always set to the same
value as the (possibly new) effective UID.
sets the real GID, effective GID, and saved set-group-ID
of the calling process (and always modifies the filesystem GID
to be the same as the effective GID),
with the same restrictions for unprivileged processes.
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 (did not have the necessary
capability in its user namespace)
and tried to change the IDs to values that are not permitted.
the necessary capability is
These calls are available under Linux since Linux 2.1.44.
These calls are nonstandard;
they also appear on HP-UX and some of the BSDs.
Under HP-UX and FreeBSD, the prototype is found in
Under Linux, the prototype is provided by glibc since version 2.3.2.
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
This page is part of release 5.04 of the Linux
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