SETRESUID(2) Linux Programmer's Manual SETRESUID(2)
setresuid, setresgid - set real, effective and saved user or group ID
#define _GNU_SOURCE /* See feature_test_macros(7) */
int setresuid(uid_t ruid, uid_t euid, uid_t suid);
int setresgid(gid_t rgid, gid_t egid, gid_t sgid);
setresuid() 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
If one of the arguments equals -1, the corresponding value is not
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.
Completely analogously, setresgid() 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 re-
strictions for unprivileged processes.
On success, zero is returned. On error, -1 is returned, and errno is
Note: there are cases where setresuid() can fail even when the caller
is UID 0; it is a grave security error to omit checking for a failure
return from setresuid().
EAGAIN 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.
EAGAIN 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 ex-
EINVAL One or more of the target user or group IDs is not valid in this
EPERM The calling process is not privileged (did not have the neces-
sary capability in its user namespace) and tried to change the
IDs to values that are not permitted. For setresuid(), the nec-
essary capability is CAP_SETUID; for setresgid(), it is CAP_SET-
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
Under HP-UX and FreeBSD, the prototype is found in <unistd.h>. Under
Linux, the prototype is provided by glibc since version 2.3.2.
The original Linux setresuid() and setresgid() system calls supported
only 16-bit user and group IDs. Subsequently, Linux 2.4 added setre-
suid32() and setresgid32(), supporting 32-bit IDs. The glibc setre-
suid() and setresgid() 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 re-
quirements by providing wrapper functions for the various system calls
that change process UIDs and GIDs. These wrapper functions (including
those for setresuid() and setresgid()) 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
getresuid(2), getuid(2), setfsgid(2), setfsuid(2), setreuid(2), se-
tuid(2), capabilities(7), credentials(7), user_namespaces(7)
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Linux 2017-09-15 SETRESUID(2)
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