int setuid(uid_t uid);
setuid() sets the effective user ID of the calling process. If the
effective UID of the caller is root (more precisely: if the caller has
the CAP_SETUID capability), the real UID and saved set-user-ID are also
Under Linux, setuid() is implemented like the POSIX version with the
_POSIX_SAVED_IDS feature. This allows a set-user-ID (other than root)
program to drop all of its user privileges, do some un-privileged work,
and then reengage the original effective user ID in a secure manner.
If the user is root or the program is set-user-ID-root, special care
must be taken. The setuid() function checks the effective user ID of
the caller and if it is the superuser, all process-related user ID's
are set to uid. After this has occurred, it is impossible for the pro-
gram to regain root privileges.
Thus, a set-user-ID-root program wishing to temporarily drop root priv-
ileges, assume the identity of an unprivileged user, and then regain
root privileges afterward cannot use setuid(). You can accomplish this
On success, zero is returned. On error, -1 is returned, and errno is
Note: there are cases where setuid() can fail even when the caller is
UID 0; it is a grave security error to omit checking for a failure
return from setuid().
EAGAIN The call would change the caller's real UID (i.e., uid does not
match the caller's real UID), but there was a temporary failure
allocating the necessary kernel data structures.
EAGAIN uid does not match the real user ID of the caller and this call
would bring the number of processes belonging to the real user
ID uid over the caller's RLIMIT_NPROC resource limit. Since
Linux 3.1, this error case no longer occurs (but robust applica-
tions should check for this error); see the description of
EAGAIN in execve(2).
EINVAL The user ID specified in uid is not valid in this user names-
EPERM The user is not privileged (Linux: does not have the CAP_SETUID
capability) and uid does not match the real UID or saved set-
user-ID of the calling process.
The original Linux setuid() system call supported only 16-bit user IDs.
Subsequently, Linux 2.4 added setuid32() supporting 32-bit IDs. The
glibc setuid() wrapper function transparently deals with the variation
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 the one for setuid()) 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
getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7), cre-
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Linux 2015-07-23 SETUID(2)
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