.\" Copyright (C), 1994, Graeme W. Wilford (Wilf).
.\" and Copyright (C) 2010, 2014, 2015, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Fri Jul 29th 12:56:44 BST 1994  Wilf. <G.Wilford@ee.surrey.ac.uk>
.\" Changes inspired by patch from Richard Kettlewell
.\"   <richard@greenend.org.uk>, aeb 970616.
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.TH setuid 2 2023-03-30 "Linux man-pages 6.05.01"
.SH NAME
setuid \- set user identity
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int setuid(uid_t " uid );
.fi
.SH DESCRIPTION
.BR setuid ()
sets the effective user ID of the calling process.
If the calling process is privileged
(more precisely: if the process has the
.B CAP_SETUID
capability in its user namespace),
the real UID and saved set-user-ID are also set.
.PP
Under Linux,
.BR setuid ()
is implemented like the POSIX version with the
.B _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.
.PP
If the user is root or the program is set-user-ID-root, special care must be
taken:
.BR setuid ()
checks the effective user ID of the caller and if it is
the superuser, all process-related user ID's are set to
.IR uid .
After this has occurred, it is impossible for the program to regain root
privileges.
.PP
Thus, a set-user-ID-root program wishing to temporarily drop root
privileges, assume the identity of an unprivileged user, and then regain
root privileges afterward cannot use
.BR setuid ().
You can accomplish this with
.BR seteuid (2).
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.PP
.IR Note :
there are cases where
.BR setuid ()
can fail even when the caller is UID 0;
it is a grave security error to omit checking for a failure return from
.BR setuid ().
.SH ERRORS
.TP
.B EAGAIN
The call would change the caller's real UID (i.e.,
.I uid
does not match the caller's real UID),
but there was a temporary failure allocating the
necessary kernel data structures.
.TP
.B EAGAIN
.I 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
.I uid
over the caller's
.B 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
.B EAGAIN
in
.BR execve (2).
.TP
.B EINVAL
The user ID specified in
.I uid
is not valid in this user namespace.
.TP
.B EPERM
The user is not privileged (Linux: does not have the
.B CAP_SETUID
capability in its user namespace) and
.I uid
does not match the real UID or saved set-user-ID of the calling process.
.SH VERSIONS
.SS 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
.BR 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
.BR nptl (7).
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, SVr4.
.PP
Not quite compatible with the 4.4BSD call, which
sets all of the real, saved, and effective user IDs.
.\" SVr4 documents an additional EINVAL error condition.
.PP
The original Linux
.BR setuid ()
system call supported only 16-bit user IDs.
Subsequently, Linux 2.4 added
.BR setuid32 ()
supporting 32-bit IDs.
The glibc
.BR setuid ()
wrapper function transparently deals with the variation across kernel versions.
.SH NOTES
Linux has the concept of the filesystem user ID, normally equal to the
effective user ID.
The
.BR setuid ()
call also sets the filesystem user ID of the calling process.
See
.BR setfsuid (2).
.PP
If
.I uid
is different from the old effective UID, the process will
be forbidden from leaving core dumps.
.SH SEE ALSO
.BR getuid (2),
.BR seteuid (2),
.BR setfsuid (2),
.BR setreuid (2),
.BR capabilities (7),
.BR credentials (7),
.BR user_namespaces (7)