Section (2) setuid
setuid — set user identity
#include <sys/types.h> #include <unistd.h>
setuid() sets the effective
user ID of the calling process. If the calling process is
privileged (more precisely: if the process has the
CAP_SETUID capability in its
user namespace), the real UID and saved set-user-ID are also
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:
setuid() checks the effective user ID of
the caller and if it is the superuser, all process-related
user ID_zsingle_quotesz_s are set to
uid. After this has occurred,
it is impossible for the program to regain root
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
setuid(). You can accomplish
this with seteuid(2).
On success, zero is returned. On error, −1 is
errno is set
there are cases where
The call would change the caller_zsingle_quotesz_s real UID (i.e.,
uiddoes not match the caller_zsingle_quotesz_s real UID), but there was a temporary failure allocating the necessary kernel data structures.
uiddoes not match the real user ID of the caller and this call would bring the number of processes belonging to the real user ID
uidover the caller_zsingle_quotesz_s
RLIMIT_NPROCresource 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 execve(2).
The user ID specified in
uidis not valid in this user namespace.
The user is not privileged (Linux: does not have the
CAP_SETUIDcapability in its user namespace) and
uiddoes not match the real UID or saved set-user-ID of the calling process.
POSIX.1-2001, POSIX.1-2008, SVr4. Not quite compatible with the 4.4BSD call, which sets all of the real, saved, and effective user IDs.
Linux has the concept of the filesystem user ID, normally
equal to the effective user ID. The
setuid() call also sets the filesystem user
ID of the calling process. See setfsuid(2).
different from the old effective UID, the process will be
forbidden from leaving core dumps.
The original Linux
system call supported only 16-bit user IDs. Subsequently,
Linux 2.4 added
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 nptl(7).
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Copyright (C), 1994, Graeme W. Wilford (Wilf).
and Copyright (C) 2010, 2014, 2015, Michael Kerrisk <mtk.manpagesgmail.com>
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Fri Jul 29th 12:56:44 BST 1994 Wilf. <G.Wilfordee.surrey.ac.uk>
Changes inspired by patch from Richard Kettlewell
<richardgreenend.org.uk>, aeb 970616.
Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com>
Added notes on capability requirements