Name

setuid — set user identity

Synopsis

#include <sys/types.h>
#include <unistd.h>
int setuid( uid_t uid);
 

DESCRIPTION

setuid() sets the effective user ID of the calling process. If the effective UID of the caller is root, the real UID and saved set-user-ID are also set.

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 program to regain root privileges.

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).

RETURN VALUE

On success, zero is returned. On error, −1 is returned, and errno is set appropriately.

ERRORS

EAGAIN

The uid does not match the current uid and uid brings process over its RLIMIT_NPROC resource limit.

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.

CONFORMING TO

SVr4, POSIX.1-2001. Not quite compatible with the 4.4BSD call, which sets all of the real, saved, and effective user IDs.

NOTES

Linux has the concept of the file system user ID, normally equal to the effective user ID. The setuid() call also sets the file system user ID of the calling process. See setfsuid(2).

If uid is different from the old effective UID, the process will be forbidden from leaving core dumps.

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.

SEE ALSO

getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7), credentials(7)

COLOPHON

This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man−pages/.


  Copyright (C), 1994, Graeme W. Wilford (Wilf).

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

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