man-pages/man2/setuid.2

.\" Copyright (C), 1994, Graeme W. Wilford (Wilf).
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.\" 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 2010-02-21 "Linux" "Linux Programmer's Manual"
.SH NAME
setuid \- set user identity
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.sp
.BI "int setuid(uid_t " uid );
.SH DESCRIPTION
.BR 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.
.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.
The
.BR 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
.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 afterwards 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 appropriately.
.SH ERRORS
.TP
.B EAGAIN
The
.I uid
does not match the current uid and
.I uid
brings process over its
.B RLIMIT_NPROC
resource limit.
.TP
.B EPERM
The user is not privileged (Linux: does not have the
.B CAP_SETUID
capability) and
.I uid
does not match the real UID or saved set-user-ID of the calling process.
.SH "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.
.\" SVr4 documents an additional EINVAL error condition.
.SH NOTES
.SS Linux Notes
Linux has the concept of file system user ID, normally equal to the
effective user ID.
The
.BR setuid ()
call also sets the file system 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)