.\" 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@gmx.net>
.\"     Added notes on capability requirements
.TH SETUID 2 2004-05-27 "Linux 2.6.6" "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
.B setuid
sets the effective user ID of the current process.
If the effective userid of the caller is root,
the real and saved user ID's are also set.
.PP
Under Linux, 
.B 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 re-engage 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 
.B 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 a non-root user, and then regain
root privileges afterwards cannot use 
.BR setuid .
You can accomplish this with the (non-POSIX, BSD) call
.BR seteuid .
.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 it's NPROC rlimit.
.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 or saved user ID of the calling process.
.SH "CONFORMING TO"
SVr4, SVID, POSIX.1.  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 "LINUX-SPECIFIC REMARKS"
Linux has the concept of filesystem user ID, normally equal to the
effective user ID.  The
.B setuid
call also sets the filesystem user ID of the current 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)