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.\"      @(#)setreuid.2	8.2 (Berkeley) 4/16/94
.\"
.Dd $Mdocdate: May 31 2007 $
.Dt SETREUID 2
.Os
.Sh NAME
.Nm setreuid
.Nd set real and effective user IDs
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn setreuid "uid_t ruid" "uid_t euid"
.Sh DESCRIPTION
The real and effective user IDs of the
current process are set according to the arguments.
The saved user ID will be set to the new value of the real
user ID if a real user ID is specified and either
the new real user ID value is different from the current value
or the effective user ID is changed to a value other than the
current saved user ID.
.Pp
Unprivileged users may change either user ID to the current value
of the real, effective, or saved user ID.
Only the superuser may make other changes.
.Pp
Supplying a value of -1 for either the real or effective
user ID forces the system to substitute the current
ID in place of the -1 parameter.
.Pp
The
.Fn setreuid
function was intended to allow swapping the real and
effective user IDs in set-user-ID programs to temporarily relinquish
the set-user-ID value.
This purpose is now better served by the use of the
.Fn seteuid
function (see
.Xr setuid 2 ) .
.Pp
When setting the real and effective user IDs to the same value, the
.Fn setuid
function is preferred.
.Sh RETURN VALUES
Upon successful completion, a value of 0 is returned.
Otherwise, a value of -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EPERM
The current process is not the superuser and a change
other than changing the effective user ID to the real user ID
was specified.
.El
.Sh SEE ALSO
.Xr getuid 2 ,
.Xr seteuid 2 ,
.Xr setresuid 2 ,
.Xr setuid 2
.Sh STANDARDS
The
.Fn setreuid
function conforms to the
.St -p1003.1-2001
and
.St -xpg4.3 .
specifications.
.Pp
Note, however, that prior to
.St -p1003.1-2001 ,
the
.Fn setreuid
function was not a part of the
.St -p1003.1
specification.
As a result, it may not be implemented on all systems.
.Sh HISTORY
The
.Fn setreuid
function call appeared in
.Bx 4.2 .
A semantically different version appeared in
.Bx 4.4 .
The current version, with the original semantics restored, appeared in
.Ox 3.3 .
.Sh CAVEATS
The
.Fn setreuid
function predates
.Tn POSIX
saved user IDs.
This implementation changes the saved user ID to the new value of
the real user ID only if the real user ID is specified and either
changed
or the effective user ID is changed to a value other than the current
saved user ID.
Other implementations may behave differently.