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+.\" Copyright (c) 1980, 1991, 1993, 1994
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
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+.\"      @(#)setreuid.2	8.2 (Berkeley) 4/16/94
+.\"
+.Dd January 29, 2003
+.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.
+If the real user ID is changed, or the effective user ID is changed
+to a value other than the real user ID, then the saved user ID will
+be set to the effective 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 -p1003.1-01
+and
+.St -xpg4.3 .
+specifications.
+.Pp
+Note, however, that prior to
+.St -p1003.1-01 ,
+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 effective user ID if the real user ID is changed.
+Other implementations may behave differently.