The setreuid() function shall set the real and effective user IDs of the
current process to the values specified by the ruid and euid
arguments. If ruid or euid is -1, the corresponding effective or
real user ID of the current process shall be left unchanged.
A process with appropriate privileges can set either ID to any value. An
unprivileged process can only set the effective user ID if the euid
argument is equal to either the real, effective, or saved user ID of the
It is unspecified whether a process without appropriate privileges is permitted
to change the real user ID to match the current real, effective, or saved
set-user-ID of the process.
The value of the ruid or euid argument is
invalid or out-of-range.
The current process does not have appropriate privileges,
and either an attempt was made to change the effective user ID to a value
other than the real user ID or the saved set-user-ID or an attempt was
made to change the real user ID to a value not permitted by the
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html