Section: POSIX Programmer's Manual (3P)
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
--- set real and effective user IDs
int setreuid(uid_t ruid, uid_t euid);
function shall set the real and effective user IDs of the current
process to the values specified by the
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
argument is equal to either the real, effective, or saved user ID of
If the real user ID is being set (ruid
is not -1), or the effective user ID is being set to a value not
equal to the real user ID, then the saved set-user-ID of the current
process shall be set equal to the new effective user ID.
It is unspecified whether a process without appropriate privileges is
permitted to change the real user ID to match the current effective user
ID or saved set-user-ID of the process.
Upon successful completion, 0 shall be returned. Otherwise, -1
shall be returned and
set to indicate the error.
function shall fail if:
The value of the
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
The following sections are informative.
Setting the Effective User ID to the Real User ID
The following example sets the effective user ID of the calling process
to the real user ID, so that files created later will be owned by the
current user. It also sets the saved set-user-ID to the real user ID,
so any future attempt to set the effective user ID back to its previous
value will fail.
Earlier versions of this standard did not specify whether the saved
set-user-ID was affected by
calls. This version specifies common existing practice that constitutes
an important security feature. The ability to set both the effective user
ID and saved set-user-ID to be the same as the real user ID means that
any security weakness in code that is executed after that point cannot
result in malicious code being executed with the previous effective user
ID. Privileged applications could already do this using just
but for non-privileged applications the only standard method available
is to use this feature of
The Base Definitions volume of POSIX.1-2008,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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
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