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 group IDs
int setregid(gid_t rgid, gid_t egid);
function shall set the real and effective group IDs of the calling
is -1, the real group ID shall not be changed; if
is -1, the effective group ID shall not be changed.
The real and effective group IDs may be set to different values in the
Only a process with appropriate privileges can set the real group ID
and the effective group ID to any valid value.
A non-privileged process can set either the real group ID to the saved
set-group-ID from one of the
family of functions, or the effective group ID to the saved
set-group-ID or the real group ID.
If the real group ID is being set (rgid
is not -1), or the effective group ID is being set to a value not
equal to the real group ID, then the saved set-group-ID of the current
process shall be set equal to the new effective group ID.
Any supplementary group IDs of the calling process remain unchanged.
Upon successful completion, 0 shall be returned. Otherwise, -1
shall be returned and
set to indicate the error, and neither of the group IDs are changed.
function shall fail if:
The value of the
argument is invalid or out-of-range.
The process does not have appropriate privileges and a change other
than changing the real group ID to the saved set-group-ID, or changing
the effective group ID to the real group ID or the saved set-group-ID,
The following sections are informative.
If a non-privileged set-group-ID process sets its effective group ID to
its real group ID, it can only set its effective group ID back to the
previous value if
was -1 in the
call, since the saved-group-ID is not changed in that case. If
was equal to the real group ID in the
call, then the saved set-group-ID will also have been changed to the
real user ID.
Earlier versions of this standard did not specify whether the saved
set-group-ID was affected by
calls. This version specifies common existing practice that constitutes an
important security feature. The ability to set both the effective group
ID and saved set-group-ID to be the same as the real group 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
group 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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