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.
--- get supplementary group IDs
int getgroups(int gidsetsize, gid_t grouplist);
function shall fill in the array
with the current supplementary group IDs of the calling process. It is
also returns the effective group ID in the
argument specifies the number of elements in the array
The actual number of group IDs stored in the array shall be returned.
The values of array entries with indices greater than or equal to the
value returned are undefined.
shall return the number of group IDs that it would otherwise return
without modifying the array pointed to by
If the effective group ID of the process is returned with the
supplementary group IDs, the value returned shall always be greater
than or equal to one and less than or equal to the value of
Upon successful completion, the number of supplementary group IDs shall
be returned. A return value of -1 indicates failure and
shall be set to indicate the error.
function shall fail if:
argument is non-zero and less than the number of group IDs that would
have been returned.
The following sections are informative.
Getting the Supplementary Group IDs of the Calling Process
The following example places the current supplementary group IDs of the
calling process into the
ngroups_max = sysconf(_SC_NGROUPS_MAX) + 1;
group = (gid_t *)malloc(ngroups_max *sizeof(gid_t));
ngroups = getgroups(ngroups_max, group);
The related function
is a privileged operation and therefore is not covered by this volume of POSIX.1-2017.
As implied by the definition of supplementary groups, the effective
may appear in the array returned by
or it may be returned only by
Duplication may exist, but the application needs to call
to be sure of getting all of the information. Various implementation
variations and administrative sequences cause the set of groups
appearing in the result of
to vary in order and as to whether the effective group ID is included,
even when the set of groups is the same (in the mathematical sense of
``set''). (The history of a process and its parents could affect the
details of the result.)
Application developers should note that
is not necessarily a constant on all implementations.
The Base Definitions volume of POSIX.1-2017,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see