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GETGROUPS(3P) POSIX Programmer's Manual GETGROUPS(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.
getgroups — get supplementary group IDs
#include <unistd.h>
int getgroups(int gidsetsize, gid_t grouplist[]);
The getgroups() function shall fill in the array grouplist with the
current supplementary group IDs of the calling process. It is
implementation-defined whether getgroups() also returns the effective
group ID in the grouplist array.
The gidsetsize argument specifies the number of elements in the array
grouplist. 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.
If gidsetsize is 0, getgroups() shall return the number of group IDs
that it would otherwise return without modifying the array pointed to
by grouplist.
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
{NGROUPS_MAX}+1.
Upon successful completion, the number of supplementary group IDs
shall be returned. A return value of −1 indicates failure and errno
shall be set to indicate the error.
The getgroups() function shall fail if:
EINVAL The gidsetsize 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 group array.
#include <sys/types.h>
#include <unistd.h>
...
gid_t *group;
int nogroups;
long ngroups_max;
ngroups_max = sysconf(_SC_NGROUPS_MAX) + 1;
group = (gid_t *)malloc(ngroups_max *sizeof(gid_t));
ngroups = getgroups(ngroups_max, group);
None.
The related function setgroups() is a privileged operation and
therefore is not covered by this volume of POSIX.1‐2008.
As implied by the definition of supplementary groups, the effective
group ID may appear in the array returned by getgroups() or it may be
returned only by getegid(). Duplication may exist, but the
application needs to call getegid() 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
getgroups() 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 {NGROUPS_MAX} is not
necessarily a constant on all implementations.
None.
getegid(3p), setgid(3p)
The Base Definitions volume of POSIX.1‐2008, sys_types.h(0p),
unistd.h(0p)
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 http://www.unix.org/online.html .
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
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 GETGROUPS(3P)
Pages that refer to this page: unistd.h(0p), id(1p), open(3p)