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.\" Modified Thu Oct 31 12:04:29 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.\" 2008-05-03, mtk, expanded and rewrote parts of DESCRIPTION and RETURN
.\"     VALUE, made style of page more consistent with man-pages style.
.\"
.TH GETGROUPS 2 2014-08-19 "Linux" "Linux Programmer's Manual"
.SH NAME
getgroups, setgroups \- get/set list of supplementary group IDs
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.sp
.BI "int getgroups(int " size ", gid_t " list []);
.sp
.B #include <grp.h>
.sp
.BI "int setgroups(size_t " size ", const gid_t *" list );
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR setgroups ():
_BSD_SOURCE
.SH DESCRIPTION
.PP
.BR getgroups ()
returns the supplementary group IDs of the calling process in
.IR list .
The argument
.I size
should be set to the maximum number of items that can be stored in the
buffer pointed to by
.IR list .
If the calling process is a member of more than
.I size
supplementary groups, then an error results.
It is unspecified whether the effective group ID of the calling process
is included in the returned list.
(Thus, an application should also call
.BR getegid (2)
and add or remove the resulting value.)

If
.I size
is zero,
.I list
is not modified, but the total number of supplementary group IDs for the
process is returned.
This allows the caller to determine the size of a dynamically allocated
.I list
to be used in a further call to
.BR getgroups ().
.PP
.BR setgroups ()
sets the supplementary group IDs for the calling process.
Appropriate privileges (Linux: the
.B CAP_SETGID
capability) are required.
The
.I size
argument specifies the number of supplementary group IDs
in the buffer pointed to by
.IR list .
.SH RETURN VALUE
On success,
.BR getgroups ()
returns the number of supplementary group IDs.
On error, \-1 is returned, and
.I errno
is set appropriately.

On success,
.BR setgroups ()
returns 0.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EFAULT
.I list
has an invalid address.
.PP
.BR getgroups ()
can additionally fail with the following error:
.TP
.B EINVAL
.I size
is less than the number of supplementary group IDs, but is not zero.
.PP
.BR setgroups ()
can additionally fail with the following errors:
.TP
.B EINVAL
.I size
is greater than
.B NGROUPS_MAX
(32 before Linux 2.6.4; 65536 since Linux 2.6.4).
.TP
.B ENOMEM
Out of memory.
.TP
.B EPERM
The calling process has insufficient privilege.
.SH CONFORMING TO
SVr4, 4.3BSD.
The
.BR getgroups ()
function is in POSIX.1-2001.
Since
.BR setgroups ()
requires privilege, it is not covered by POSIX.1-2001.
.SH NOTES
A process can have up to
.B NGROUPS_MAX
supplementary group IDs
in addition to the effective group ID.
The constant
.B NGROUPS_MAX
is defined in
.IR <limits.h> .
The set of supplementary group IDs
is inherited from the parent process, and preserved across an
.BR execve (2).

The maximum number of supplementary group IDs can be found at run time using
.BR sysconf (3):
.nf

    long ngroups_max;
    ngroups_max = sysconf(_SC_NGROUPS_MAX);

.fi
The maximum return value of
.BR getgroups ()
cannot be larger than one more than this value.
Since Linux 2.6.4, the maximum number of supplementary group IDs is also
exposed via the Linux-specific read-only file,
.IR /proc/sys/kernel/ngroups_max .

The original Linux
.BR getgroups ()
system call supported only 16-bit group IDs.
Subsequently, Linux 2.4 added
.BR getgroups32 (),
supporting 32-bit IDs.
The glibc
.BR getgroups ()
wrapper function transparently deals with the variation across kernel versions.
.SH SEE ALSO
.BR getgid (2),
.BR setgid (2),
.BR getgrouplist (3),
.BR group_member (3),
.BR initgroups (3),
.BR capabilities (7),
.BR credentials (7)
.SH COLOPHON
This page is part of release 3.74 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%http://www.kernel.org/doc/man\-pages/.