man-pages/man2/getgroups.2

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.TH GETGROUPS 2 2008-06-02 "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
.\" FIXME. The layout of the DESCRIPTION and RETURN VALUE sections
.\" of this page are somewhat inconsistent.
.TP
.BR getgroups ()
Up to
.I size
supplementary group IDs (of the calling process) are returned in
.IR list .
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.
.TP
.BR setgroups ()
Sets the supplementary group IDs for the process.
Appropriate privileges (Linux: the
.B CAP_SETGID
capability) are required.
.SH "RETURN VALUE"
.TP
.BR getgroups ()
On success, the number of supplementary group IDs is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.TP
.BR setgroups ()
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EFAULT
.I list
has an invalid address.
.TP
.B EINVAL
For
.BR setgroups (),
.I size
is greater than
.B NGROUPS_MAX
(32 before Linux 2.6.4; 65536 since Linux 2.6.4).
For
.BR getgroups (),
.I size
is less than the number of supplementary group IDs, but is not zero.
.TP
.B EPERM
The calling process has insufficient privilege to call
.BR setgroups ().
.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 at least
.B NGROUPS_MAX
supplementary group IDs
in addition to the effective group ID.
The set of supplementary group IDs
is inherited from the parent process and may be changed using
.BR setgroups ().
The maximum number of supplementary group IDs can be found using
.BR sysconf (3):
.nf

    long ngroups_max;
    ngroups_max = sysconf(_SC_NGROUPS_MAX);

.fi
The maximal return value of
.BR getgroups ()
cannot be larger than one more than the value obtained this way.
.SH "SEE ALSO"
.BR getgid (2),
.BR setgid (2),
.BR getgrouplist (3),
.BR initgroups (3),
.BR capabilities (7),
.BR credentials (7)