1.\"- 2.\" SPDX-License-Identifier: BSD-3-Clause 3.\" 4.\" Copyright (c) 1983, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" Copyright (c) 2025 The FreeBSD Foundation 7.\" 8.\" Portions of this documentation were written by Olivier Certner 9.\" <olce@FreeBSD.org> at Kumacom SARL under sponsorship from the FreeBSD 10.\" Foundation. 11.\" 12.\" Redistribution and use in source and binary forms, with or without 13.\" modification, are permitted provided that the following conditions 14.\" are met: 15.\" 1. Redistributions of source code must retain the above copyright 16.\" notice, this list of conditions and the following disclaimer. 17.\" 2. Redistributions in binary form must reproduce the above copyright 18.\" notice, this list of conditions and the following disclaimer in the 19.\" documentation and/or other materials provided with the distribution. 20.\" 3. Neither the name of the University nor the names of its contributors 21.\" may be used to endorse or promote products derived from this software 22.\" without specific prior written permission. 23.\" 24.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 25.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 26.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 27.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 28.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 29.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 30.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 31.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 32.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 33.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 34.\" SUCH DAMAGE. 35.\" 36.Dd September 17, 2025 37.Dt GETGROUPS 2 38.Os 39.Sh NAME 40.Nm getgroups 41.Nd get the calling process' supplementary groups 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In unistd.h 46.Ft int 47.Fn getgroups "int gidsetlen" "gid_t *gidset" 48.Sh DESCRIPTION 49The 50.Fn getgroups 51system call gets the calling process' supplementary groups and stores them in 52the 53.Fa gidset 54array in strictly ascending order. 55The value of 56.Fa gidsetlen 57indicates the maximum number of entries that may be placed in 58.Fa gidset . 59.Pp 60If 61.Fa gidsetlen 62is zero, 63.Fn getgroups 64returns the cardinal of the calling process' supplementary groups set and 65ignores argument 66.Fa gidset . 67.Pp 68No more than 69.Dv {NGROUPS_MAX} 70values may ever be returned. 71The value of 72.Dv {NGROUPS_MAX} 73should be obtained using 74.Xr sysconf 3 75to avoid hard-coding it into the executable. 76.Sh RETURN VALUES 77On success, the 78.Fn getgroups 79system call returns the cardinal of the supplementary groups set. 80It always succeeds if argument 81.Fa gidsetlen 82is zero. 83.Pp 84A value of -1 indicates that an error occurred, and the error 85code is stored in the global variable 86.Va errno . 87.Sh ERRORS 88The possible errors for 89.Fn getgroups 90are: 91.Bl -tag -width Er 92.It Bq Er EINVAL 93The argument 94.Fa gidsetlen 95is smaller than the number of supplementary groups 96.Pq but not zero . 97.It Bq Er EFAULT 98An invalid address was encountered while reading from the 99.Fa gidset 100array. 101.El 102.Sh SEE ALSO 103.Xr setgroups 2 , 104.Xr initgroups 3 , 105.Xr sysconf 3 106.Sh STANDARDS 107The 108.Fn getgroups 109system call conforms to 110.St -p1003.1-2008 111with the additional properties that supplementary groups are reported in 112strictly ascending order and the returned size coincides with the cardinal of 113the set. 114.Sh HISTORY 115The 116.Fn getgroups 117system call appeared in 118.Bx 4.2 . 119.Pp 120Since 121.Fx 14.3 , 122the 123.Fn getgroups 124system call has treated the supplementary groups as a set, reporting them in 125strictly ascending order and returning the cardinal of the set. 126.Pp 127Before 128.Fx 15.0 , 129the 130.Fn getgroups 131system call would additionally return the effective group ID as the first 132element of the array, before the supplementary groups. 133.Sh SECURITY CONSIDERATIONS 134The 135.Fn getgroups 136system call gets the supplementary groups set in the 137.Fa gidset 138array. 139In particular, as evoked in 140.Sx HISTORY , 141it does not anymore retrieve the effective GID in the first slot of 142.Fa gidset . 143Programs should not make any assumption about which group is placed in the first 144slot of 145.Fa gidset 146other than it being the supplementary group with smallest GID. 147.Pp 148The effective GID is present in the supplementary groups set if and only if it 149was explicitly set as a supplementary group. 150The function 151.Fn initgroups 152enforces that, while the 153.Fn setgroups 154system call does not. 155Please consult the 156.Xr initgroups 3 157manual page for the rationale. 158