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 October 10, 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 , 111not reporting the effective group ID. 112.Sh HISTORY 113The 114.Fn getgroups 115system call appeared in 116.Bx 4.2 . 117.Pp 118Since 119.Fx 14.3 , 120the 121.Fn getgroups 122system call has been reporting the supplementary groups in strictly ascending 123order. 124.Pp 125Before 126.Fx 15.0 , 127the 128.Fn getgroups 129system call would additionally return the effective group ID as the first 130element of the array, before the supplementary groups. 131.Sh SECURITY CONSIDERATIONS 132The 133.Fn getgroups 134system call gets the supplementary groups set in the 135.Fa gidset 136array. 137In particular, as evoked in 138.Sx HISTORY , 139it does not anymore retrieve the effective group ID in the first slot of 140.Fa gidset . 141Programs that process this slot in a specific way must be modified to obtain the 142effective group ID through other means, such as a call to 143.Xr getegid 2 . 144.Pp 145The effective group ID is present in the supplementary groups set if and only if 146it was explicitly set as a supplementary group. 147The function 148.Fn initgroups 149enforces that, while the 150.Fn setgroups 151system call does not. 152Please consult the 153.Xr initgroups 3 154manual page for the rationale. 155