.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) .\" and Copyright (C) 2008, 2010, 2015, Michael Kerrisk .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" Modified Thu Oct 31 12:04:29 1996 by Eric S. Raymond .\" Modified, 27 May 2004, Michael Kerrisk .\" 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 2023-10-31 "Linux man-pages 6.7" .SH NAME getgroups, setgroups \- get/set list of supplementary group IDs .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .P .BI "int getgroups(int " size ", gid_t " list []); .P .B #include .P .BI "int setgroups(size_t " size ", const gid_t *_Nullable " list ); .fi .P .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .P .BR setgroups (): .nf Since glibc 2.19: _DEFAULT_SOURCE glibc 2.19 and earlier: _BSD_SOURCE .fi .SH DESCRIPTION .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. .P 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.) .P 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 (). .P .BR setgroups () sets the supplementary group IDs for the calling process. Appropriate privileges are required (see the description of the .B EPERM error, below). The .I size argument specifies the number of supplementary group IDs in the buffer pointed to by .IR list . A process can drop all of its supplementary groups with the call: .P .in +4n .EX setgroups(0, NULL); .EE .in .SH RETURN VALUE On success, .BR getgroups () returns the number of supplementary group IDs. On error, \-1 is returned, and .I errno is set to indicate the error. .P On success, .BR setgroups () returns 0. On error, \-1 is returned, and .I errno is set to indicate the error. .SH ERRORS .TP .B EFAULT .I list has an invalid address. .P .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. .P .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 (the caller does not have the .B CAP_SETGID capability in the user namespace in which it resides). .TP .BR EPERM " (since Linux 3.19)" The use of .BR setgroups () is denied in this user namespace. See the description of .IR /proc/ pid /setgroups in .BR user_namespaces (7). .SH VERSIONS .SS C library/kernel differences At the kernel level, user IDs and group IDs are a per-thread attribute. However, POSIX requires that all threads in a process share the same credentials. The NPTL threading implementation handles the POSIX requirements by providing wrapper functions for the various system calls that change process UIDs and GIDs. These wrapper functions (including the one for .BR setgroups ()) employ a signal-based technique to ensure that when one thread changes credentials, all of the other threads in the process also change their credentials. For details, see .BR nptl (7). .SH STANDARDS .TP .BR getgroups () POSIX.1-2008. .TP .BR setgroups () None. .SH HISTORY .TP .BR getgroups () SVr4, 4.3BSD, POSIX.1-2001. .TP .BR setgroups () SVr4, 4.3BSD. Since .BR setgroups () requires privilege, it is not covered by POSIX.1. .P 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 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 . The set of supplementary group IDs is inherited from the parent process, and preserved across an .BR execve (2). .P The maximum number of supplementary group IDs can be found at run time using .BR sysconf (3): .P .in +4n .EX long ngroups_max; ngroups_max = sysconf(_SC_NGROUPS_MAX); .EE .in .P 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 . .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)