getgroups(2) — Linux manual pages

Linux manual pages	Section 2

Name

getgroups, setgroups — get/set list of supplementary group IDs

Synopsis

#include <sys/types.h>
#include <unistd.h>

`int getgroups(`	int `size`,
	gid_t `list[])`;

#include <grp.h>

`int setgroups(`	size_t `size`,
	const gid_t *`list)`;

Note

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
`setgroups`():
Since glibc 2.19:
`_DEFAULT_SOURCE` Glibc 2.19 and earlier:
`_SVID_SOURCE`

DESCRIPTION

getgroups() returns the supplementary group IDs of the calling process in list. The argument size should be set to the maximum number of items that can be stored in the buffer pointed to by list. If the calling process is a member of more than 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 getegid(2) and add or remove the resulting value.)

If size is zero, 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 list to be used in a further call to getgroups().

setgroups() sets the supplementary group IDs for the calling process. Appropriate privileges (Linux: the CAP_SETGID capability) are required. The size argument specifies the number of supplementary group IDs in the buffer pointed to by list.

RETURN VALUE

On success, getgroups() returns the number of supplementary group IDs. On error, −1 is returned, and errno is set appropriately.

On success, setgroups() returns 0. On error, −1 is returned, and errno is set appropriately.

ERRORS

EFAULT: list has an invalid address.

getgroups() can additionally fail with the following error:

EINVAL: size is less than the number of supplementary group IDs, but is not zero.

setgroups() can additionally fail with the following errors:

EINVAL: size is greater than NGROUPS_MAX (32 before Linux 2.6.4; 65536 since Linux 2.6.4).
ENOMEM: Out of memory.
EPERM: The calling process has insufficient privilege (it does not have the CAP_SETGID capability).
EPERM (since Linux 3.19): The use of setgroups() is denied in this user namespace. See the description of /proc/[pid]/setgroups in user_namespaces(7).

CONFORMING TO

SVr4, 4.3BSD. The getgroups() function is in POSIX.1-2001 and POSIX.1-2008. Since setgroups() requires privilege, it is not covered by POSIX.1.

NOTES

A process can have up to NGROUPS_MAX supplementary group IDs in addition to the effective group ID. The constant NGROUPS_MAX is defined in <limits.h> The set of supplementary group IDs is inherited from the parent process, and preserved across an execve(2).

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

    long ngroups_max;
    ngroups_max = sysconf(_SC_NGROUPS_MAX);

The maximum return value of 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, /proc/sys/kernel/ngroups_max.

The original Linux getgroups() system call supported only 16-bit group IDs. Subsequently, Linux 2.4 added getgroups32(), supporting 32-bit IDs. The glibc getgroups() wrapper function transparently deals with the variation across kernel versions.

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 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 nptl(7).

COLOPHON

This page is part of release 4.07 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.

  Copyright 1993 Rickard E. Faith (faithcs.unc.edu)
and Copyright (C) 2008, 2010, 2015, Michael Kerrisk <mtk.manpagesgmail.com>

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Modified Thu Oct 31 12:04:29 1996 by Eric S. Raymond <esrthyrsus.com>
Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.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.