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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
GETGROUPS(2) Linux Programmer's Manual GETGROUPS(2)
getgroups, setgroups - get/set list of supplementary group IDs
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
setgroups():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc 2.19 and earlier:
_BSD_SOURCE
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 are required (see the description of the EPERM
error, below). The size argument specifies the number of
supplementary group IDs in the buffer pointed to by list.
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.
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 (the caller
does not have the CAP_SETGID capability in the user namespace
in which it resides).
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).
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.
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).
getgid(2), setgid(2), getgrouplist(3), group_member(3),
initgroups(3), capabilities(7), credentials(7)
This page is part of release 4.12 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/.
Linux 2016-10-08 GETGROUPS(2)
Pages that refer to this page: capsh(1), gawk(1), groups(1@@shadow-utils), ps(1), unshare(1), syscalls(2), getgrouplist(3), group_member(3), initgroups(3), credentials(7), nptl(7), path_resolution(7), signal-safety(7), user_namespaces(7)