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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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getgroups(2) System Calls Manual getgroups(2)
getgroups, setgroups - get/set list of supplementary group IDs
Standard C library (libc, -lc)
#include <unistd.h>
int getgroups(int size, gid_t list[_Nullable size]);
#include <grp.h>
int setgroups(size_t size, const gid_t list[_Nullable size]);
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. A process can drop all of its supplementary groups with the
call:
setgroups(0, NULL);
On success, getgroups() returns the number of supplementary group
IDs. On error, -1 is returned, and errno is set to indicate the
error.
On success, setgroups() returns 0. On error, -1 is returned, and
errno is set to indicate the error.
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).
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).
getgroups()
POSIX.1-2008.
setgroups()
None.
getgroups()
SVr4, 4.3BSD, POSIX.1-2001.
setgroups()
SVr4, 4.3BSD. Since setgroups() requires privilege, it is
not covered by POSIX.1.
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.
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.
#include <err.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#define MALLOC(n, T) ((T *) reallocarray(NULL, n, sizeof(T)))
static gid_t *agetgroups(size_t *ngids);
int
main(void)
{
gid_t *gids;
size_t n;
gids = agetgroups(&n);
if (gids == NULL)
err(EXIT_FAILURE, "agetgroups");
if (n != 0) {
printf("%jd", (intmax_t) gids[0]);
for (size_t i = 1; i < n; i++)
printf(" %jd", (intmax_t) gids[i]);
}
puts("");
free(gids);
exit(EXIT_SUCCESS);
}
static gid_t *
agetgroups(size_t *ngids)
{
int n;
gid_t *gids;
n = getgroups(0, NULL);
if (n == -1)
return NULL;
gids = MALLOC(n, gid_t);
if (gids == NULL)
return NULL;
n = getgroups(n, gids);
if (n == -1) {
free(gids);
return NULL;
}
*ngids = n;
return gids;
}
getgid(2), setgid(2), getgrouplist(3), group_member(3),
initgroups(3), capabilities(7), credentials(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 getgroups(2)
Pages that refer to this page: capsh(1), groups(1@@shadow-utils), ps(1), unshare(1), syscalls(2), cap_get_proc(3), getgrouplist(3), group_member(3), id_t(3type), initgroups(3), credentials(7), nptl(7), path_resolution(7), signal-safety(7), user_namespaces(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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