.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH getgroups 2 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
getgroups, setgroups \- get/set list of supplementary group IDs
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.P
.BI "int getgroups(int " size ", gid_t " list "[_Nullable " size ]);
.P
.B #include <grp.h>
.P
.BI "int setgroups(size_t " size ", const gid_t " list "[_Nullable " size ]);
.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 <limits.h> .
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 EXAMPLES
.\" SRC BEGIN (agetgroups.c)
.EX
#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;
}
.EE
.\" SRC END
.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)