getgroups(2) System Calls Manual getgroups(2)
NAME
getgroups, setgroups - get/set list of supplementary group IDs
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int getgroups(int size, gid_t list[]);
#include <grp.h>
int setgroups(size_t size, const gid_t *_Nullable 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
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 er-
ror 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 supple-
mentary 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);
RETURN VALUE
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.
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 (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).
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 re-
quirements 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).
STANDARDS
getgroups()
POSIX.1-2008.
setgroups()
None.
HISTORY
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.
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 <lim-
its.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.
SEE ALSO
getgid(2), setgid(2), getgrouplist(3), group_member(3), initgroups(3),
capabilities(7), credentials(7)
Linux man-pages 6.9.1 2024-05-02 getgroups(2)
Generated by dwww version 1.16 on Tue Dec 16 04:53:58 CET 2025.