getgroups, setgroups - get/set list of supplementary group IDs
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
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
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.)
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
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:
list has an invalid address.
getgroups() can additionally fail with the following error:
size is less than the number of supplementary group IDs, but
is not zero.
setgroups() can additionally fail with the following errors:
size is greater than NGROUPS_MAX (32 before
Linux 2.6.4; 65536 since Linux 2.6.4).
Out of memory.
The calling process has insufficient privilege (the caller does not have the CAP_SETGID capability in the user namespace in which it resides).
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
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
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.
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).
This page is part of release 5.10 of the Linux
project. A description of the project, information about reporting bugs,
and the latest version of this page, can be found at