int getgroups(int size, gid_t list);
int setgroups(size_t size, const gid_t *list);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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
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 (Linux: the CAP_SETGID capability) are required.
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
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.
A process can have up to NGROUPS_MAX supplementary group IDs in addi-
tion 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
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 supplemen-
tary group IDs is also exposed via the Linux-specific read-only file,
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
getgid(2), setgid(2), getgrouplist(3), group_member(3), initgroups(3),
This page is part of release 4.04 of the Linux man-pages project. A
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latest version of this page, can be found at
Linux 2015-07-23 GETGROUPS(2)
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