getgroups, setgroups - get/set list of supplementary group IDs
#include <sys/types.h>
#include <unistd.h>
int getgroups(int size, gid_t list[]);
#include <grp.h>
int setgroups(size_t size, const gid_t
*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
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 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 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).
getgroups(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
setgroups(): SVr4, 4.3BSD. Since
setgroups() requires privilege,
it is not covered by POSIX.1.
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.
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).
getgid(2),
setgid(2),
getgrouplist(3),
group_member(3),
initgroups(3),
capabilities(7),
credentials(7)