GETGROUPS
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
getgroups, setgroups - get/set list of supplementary group IDs
SYNOPSIS
#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
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 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.
RETURN VALUE
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.
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).
CONFORMING TO
SVr4, 4.3BSD.
The
getgroups()
function is in POSIX.1-2001 and POSIX.1-2008.
Since
setgroups()
requires privilege, it is not covered by POSIX.1.
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
<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.
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
nptl(7).
SEE ALSO
getgid(2),
setgid(2),
getgrouplist(3),
group_member(3),
initgroups(3),
capabilities(7),
credentials(7)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- C library/kernel differences
-
- SEE ALSO
-
- COLOPHON
-
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