getgroups(2) - Linux manual page (original) (raw)
getgroups(2) System Calls Manual getgroups(2)
NAME top
getgroups, setgroups - get/set list of supplementary group IDsLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <unistd.h>**
**int getgroups(int** _size_**, gid_t** _list_**[_Nullable .**_size_**]);**
**#include <grp.h>**
**int setgroups(size_t** _size_**, const gid_t** _list_**[_Nullable .**_size_**]);**Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
**setgroups**():
Since glibc 2.19:
_DEFAULT_SOURCE
glibc 2.19 and earlier:
_BSD_SOURCEDESCRIPTION top
**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)](../man2/getegid.2.html) 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);RETURN VALUE top
On success, **getgroups**() returns the number of supplementary group
IDs. On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the
error.
On success, **setgroups**() returns 0. On error, -1 is returned, and
_[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**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)](../man7/user%5Fnamespaces.7.html).VERSIONS top
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).
STANDARDS top
**getgroups**()
POSIX.1-2008.
**setgroups**()
None.HISTORY top
**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 top
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)](../man2/execve.2.html).
The maximum number of supplementary group IDs can be found at run
time using [sysconf(3)](../man3/sysconf.3.html):
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/ngroupsmax_.EXAMPLES top
#include <err.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#define MALLOC(n, T) ((T *) reallocarray(NULL, n, sizeof(T)))
static gid_t *agetgroups(size_t *ngids);
int
main(void)
{
gid_t *gids;
size_t n;
gids = agetgroups(&n);
if (gids == NULL)
err(EXIT_FAILURE, "agetgroups");
if (n != 0) {
printf("%jd", (intmax_t) gids[0]);
for (size_t i = 1; i < n; i++)
printf(" %jd", (intmax_t) gids[i]);
}
puts("");
free(gids);
exit(EXIT_SUCCESS);
}
static gid_t *
agetgroups(size_t *ngids)
{
int n;
gid_t *gids;
n = getgroups(0, NULL);
if (n == -1)
return NULL;
gids = MALLOC(n, gid_t);
if (gids == NULL)
return NULL;
n = getgroups(n, gids);
if (n == -1) {
free(gids);
return NULL;
}
*ngids = n;
return gids;
}SEE ALSO top
[getgid(2)](../man2/getgid.2.html), [setgid(2)](../man2/setgid.2.html), [getgrouplist(3)](../man3/getgrouplist.3.html), [group_member(3)](../man3/group%5Fmember.3.html),
[initgroups(3)](../man3/initgroups.3.html), [capabilities(7)](../man7/capabilities.7.html), [credentials(7)](../man7/credentials.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-12-14 getgroups(2)
Pages that refer to this page:capsh(1), groups(1@@shadow-utils), ps(1), unshare(1), syscalls(2), cap_get_proc(3), getgrouplist(3), group_member(3), id_t(3type), initgroups(3), credentials(7), nptl(7), path_resolution(7), signal-safety(7), user_namespaces(7)