getgroups, setgroups - get/set list of supplementary group IDs
int getgroups(int size, gid_t list);
int setgroups(size_t size, const gid_t
Feature Test Macro Requirements for glibc (see feature_test_macros(7)
Since glibc 2.19:
Glibc 2.19 and earlier:
() returns the supplementary group IDs of the calling process in
. 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
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.)
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
() 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
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
is set appropriately.
- list has an invalid address.
() can additionally fail with the following error:
- size is less than the number of supplementary group
IDs, but is not zero.
() 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).
- EPERM (since Linux 3.19)
- The use of setgroups() is denied in this user
namespace. See the description of /proc/[pid]/setgroups in
(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
(): 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
. 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
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,
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)
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