Linux man pages : getgroups (2)
GETGROUPS(2) Linux Programmer's Manual GETGROUPS(2)
getgroups, setgroups - get/set list of supplementary group IDs
int getgroups(int size, gid_t list);
int setgroups(size_t size, const gid_t *list);
Up to size supplementary group IDs are returned in list. It is
unspecified whether the effective group ID of the calling pro-
cess 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.
Sets the supplementary group IDs for the process. Only the
super-user may use this function.
On success, the number of supplementary group IDs is returned.
On error, -1 is returned, and errno is set appropriately.
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
EFAULT list has an invalid address.
EPERM For setgroups, the user is not the super-user.
EINVAL For setgroups, size is greater than NGROUPS (32 for Linux
2.0.32). For getgroups, size is less than the number of supple-
mentary group IDs, but is not zero.
A process can have up to at least NGROUPS_MAX supplementary group IDs
in addition to the effective group ID. The set of supplementary group
IDs is inherited from the parent process and may be changed using set-
groups. The maximum number of supplementary group IDs can be found
ngroups_max = sysconf(_SC_NGROUPS_MAX);
The maximal return value of getgroups cannot be larger than one more
than the value obtained this way.
The prototype for setgroups is only available if _BSD_SOURCE is defined
(either explicitly, or implicitly, by not defining _POSIX_SOURCE or
compiling with the -ansi flag).
SVr4, SVID (issue 4 only; these calls were not present in SVr3),
X/OPEN, 4.3BSD. The getgroups function is in POSIX.1. Since setgroups
requires privilege, it is not covered by POSIX.1.
initgroups(3), getgid(2), setgid(2)
Linux 2.0.32 1997-12-10 GETGROUPS(2)