|GETGROUPS(2)||Linux Programmer's Manual||GETGROUPS(2)|
int getgroups(int size, gid_t list);
int setgroups(size_t size, const gid_t *list);
Since glibc 2.19: _DEFAULT_SOURCE Glibc 2.19 and earlier: _BSD_SOURCE
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:
On success, setgroups() returns 0. On error, -1 is returned, and errno is set to indicate the error.
- list has an invalid address.
getgroups() can additionally fail with the following error:
- size is less than the number of supplementary group IDs, but is not zero.
setgroups() 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 user_namespaces(7).
setgroups(): SVr4, 4.3BSD. Since setgroups() requires privilege, it is not covered by POSIX.1.
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.