redhat man page for getgroups

GETGROUPS(2)						     Linux Programmer's Manual						      GETGROUPS(2)

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);

DESCRIPTION
       getgroups
	      Up to size supplementary group IDs are returned in list.	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.

       setgroups
	      Sets the supplementary group IDs for the process.  Only the super-user may use this function.

RETURN VALUE
       getgroups
	      On success, the number of supplementary group IDs is returned.  On error, -1 is returned, and errno is set appropriately.

       setgroups
	      On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       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 supplementary
	      group IDs, but is not zero.

NOTES
       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 setgroups.	The maximum number of supplementary group IDs can be found
       using sysconf(3):
	   long ngroups_max;
	   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).

CONFORMING TO
       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.

SEE ALSO
       initgroups(3), getgid(2), setgid(2)

Linux 2.0.32							    1997-12-10							      GETGROUPS(2)