bsd man page for getgroups

GETGROUPS(2)							System Calls Manual						      GETGROUPS(2)

NAME
       getgroups - get group access list

SYNOPSIS
       #include <sys/param.h>

       ngroups = getgroups(gidsetlen, gidset)
       int ngroups, gidsetlen, *gidset;

DESCRIPTION
       Getgroups  gets the current group access list of the user process and stores it in the array gidset.  The parameter gidsetlen indicates the
       number of entries that may be placed in gidset.	Getgroups returns the actual number of groups returned in gidset.  No more  than  NGROUPS,
       as defined in <sys/param.h>, will ever be returned.

RETURN VALUE
       A  successful  call  returns  the number of groups in the group set.  A value of -1 indicates that an error occurred, and the error code is
       stored in the global variable errno.

ERRORS
       The possible errors for getgroup are:

       [EINVAL]       The argument gidsetlen is smaller than the number of groups in the group set.

       [EFAULT]       The argument gidset specifies an invalid address.

SEE ALSO
       setgroups(2), initgroups(3X)

BUGS
       The gidset array should be of type gid_t, but remains integer for compatibility with earlier systems.

4.2 Berkeley Distribution					   May 13, 1986 						      GETGROUPS(2)