The getgroups function gets the current supplementary group IDs
    of the calling process. The list is stored in the array pointed
    to by the grouplist parameter. The gidsetsize parameter indicates
    the number of entries that can be stored in this array.

    The getgroups function never returns more IDs than the value
    indicated by the sysconf parameter _SC_NGROUPS_MAX.

    See also getgid and setsid.