n The number of elements stored in the array
pointed to by the grouplist parameter.
-1 Indicates failure. errno might be set to one
of the following values:
o EFAULT - The gidsetsize and grouplist
parameters specify an array that is
partially or completely outside of the
allocated address space of the process.
o EINVAL - The gidsetsize parameter is
nonzero and smaller than the number of
supplementary group IDs.