These are the sync(2) semantics that I always have seen.

	I do not remember the details, but I thought that the second
sync(2) call was not allowed to return until the first one had completed,
i.e. that only one could be queued at a time and the first one needed to
complete before the second one could be queued.  That, not the speed of
teletypes, was the reason for issuing multiple sync(8) commands -- one
only needed two but the third one was added for good measure.

David