What does sync() _really_ do?

[Gordon Burditt]

>:If one insists on believing that people are being rational in typing sync
>:three times, that's a great hypothesis.  However, it has no basis in fact.
>:While there may a such a sync queue in some kernel somewhere, there isn't one
>:in any of the common kernels that have had triple syncs directed at them.

There were semi-valid reasons for three sync's on a PDP-11/70 running PWB
UNIX (a variant of UNIX V6).  This was, of course, a long time ago.

(1) The length of time it took us fumble-fingered superusers to type
    "sync<CR>" 3 times ensured enough time for the first sync() to 
    complete.  As far as I know, this depended exclusively on the delay
    introduced by human typing speed.
(2) Due to the placement of the keys on a DecWriter, it was distressingly
    common to type "sync<DEL>" instead of "sync<CR>", (<DEL> being the 
    interrupt character) but you usually wouldn't make the same mistake 
    3 times in a row.
(3) It prevented one guy from typing "sync", then putting one finger on
    the <CR> key, one finger on the other hand on the CPU HALT button (or 
    whatever it was labelled), and pressing them simultaneously.

					Gordon L. Burditt
					sneaky.lonestar.org!gordon