[net.unix-wizards] Why sync three times

mo@seismo.UUCP (Mike O'Dell) (05/23/85)

People have been asking why type sync three times before halting a machine.
I believe the answer is the same as why people say

	"Testing Testing 1 2 3" (ie, count to three) into microphones.

	Yours for better rumors,
	-Mike

trt@rti-sel.UUCP (Tom Truscott) (05/24/85)

On UNIX V7, it was possible to type 'sync' followed by interrupt (e.g. DEL)
rather than newline and not be able to tell the difference.
This is still possible if control characters (e.g. interrupt)
are not made visible via 'ctlecho' or some such.
Typing 'sync' several times
increases the probability that the command gets done.
(But anyone who has had an important 'cp' finish early due to a noisy
phone line will appreciate the merits of 'ctlecho'.)

Also, the fact that the 'sync' system call is not synchronous
makes people nervous, and with good reason.
Is this bug fixed in any version of UNIX?
	Tom Truscott

geoff@burl.UUCP (geoff) (05/24/85)

> People have been asking why type sync three times before halting a machine.
> I believe the answer is the same as why people say
> 
> 	"Testing Testing 1 2 3" (ie, count to three) into microphones.
> 
> 	Yours for better rumors,
> 	-Mike

How about this one?  The 'sync' program (what runs when you type 'sync')
just calls the 'sync' system call (I peeked).  It says in the manual under
sync(2) that the writing to disk may not be completed when sync returns.
Hence typing sync 3 times provides a delay after which the file system
should be up to date.  Running 'fortune' the second and third times would
be as effective and a whole lot more fun.

I think the above is true, but am not positive.  Comments?

	geoff sherwood

dhb@rayssd.UUCP (05/28/85)

Isn't "sync three times" the title of a hit song from
the late sixties/early seventies?

	"Sync three times on the console if you want me"

roy@phri.UUCP (Roy Smith) (06/01/85)

> 	"Sync three times on the console if you want me"

..... Seek(II) on the pipes(II), if(I) the answer is who(I)...
-- 
allegra!phri!roy (Roy Smith)
System Administrator, Public Health Research Institute