Are terminal writes atomic?

[Clyde Smith-Stubbs]

>From article <186 at promark.UUCP>, by mark at promark.UUCP (Mark J. DeFilippis):
> In article <1118 at cs.yale.edu>, spolsky-joel at CS.YALE.EDU (Joel Spolsky) writes:
>> I think this is safe. A while ago I wrote a little program that runs
> 
> How about we take this further.  The answer to this question is an
> undocumented feature of write() under Unix.  Several Authors note
> that write() is atomic under Unix and that it is undocumented.

Rubbish! Writes to terminals under Unix are not, and have never been,
in any way atomic. In many cases they will be atomic, but there
is nothing in the structure of the system to guarantee this. It is easy
to demonstrate a situation where they are not atomic. Try the following
program: (If run on e.g. the memory mapped display of a 386 PC then the
writes will be atomic - but on a serial terminal, or anything that uses
buffering and interrupts, they will not be).

char x[25][61];
main()
{
	char	c;
	int	i, j;

	if(fork())
		c = 'A';
	else
		c = '.';
	for(i = 0 ; i != 25 ; i++) {
		for (j = 0 ; j != 60 ; j++)
			x[i][j] = c;
		x[i][j] = '\n';
	}
	for(;;)
		write(1, x, sizeof(x));
}

For the explanation, here is the mail I sent to Jim Stratton after his
original posting:

>From clyde Tue Sep 26 14:32:58 1989
To: stratton at hpcupt1.HP.COM

Subject: Re: Are terminal writes atomic?

stratton at hpcupt1.HP.COM (Jim Stratton):
> I have two processes doing simultaneous output to the same terminal using the
> write system call.  Assume A does a write of 10 bytes while B does a write
> of 30 bytes.  Can I be assured that A's and B's output will not be inter-
> mixed?  I understand that for pipes, they won't be mixed.  How about terminals?

No, you can't assume that. It depends on how much output is going to the
terminal. If the terminal output queue reaches the high water mark the current
process will be suspended. When the queue drops to the low water mark all
processes waiting on that terminal will be woken. Which one gets to write some
more output first is indeterminate.
------------------------
Clyde Smith-Stubbs
HI-TECH Software, P.O. Box 103, ALDERLEY, QLD, 4051, AUSTRALIA.

ACSnet:		clyde at hitech.ht.oz
INTERNET:	clyde at hitech.ht.oz.au		PHONE:	+61 7 300 5011
UUCP:		uunet!hitech.ht.oz.au!clyde	FAX:	+61 7 300 5246
-- 
Clyde Smith-Stubbs
HI-TECH Software, P.O. Box 103, ALDERLEY, QLD, 4051, AUSTRALIA.
INTERNET:	clyde at hitech.ht.oz.au		PHONE:	+61 7 300 5011
UUCP:		uunet!hitech.ht.oz.au!clyde	FAX:	+61 7 300 5246