Printf sets errno to 25?

[Paul E. Olson]

*** REPLACE THIS LINE WITH YOUR MESSAGE ***

Could someone please explain to me why the printf() family of
functions cause errno to be set to 25 (not a typewritter) when output
is redirected to either a file or a pipe. I know that the problem comes
from the C lib function isatty() when ioctl() fails. So, maybe the real
question should be - Is this an oversight? It would seem to me that redirecting
printf output does not constitute an error and that the functions _findbuf()
and setbuf() should take care to reset errno to 0 if line buffering is not
needed. At first I thought it was a bug, but now I'm not so sure. I get the 
same results using:
	SYS 5.0.5 on a 3B20
	SYS 5.2 on a 3B5
	SYS 5.2 on a 3B2
	Xenix 3.0 on an Altos
	4.2BSD on an 11/780
	AT&T PC7300
The following illustrates the problem.

/* foo.c */
extern int	errno;
main()
{
	printf("errno=%d\n", errno);
	printf("errno=%d\n", errno);
}

$ cc foo.c -o foo
$ foo
errno=0
errno=0
$ foo >bar
$ cat bar
errno=0
errno=25
$

All this started while chasing a bug is some software I was porting.
Thanks in advance for any and all comments. 

			Paul E. Olson
			ihnp4!cpsc6a!peo