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From: apm@iclbra.UUCP (Andy Merritt)
Newsgroups: net.unix-wizards
Subject: Re: Printf sets errno to 25?
Message-ID: <212@iclbra.UUCP>
Date: Tue, 17-Dec-85 13:29:13 EST
Article-I.D.: iclbra.212
Posted: Tue Dec 17 13:29:13 1985
Date-Received: Sat, 21-Dec-85 06:23:27 EST
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> *** 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

The problem is a common misunderstanding about the use of errno. The value
in errno is useful if, and only if, the call has returned a value indicating
failure. This value is commonly -1, and sometimes NULL for functions 
returning pointers. The Intro(2) manual page says: "Errno is not cleared on
successful calls, so it should be tested only after an error has been indicated"

The documentation I have for printf further states that it, fprintf and sprintf
return the number of characters transmitted, or a negative value if an output
error was encountered.
 
Hope this helps.
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