Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site brl-tgr.ARPA
Path: utzoo!watmath!clyde!cbosgd!gatech!seismo!brl-tgr!gwyn
From: gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>)
Newsgroups: net.unix-wizards
Subject: Re: Printf sets errno to 25?
Message-ID: <952@brl-tgr.ARPA>
Date: Fri, 20-Dec-85 17:39:04 EST
Article-I.D.: brl-tgr.952
Posted: Fri Dec 20 17:39:04 1985
Date-Received: Sat, 21-Dec-85 14:42:46 EST
References: <119@cpsc6a.UUCP> <212@iclbra.UUCP>
Distribution: net
Organization: Ballistic Research Lab
Lines: 13

> 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.

Close, but not quite.  `errno' is only set by unsuccessful SYSTEM CALLS
(and some math library routines, alas).  Therefore, failure of printf()
does not imply that errno will correctly indicate the reason for failure.