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From: john@frog.UUCP (John Woods)
Newsgroups: net.bugs.4bsd,net.unix-wizards,net.bugs.usg
Subject: Re: Bug in isatty (all systems!)
Message-ID: <247@frog.UUCP>
Date: Tue, 30-Jul-85 17:55:58 EDT
Article-I.D.: frog.247
Posted: Tue Jul 30 17:55:58 1985
Date-Received: Thu, 1-Aug-85 07:42:56 EDT
References: <6367@ucla-cs.ARPA> <997@sdcsvax.UUCP> <1017@ulysses.UUCP> <1730@reed.UUCP>
Organization: Charles River Data Systems, Framingham MA
Lines: 25
Xref: linus net.bugs.4bsd:1342 net.unix-wizards:11292 net.bugs.usg:257

>   MY 4.2bsd manual says:
>       ... An
>      error number is also made available in the external variable
>      _errno_, which is not cleared on successful calls.  Thus _errno_
>      should be tested only after an error has occurred.
> 
>   What does "not cleared on successful calls" mean to you?  To me, it
> means that if I set errno = 0 in the beginning of my program, it will
> stay zero until a system call _fails_, then it will stay non-zero long
> enough for me to do something about it.  Now if a perfectly successful
> operation sets errno and does not bother to clear it, how can I depend
> on errno as an indication that something went _wrong_?
> 
You can't.  For once, the UNIX manual was CRYSTAL CLEAR on something, and you
even quoted it!

What does "Thus _errno_ should be tested only after an error has occurred."
mean to you?

Repeat it softly to yourself until you either understand it or are hauled away
in a butterfly net.

--
John Woods, Charles River Data Systems, Framingham MA, (617) 626-1101
...!decvax!frog!john, ...!mit-eddie!jfw, jfw%mit-ccc@MIT-XX.ARPA