Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!mcsun!ukc!edcastle!hwcs!zen!frank
From: frank@zen.co.uk (Frank Wales)
Newsgroups: comp.unix.wizards
Subject: Re: ENOTTY error when executing cp(1)
Message-ID: <1708@zen.co.uk>
Date: 15 Sep 89 15:52:22 GMT
References: <507@fdmetd.uucp> <2556@taux01.UUCP> <301@6sigma.UUCP> <109@harald.UUCP>
Reply-To: frank@zen.co.uk (Frank Wales)
Organization: Zengrange Limited, Leeds, England
Lines: 37

In article <109@harald.UUCP> jba@harald.ruc.dk (Jan B. Andersen) writes:
>As I see it, the "correct" way to check for errors is by coding something
>like this:
>
>  if( some_system_call() ) {
>	printf( "errno %d has occured in some_system_call()\n", errno );
>	exit( errno );
>  }

Assuming some_system_call() returns true for failure, yes; this is
not always the case, however.

>If you don't like if'ing the call do something like this:
>
>  errno = 0;
>  some_system_call();
>  if( errno ) {
>	...
>  }

This isn't equivalent to the previous code fragment, and furthermore is
not a valid use of errno; if you don't like the former style, then an
alternative is:

  returnval=some_system_call();
  if (returnval) {
    /* appropriate failure action... */
  }

It bears repeating that the purpose of errno is *not* to indicate 
system call failure, but to elaborate on the reason for such a failure.
Examining it without due cause, such as some_system_call() returning
a failure value, is wrong and dangerous, precisely because it sometimes
seems to generate the right behaviour.
--
Frank Wales, Systems Manager,        [frank@zen.co.uk<->mcvax!zen.co.uk!frank]
Zengrange Ltd., Greenfield Rd., Leeds, ENGLAND, LS9 8DB. (+44) 532 489048 x217