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From: dgk@ulysses.UUCP (David Korn)
Newsgroups: net.unix,net.unix-wizards
Subject: Re: Not a typewriter
Message-ID: <1296@ulysses.UUCP>
Date: Sun, 15-Jun-86 23:35:20 EDT
Article-I.D.: ulysses.1296
Posted: Sun Jun 15 23:35:20 1986
Date-Received: Tue, 17-Jun-86 22:29:14 EDT
References: <1683@homxb.UUCP> <542@codas.ATT.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 31
Xref: watmath net.unix:8164 net.unix-wizards:18413

> > What does the sys_errlist message 'Not a typewriter' *really* mean??
> 
> Errno 25 is usually generated when a program tries to execute an
> ioctl command that expects the file descriptor to be a terminal.
> (ie: you can't set the baud rate on a file).
> 
> However I have also noticed that some aplications aren't very
> carefull when they use errno. For example, Informix may give
> you this message if it cannot open a database file because it
> does not exist!
> 
> I think there may be one or two system calls that will alter
> errno to be 25, without returning a non-zero value, when in
> fact there was no error.
> -- 
> 			Mikel Manitius @ AT&T-IS Altamonte Springs, FL
> 			...{seismo!akgua|ihnp4|cbosgd}!codas!mikel.ATT.UUCP

Things can be even worse than this.  I recently ran across a problem where
an ioctl worked fine and I was able to change the tty mode.  When I went
to restore the mode, I got an ENOTTY error from ioctl().  Appearantly,
being a 'tty' is not an invariant of a file stream.  The line was
actually a pseudo-tty and the process which was processing characters
died.

It appears that you can't do an isatty at the beginning of a program and
trust the result.  The only safe thing that you can do is to check the
error status of ioctl() and see if it changes.

David Korn
ulysses!dgk