Sender: vheyndri AT rug DOT ac DOT be
Message-Id: <35403B8A.776D@rug.ac.be>
Date: Fri, 24 Apr 1998 09:13:14 +0200
From: Vik Heyndrickx <Vik DOT Heyndrickx AT rug DOT ac DOT be>
Mime-Version: 1.0
Newsgroups: comp.os.msdos.djgpp
To: Matti Haavikko <mhaavi AT sci DOT fi>
Cc: djgpp AT delorie DOT com
Subject: Re: Interfacing with printer
References: <353ec46f DOT 9950927 AT news DOT sci DOT fi>
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Precedence: bulk

Matti Haavikko wrote:
> 
> I have run in trouble with my printer.
> When I fopen() "lpt1", it gives no error even if no printer is
> connected.

This happens because lpt1 is not really the printer, but the printer
port which *is* present. Technically, you don't need a printer connected
to the other end of the cable. You could also have e.g. a computer
connected to it. Since the hardware port doesn't necessarily see whether
something is connected to it, DOS never fails to open it.

> When I fprintf() to the *FILE fopen gave me, it happily returns the
> number of characters written, whether they really print or not.

Does someone know whether full buffering is enabled for devices not
related to tty? It probably is, which means that the bytes you send to
the printer first will end up into a memory buffer associated with that
file handle. Until the buffer is full, nothing will be effectively
written (unless you call fflush), which of course cannot yield an error
situation.

> How can I
> * output some text to the printer and see, if the print operation
> actually succeeded?

Try to switch FULL buffering off temporarily (function setvbuf), and
then write something to the printer. I cannot say I know this works,
however...

-- 
 \ Vik /-_-_-_-_-_-_/
  \___/ Heyndrickx /
   \ /-_-_-_-_-_-_/  Knight in the Order of the Unsigned Types