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From: nboogaar@ruunsa.fys.ruu.nl (Martin v.d. Boogaard)
Newsgroups: comp.lang.pascal
Subject: Re: Problem with TP 5.5 Reset
Message-ID: <1407@ruunsa.fys.ruu.nl>
Date: 10 Aug 90 06:45:55 GMT
References: <8096@ucrmath.ucr.edu>
Organization: University of Utrecht, Dept. of Physics
Lines: 48

In <8096@ucrmath.ucr.edu> pdm@ucrmath.ucr.edu (paul moore) writes:

%  I've been having a problem using the Reset function to determine
%  if a file exists, and was wondering if anybody has seen anything similar.
%   The code is basically:
%    var
%       F : file of MyOwn_type;
%    [...]
%    assign( F, Fname );
%    {$I-}
%    reset( F );
%    {$I+}
%    The program produces a runtime 002 error (file not found) and dumps
%  me out to DOS.
[...]
%   Any thoughts?

Well, the obvious one: supposing the file Fname really doesn't exist,
do you read the ioresult value that is set when you use an I/O function like
reset? Something like

  [...] {$I-} reset ( F ); {$I+}
  if ioresult > 0
  then
    [handle error yourself]
  else
    [whatever you planned to do with the file]

gives you control off the error handling and resets ioresult to 0.
Beware of the `logical shortcircuit' compiler option combined with code like

  if Flag and ( ioresult = 0 ) [...]

If Flag equals FALSE ioresult is not called and the run-time error you thought
you had prevented by using {$I+} [...] {$I-} shows up at the next, probably
very different, I/O statement.

Off course, since Turbo Pascal is non-portable anyway, you might just as well
use the built-in findfirst and findnext functions if the only purpose of the
operation is to determine a file's existence.


Martin J. van den Boogaard         | Dept. of Atomic & Interface Physics
                                   | Debye Institute--Utrecht University
P.O. Box 80.000                    |
NL-3508 TA  Utrecht                | decnet:   ruunsc::boogaard
the Netherlands                    | bitnet:   boogaard@hutruu51
+31 30 532904                      | internet: nboogaar@fys.ruu.nl