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From: winfave@dutrun.UUCP (Alexander Verbraeck)
Newsgroups: comp.lang.pascal
Subject: Re: Cursor On/Off Code plus ...
Message-ID: <15647@dutrun.UUCP>
Date: 17 Apr 91 10:59:32 GMT
References: <1991Apr8.192802.27683@noose.ecn.purdue.edu>
Reply-To: winfave@dutrun.tudelft.nl.UUCP (A.Verbraeck)
Organization: Delft University of Technology, The Netherlands
Lines: 67

The FileExists function as posted in the "Cursor On/Off Code Plus"
posting (originating from Dave Sisson) contains a slight but extremely
boresome error, as far as I can see. My first attempt to write a
FileExists function contained the same error, and it took me quite a
while to find it. The original FE routing looked something like:

function FE(n:string):boolean; {a little bit abbreviated}
var f:text;
begin
  {$I-} assign(f,n); reset(f);
        if IOresult<>0 then FE:=false else FE:=true;
  {$I+}
end;

When I include the above function in a program like:

program TestFE;

var
  i : integer;

function FE { as above }

begin
  writeln;
  for i:=1 to 40 do if FE('C:\CONFIG.SYS') then 
    write('E')
  else
    write('N');
  writeln;
end.

The output of this little program is (when C:\CONFIG.SYS exists):

EEEEEEEEEEEEEEENNNNNNNNNNNNNNNNNNNNNNNNN

which means that the file existed the first 15 times, and not anymore
afterward. What's the flaw? Not closing the file in FileExists, which
occupies a DOS file handler when it exists. What's the solution? Either
always closing the file between the {$I-} and {$I+} directives, or 
closing it after testing if IOresult reports that the file exists and
THEN closing the file. 
I used the first method, resulting in the following function:

function FileExists(fn:string) : boolean;

var
  f : text;

begin
  {$I-} 
    assign(f,fn);
    reset(f);
    close(f);
    FileExists:=(IOresult=0);
  {$I+}
end;

Sincerely,
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Alexander Verbraeck                    e-mail: winfave@duticai.tudelft.nl
Delft University of Technology                 winfave@hdetud1.bitnet
Department of Information Systems              winfave@duticai.uucp
PO Box 356, 2600 AJ  The Netherlands           winfave@dutrun.tudelft.nl
Tel: +31 15 783805  Fax: +31 15 786632/787022  winfave@dutrun.uucp
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