Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!rpi!uupsi!cmcl2!adm!news
From: CDCKAB%EMUVM1.BITNET@cunyvm.cuny.edu ( Karl Brendel)
Newsgroups: comp.lang.pascal
Subject: FileExists (was Re: Cursor On/Off Code plus ...)
Message-ID: <26609@adm.brl.mil>
Date: 19 Apr 91 16:00:46 GMT
Sender: news@adm.brl.mil
Lines: 69

In article <15647@dutrun.UUCP>, winfave@dutrun.uucp (Alexander
  Verbraeck) wrote:

>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:

[...deleted...]

>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;

(Alexander Verbraeck's posting provides an opportunity to discuss a
FileExists function. My posting should in no way be taken as a
criticism of his excellent discussion or of his particular coding.)

FileExists functions seem to be enjoying a spurt of popularity on
the net for some reason. :) As Alexander Verbraeck has pointed out,
some popular FileExists functions may have unexpected results. They
may also fail unexpectedly. E.g., if no file handles are available,
Reset will fail, and a Reset-based function will return False. (Does
one ever want to know if a file exists without caring if it can be
opened. Yes. One example is a network semaphore.)

Consider the use of FindFirst for a FileExists function. It works
without requiring a file handle, and can easily be used to check the
existance of directories, volume IDs, etc.:

uses DOS;

function FileExists(var fn : string; attr : word) : Boolean;
var
  sr : SearchRec;
begin {Exists}
  FindFirst(fn,attr,sr);
  FileExists := DosError = 0;
end; {Exists}

If you want this strictly for determining whether "real" files
exist, you can drop attr from the parameters and include an
appropriate value in the body of the function.

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| Karl Brendel                           Centers for Disease Control |
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