Newsgroups: comp.lang.pascal
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!caen!umich!sharkey!tygra!dave
From: dave@tygra.Michigan.COM (David Conrad)
Subject: Re: TP error while opening R/O files
Organization: CAT-TALK Conferencing System, Detroit, MI
Date: Thu, 9 May 91 10:00:31 GMT
Message-ID: <1991May9.100031.25333@tygra.Michigan.COM>
Keywords: r/o turbo pascal 6.0 borland files opening error
References: <1991May02.160542.28060@cs.ruu.nl> <9144@mcshh.hanse.de>

In article <9144@mcshh.hanse.de> frimp@mcshh.hanse.de (Frank Heinzius) writes:
>
>[In response to a question about accessing read only files]
>
>You must reassign the "filemode" system variable like this:
>
>temp := filemode;
>filemode := 0;
>reset(read_only_file);
>{other file operations}
>{...}
>close(read_only_file);
>filemode := temp;

Incidentally, filemode need not be changed the whole time the read only file
is opened, only while opening (i.e., resetting,) it.  It is best to always
open files you only intend to read with filemode set to 0, e.g.

  filemode := 0;  (* open input file in read mode *)
  reset (infile);
  filemode := 2;  (* open output file in read/write mode *)
  reset (outfile);
  while not eof(infile) do
    (* do it! *)
  close (infile);
  close (outfile);

If you try to open a file with filemode = 2, even if you reset, not rewrite
it, even if you only read from it, you will get a runtime error (access
denied).  DOS won't allow a read only file to be opened in read/write mode,
and rightly so.  (BTW, 2 is the default for filemode, so if you don't set it
to anything, 2 (read/write) is what you get.)  Even if you expect a file not
to be read only, if you only want to read it, it's best to let the operating
system know that.

Why?  Well, a user may mark a file which she knows is strictly an input file
read only to prevent it getting clobbered.  Do you want your application to
crash then?  Or perhaps your program gets loaded onto a network.  A user
might not have write priviledges in the current directory, which means that
your program will crash even if the specific file isn't marked read only,
because the network won't allow the current user to open *any* of the files
in the current directory for writing.
 
Also, for networks and file sharing, it's important that the network know
just exactly how the file is to be used, but that is, as they say, beyond
the scope of this article.  Also I should mention that this article is
mostly not a response to Frank's article, but just general info.

David R. Conrad     |   "A dream unthreatened by the morning light
dave@michigan.com   |   could blow this soul right through the roof
...!tygra!dave      |   of the night." -- Pink Floyd, "Learning to Fly"
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