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From: IO92203@MAINE.BITNET (Scott Maxell)
Newsgroups: comp.lang.pascal
Subject: Re: FILES and pointers
Message-ID: <91112.130537IO92203@MAINE.BITNET>
Date: 22 Apr 91 17:05:37 GMT
References: <1991Apr22.014013.7390@usenet.ins.cwru.edu>
Organization: University of Maine System
Lines: 77

>Say I want to save a dynamically sized structure.  People
>have suggested linked lists, but those can be very slow.
>Someone else suggested the following:
>
>type
>  bufferptr=^buffer;
>  buffer=array[1..64000] of byte;
>var
>  p: bufferptr;
>begin
>  getmem(p,(required size));
>  ...
>  freemem(p,(required size));
>  ...
>end.
>
>The problem is that I need to save this to disk so that it can
>be recalled as quickly as possible.  Instead of a text file
>with numbers written on it (way too slow and too big), I need
>a FILE of something.  But a FILE of what and how large will the
>file be?
>
>Ideas:
>
>FILE of bufferptr
>   dumb, it only saves address
>
>FILE of bufferptr^
>   is this even legal?  if it is, this looks like the
>   proper solution
>
>FILE of buffer
>   that has way to much disk storage space.  I need to
>   minimize both disk space and heap space required
>   although minimizing disk storage space seems a bit
>   more important
>
>Any suggestions or ideas?
 
 
  You might try using an untyped file and BlockRead/BlockWrite.
To open the untyped file, you should specify a record length of 1
since you want a file of BYTE.
 
File is declared:  FileVar : File;
 
ASSIGN (FileVar, 'filename.ext');
RESET  (FileVar, 1);
 
To read the file, you would use:
 
BLOCKREAD (FileVar, Buffer, SizeOf (Buffer), Result)
 
Result:            Result  : Word;
 
Result returns the number of bytes actually read from the file if it
differs from SizeOf (Buffer).
 
BlockWrite works the same way:
 
BLOCKWRITE (FileVar, Buffer, SizeOf (Buffer), Result)
 
Result returns the number of bytes actually written.
 
Result in both BlockRead/write is optional. If you don't specify it, and
the number of bytes is not an exact multiple of the record size that
you specified in your Reset/Rewrite, you'll get an I/O error.
 
P.S. This is all done from memory, so you should look things up for more
     specific information on it.
 
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