From: Greg Willits <lists@...>
Date: 2009-07-06T02:08:11+09:00
Subject: Re: file.seek and unused bytes

Brian Candler wrote:
> Eleanor McHugh wrote:
>>> Can I count on those unused bytes always being zero?
>> 
>> Unfortunately you're getting lucky. A seek adjusts the file pointer
>> but doesn't write anything to disk so whilst your 'unused' bytes won't
>> be changing value as a result of writing data to the file unless you
>> write the full record, you can't rely on them not having a value other
>> than zero if you don't.
> 
> I don't believe that's the case today. If it were, then you would have a 
> very easy way to examine the contents of unused sectors on the disk - 
> which would allow you to see other people's deleted files, passwords 
> etc.
> 
> It was possible on old mainframe systems in the 80's though :-)

80's micros too with BASIC :-P

> But today, if you extend a file using seek, you should always read 
> zeros.

That makes a great deal of sense, and would be consistent with what I 
was seeing. I was wondering why the values being returned were zeros 
instead of nil or something else.

Either way, I know its a better practice to pack the rows, but I had a 
moment of laziness because I'm dealing will a couple million rows and 
figured if there was some processing time to be saved, I'd take 
advantage of it.

I would have experimented, but I don't know how to ensure that the 
various file contents are in fact being written to the exact same disk 
space.

-- gw

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