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Date: Fri, 30 Jan 1998 16:53:25 -0500 (EST)
From: Thomas David Rivers <rivers@dignus.com>
Message-Id: <199801302153.QAA29966@lakes.dignus.com>
To: atrens@nortel.ca, hackers@FreeBSD.ORG, joelh@gnu.org
Subject: Re: sharable static arrays?
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> 
> 
> >Hmmmm.... Either way, it's illegal to define 'foobared' twice.
> 
> Sorry, bad example.
> 
> >If these were auto variables, then strategy a would require both stack
> >space for the array at runtime, and text space for the initial value
> >in the executable, whereas strategy b should only require one copy of
> >its space in either text or data, I still haven't determined which.
> 
> The initializer on the right of the `=' is *always* a const. and gets
> stored in the text segment. Fortunately, if you make the thing on the
> left of the `=' a const most compilers will not duplicate the store.
> 
> Therefore, 
> 
> const char foob[10] = "0123456789";
> 
> will use 10 bytes ( all in text ) , and
> 
> char fooba[10] = "0123456789";
> 
> will use 20 bytes ( ten in text, ten in data ). It's that simple. ;)
> Now, what's interesting is that
> 
> char foobar[10] = " ";
> 
> will *also* use 20 bytes - the compiler will pad the initializer with
> zeroes.
> 
> So if you have a big dataset, make it const. You'll only get one copy (which
> is all you really want) and it'll be in a shareable text segment :)

 Yes - this is a good explanation...

 Just to add to the one example, 

   char foobar[10] = " ";

 doesn't necessarily have to consume 20 bytes in the object file.
Many systems "fill in" at module startup.. so you may have simply two
bytes for the string, and a descriptor of some kind which indicates the 
rest of the array should be filled with zeros.

> 
> Andrew
> 
> 
	
	- Dave Rivers -