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From Luke Kenneth Casson Leighton <l...@samba-tng.org>
Subject Re: Shared Memory Hash Table / SMS
Date Thu, 05 Jul 2001 12:03:12 GMT
On Mon, Jul 02, 2001 at 07:10:27PM +0200, Sander Striker wrote:
> > On Mon, Jul 02, 2001 at 09:38:37AM -0700, Ian Holsman wrote:
> > > Just wondering if any has got one of these going, 
> > > 
> > > how does the proposed SMS shared library handle pointers?
> > 
> > Can you please expand a bit further on what you mean?  
> > 
> > Oh, do you mean, "How does the proposed SMS shared memory system 
> > handle pointers in data structures that it has?"  Oh, that, I dunno.  
> > Prolly with some type of offset/fixup routine.  But, that may not 
> > be ideal.  -- justin
> 
> We basically still need to work that out :)
> I'm personally still not sold on the offsets idea. I'm still not
> getting _why_ we would need offsets instead of a simple pointer
> to a piece of mem.

when you request that a shared memory space be mapped into real
memory on different processes, it is extremely unlikely that it
will be mapped to the *same* physical location in memory on
each process (probability: 2^^-32 or 2^^-64...)

so therefore, if you store any pointers from one process _into_
the shared memory block, when they are retrieved by _another_
process, they will of course point to garbage.

there are a number of techniques for dealing with this, but
basically they boil down to structure-packing, effectively
treating the memory exactly as you would a very very fast file.

all best,

luke

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