OpenEuphoria

Re: detecting if an atom is a pointer

• Posted by Chris Bensler <bensler at nt.net> Nov 16, 2006
• 550 views

Pete Lomax wrote:
> 
> On Wed, 15 Nov 2006 15:45:06 -0800, Chris Bensler
> <guest at RapidEuphoria.com> wrote:
> 
> >In my testing it appears that the pointers never use all the high 
> >bits. Is this because Eu's heaps? or OS heaps? Would it be safe to use 
> >those bits to detect identifiers?
> The windows api GlobalAlloc is documented somewhere as always
> returning 8-byte aligned addresses, but occasionally it does not.
> On a 32-bit machine, all native allocation methods will be 32-bit
> aligned. However:
> }}}
<eucode>
> atom ptr
> ptr = allocate(10)
> ?ptr
> poke(ptr,"0123456789")
> while peek(ptr)<'3' do
>   ptr+=1
> end while
> ?ptr
> </eucode>
{{{

> clearly results in a non-4-byte-aligned pointer, and likewise any
> pointer returned from a c function is probably 4-byte-aligned but in
> no way guaranteed to be. GetLogicalDriveStrings springs to mind as a
> c/winapi function which packs strings back-to-back, leading to code
> like the above.
> 
> So the answer is that you probably could get something working by
> mangling/mapping C_INT etc to always have one or both of the two least
> significant bits set, but one day it will break.
> 
> I also remember once assuming that allocate (or api) won't return
> memory addresses below something like #00100000 because that is where
> the OS or something similar lives on win98. Of course it went belly up
> the first time anyone ran it on XP.
> 
> My advice: pass an extra boolean flag to indicate what it is.
> 
> Regards,
> Pete

Thanks Pete. I've worked around it.


Chris Bensler
~ The difference between ordinary and extraordinary is that little extra ~
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