OpenEuphoria

Re: detecting if an atom is a pointer

• Posted by Pete Lomax <petelomax at blueyonder.co.uk> Nov 16, 2006
• 570 views

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:

atom ptr
ptr = allocate(10)
?ptr
poke(ptr,"0123456789")
while peek(ptr)<'3' do
  ptr+=1
end while
?ptr

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