[FFI] Pointer arithmetic impossible on ByteArray?

Andreas Raab andreas.raab at gmx.de
Sat May 20 01:03:43 UTC 2006

nicolas cellier wrote:
> Le Samedi 20 Mai 2006 02:30, Andreas Raab a écrit :
>> Not a chance. The "base address" of an object isn't constant; the object
>> will be moved by a GC and therefore you simply can't guarantee that "foo
>> baseAddress" is a meaningful value for any period of time other than  in
>> the primitive call itself (even the return from the primitive could
>> cause a process switch, which could cause GC somewhere, which could move
>> the object, which invalidates the base address).
>> Cheers,
>>    - Andreas
> Of course, i want it in a primitive to avoid this kind of problem.

But even if it's in a primitive, it's no longer immune from GC when you 
return from it. So you can neither return "foo baseAddress" or "foo 
baseAddress+X" from a primitive in a meaningful way.  You'd have to do 
the pointer calculation and the call from the same primitive at which 
point you're in plugin (and not FFI) land. And yes, in a plugin you can 
do that just fine.

> Is a plugin immune to that kind of moving GC ?

No, of course not. You can't keep references to objects across primitive 
calls for example. Inside primitive calls it depends on what the 
primitive does - various operations (like allocation) may cause GC and 
an oop you referred to before the GC will be no longer valid after it 
unless you explicitly remap it.

   - Andreas

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