[Vm-dev] nested structures with FFI?
Esteban Lorenzano
estebanlm at gmail.com
Fri Aug 21 02:02:46 UTC 2015
> On 20 Aug 2015, at 22:47, Esteban Lorenzano <estebanlm at gmail.com> wrote:
>
>>
>> On 20 Aug 2015, at 21:48, Max Leske <maxleske at gmail.com <mailto:maxleske at gmail.com>> wrote:
>>
>>
>>> On 20 Aug 2015, at 21:28, vm-dev-request at lists.squeakfoundation.org <mailto:vm-dev-request at lists.squeakfoundation.org> wrote:
>>>
>>> Message: 1
>>> Date: Thu, 20 Aug 2015 17:51:43 +0200
>>> From: Esteban Lorenzano <estebanlm at gmail.com <mailto:estebanlm at gmail.com>>
>>> Subject: [Vm-dev] nested structures with FFI?
>>> To: Squeak Virtual Machine Development Discussion
>>> <vm-dev at lists.squeakfoundation.org <mailto:vm-dev at lists.squeakfoundation.org>>
>>> Message-ID: <4F31ED4B-6246-43D6-B85B-7215D8570D28 at gmail.com <mailto:4F31ED4B-6246-43D6-B85B-7215D8570D28 at gmail.com>>
>>> Content-Type: text/plain; charset=utf-8
>>>
>>> Hi,
>>>
>>> I’m doing some tests with nested structures (to handle the conversion of NativeBoost-FFI to regular FFI) and I’m having some problems when accessing fields in the nested structure.
>>> For example, this tests:
>>>
>>> FFITestNestingStructure class>>#fieldsDesc
>>> ^ #(
>>> byte one;
>>> FFITestStructure nested;
>>> )
>>>
>>> FFITestStructure class>>#fieldsDesc
>>> ^ #(
>>> byte byte;
>>> short short;
>>> long long;
>>> float float;
>>> double double;
>>> int64 int64;
>>> )
>>>
>>> (definition is NB-compatible, so do not take the differences with regular FFI into account)
>>>
>>> testNestedStructure
>>> | s1 |
>>>
>>> s1 := FFITestNestingStructure new.
>>> s1 nested byte: 42.
>>> self assert: s1 nested byte = 42
>>>
>>> testExternallyAllocatedNestedStructure
>>> | s1 |
>>>
>>> s1 := FFITestNestingStructure externalNew.
>>> self assert: s1 getHandle class = ExternalAddress.
>>> s1 nested byte: 42.
>>> self assert: s1 nested byte = 42
>>>
>>>
>>> fails always because
>>>
>>> s1 nested byte = 0.
>>>
>>> (which is obviously bad).
>>>
>>> I remember Eliot saying that structures with FFI was not being optimal, and I wonder (before start digging more in deep) if this could have something to do?
>>
>> I use nested structs in NB for the libgit2 bindings and they work as advertised. There can be problems if some structure is packed (i.e. uses non-4-byte alignment for some field) but that shouldn’t apply in your test case.
>>
>> Not sure if this helps you in any way but I thought I’d let you know :)
>
> I do not think so :)
> I’m quite sure this is not going to be a problem when using the ThreadedFFI backend instead ASMJIT… now my problem is with this backend, for the moment :)
I’m just realised that this behaviour is correct according current ExternalStructure implementation, because:
nested
"This method was automatically generated"
^FFITestStructure fromHandle: (handle structAt: 2 length: 27)
and
ByteArray>>#structAt: byteOffset length: length
"Return a structure of the given length starting at the indicated byte offset."
| value |
value := ByteArray new: length.
1 to: length do:[:i|
value unsignedByteAt: i put: (self unsignedByteAt: byteOffset+i-1)].
^value
So it is giving me always a copy…
So we need a better solution (a non copy way to access)? I’m thinking on passing a “MappedByteArray” something that can work with the same original ByteArray (or ExternalAddress, is the same), to affect the referenced data… what do you think?
cheers!
Esteban
>
> Esteban
>
>>
>> Cheers,
>> Max
>>
>>>
>>> cheers,
>>> Esteban
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