[Vm-dev] nested structures with FFI?

[Esteban Lorenzano]

> 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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