[Vm-dev] nested structures with FFI?

[Max Leske]

> On 20 Aug 2015, at 21:28, 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 :)

Cheers,
Max

> 
> cheers, 
> Esteban

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