On 7 October 2010 01:27, Schwab,Wilhelm K <bschwab@anest.ufl.edu> wrote:
Sig,
Am I correct in taking this as a description of how to solve it with NB, not Squeak/Pharo FFI? Â That's ok, just want to know if there is an applicable lesson on FFI to be extracted.
You say the NB callout generator knows nothing about types. Â Is that how it should be? Â The scenario I have in mind is a bogus type and whether that can/should be flagged at compile time vs. runtime. Â Maybe all it takes is a scan for a suitable handler before or after the method is compiled. Â I ask because I hit a related snag with FFI runtime errors being reported as coercion errors when in fact I had used errant types in the callout definitions. Â IIRC, Andreas patched the compiler to report the bogus types.
"Or, if you want make double* type to use it as a DoubleArray, then you can add an alias:
MyClass>>externalTypeAlias: aTypeName  aTypeName = 'double*' ifTrue: [ ^ #DoubleArray ].  ^ nil
Then, any method which contains an FFI callout in MyClass, will use that alias."
What is MyClass in the above? Â Is it the class holding the callout method, or it elsewhere in the chain? Â I ask because it could be a sore spot for maintenance/pluggability. Â I'm wondering how you and I would independently add such instructions. Â If it goes in the external type subclass, then it is probably fine; if it goes in a class that makes the call, we might stomp on each other. Â Make sense? Â My hunch is that you have it handled.
Its in an any class, which using FFI callouts. We can stomp on each other if you define aliases in Object class :) But this won't be a good idea, isnt? NBExternalType subclasses defining coercions between ST oops and concrete C type, they are not for defining aliases.
Bill
________________________________________ From: pharo-project-bounces@lists.gforge.inria.fr [pharo-project-bounces@lists.gforge.inria.fr] On Behalf Of Igor Stasenko [siguctua@gmail.com] Sent: Monday, October 04, 2010 7:36 AM To: Pharo-project@lists.gforge.inria.fr Subject: Re: [Pharo-project] Pointer types
On 4 October 2010 06:15, Schwab,Wilhelm K <bschwab@anest.ufl.edu> wrote:
Sig,
You mentioned something that might be useful, depending on where it lives. Â Our ability to use underscores now leaves me thinking I really should rework some things that suffered mightily: there are some external things that are named assuming underscores, and in mixed case, they turn into an unreadable mess. Â Along with this, I have a large number of FFI calls that take double pointers, and I have for some time been demoralized into (or stupid enough to<g>) expressing them as void pointers to allow me to pass byte arrays into them.
Is there a way that I can get FFI to recognize DOUBLEArray (which uses a ByteArray to hold its data) as something that should be acceptable for a double* call? Â If not FFI, how does NB handle it?
In NB, there is a type system, which implements C<->Smalltalk coercions. The responsibility of type coercions taken by a subclasses of NBExternalType, so NB callout code generator actually knows nothing about types and how to coerce them.
To create own custom type, you need to subclass from NBExternalType and override methods which responsible for coercions. There you can define any kind of operations , what needs to be done in order to push value on stack, or convert it to some smalltalk object.
Then, in your DoubleArray class, simply implement a message on a class side:
asNBExternalType: gen
    ^ MyDoubleArrayExternalType new
And then, in FFI callouts you can use a class name directly:
DoubleArray foo (DoubleArray arr, DoubleArray * arrPtr)
etc.
Or, if you want make double* type to use it as a DoubleArray, then you can add an alias:
MyClass>>externalTypeAlias: aTypeName  aTypeName = 'double*' ifTrue: [ ^ #DoubleArray ].  ^ nil
Then, any method which contains an FFI callout in MyClass, will use that alias. So, you can write callout as:
double* foo (double *arr, double ** arrPtrPtr)
but it will be understood as:
DoubleArray foo (DoubleArray arr, DoubleArray * arrPtr)
About handling pointers.. well for smalltalk there is no pointers, just objects. So, in NB NBExternalType keeps two things:  - a value type  - a pointer arity
So, for example, when you specifying type, like: char ***
a value type will be char and pointer arity will be 3.
It is up to NBExternalType subclass how to push values whose pointer arity > 0. By default, if pointerArity > 0, then it is treated as a pointer, which can be either: - nil, ByteArray, or NBExternalAddress
Bill
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