[Pharo-project] Pointer types

Igor Stasenko siguctua at gmail.com
Mon Oct 4 07:36:03 EDT 2010


On 4 October 2010 06:15, Schwab,Wilhelm K <bschwab at anest.ufl.edu> wrote:
> Sig,
>
> You mentioned something that might be useful, depending on where it lives.  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.  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?  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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-- 
Best regards,
Igor Stasenko AKA sig.




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