Andreas,

#getProcAddress: is a Smalltalk message, so it does not have a return type.  It does return an ExternalData because it it ultimately calls

 <cdecl: void* 'FindSymbol' ( long char* ) >

I had to create FindSymbol as a simply wrapper around dlsym() because the vm dies a quick death over (what I believe to be) linking hassles similar to recent problems in skype.  Of course, I am referring the Linux vm or I would be calling GetProcAddress().

One thing that I considered is that the aspects of the ExternalData might be incorrect.  The handle is an external address and type is an ExternalType that prints itself as void*.  With many variations, if the call was made, the vm crashed.

Bill


-----Original Message-----
From: pharo-project-bounces at lists.gforge.inria.fr [mailto:pharo-project-bounces at lists.gforge.inria.fr] On Behalf Of Andreas Raab
Sent: Saturday, April 03, 2010 11:54 AM
To: The general-purpose Squeak developers list
Cc: Pharo-project at lists.gforge.inria.fr
Subject: Re: [Pharo-project] FFI - structure pointer

On 4/3/2010 6:36 AM, Schwab,Wilhelm K wrote:
> Andreas, John, anybody,
>
> I need to find the address of a "function" (it is in fact a global structure pointer somewhere in memory) and pass it to another function.
>
> Suppose the structure is called Type and the function allocate_one().  
> I call the function something like
>
> allocateOne:type size:anInt
>    <cdecl: SomeOtherStruct* 'allocate_one' ( Type* long )>
>
>
> but I am having an awful time getting the pointer to Type.  Given that 
> dlsym() will return a pointer to it, how can I represent that in 
> Smalltalk?  I have tried variations on
>
>    "Indirectly call dlsym()"
>    address := Library default getProcAddress:'name_of_global_struct'.
>
>    "try to create Type* from the resulting address"
>    type := Type fromHandle:address.
>    type := Type fromHandle:address getHandle.
>    type := ExternalData fromHandle:address type:Type externalType asPointerType.
>
>    Something := Library default allocateOne:type size:2.
>
> and it either complains about bad arguments during the call or segment faults.  Any ideas or similar examples?

The return type of getProcAddress: is likely wrong. It should be declared to return void* which will cause it to return an ExternalData. 
With an ExternalData your second variant "Type fromHandle: data getHandle" will work.

Cheers,
   - Andreas

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