[squeak-dev] Re: [Pharo-project] FFI - structure pointer

Andreas Raab andreas.raab at gmx.de
Sat Apr 3 18:16:19 UTC 2010

On 4/3/2010 10:10 AM, Schwab,Wilhelm K wrote:
> 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.

Can you verify the correctness of both pointer as well as function 
signature? Depending on platform and/or entity in question you just 
*might* get an extra indirection.

If you can't get the code to work as-is I would recommend compiling your 
own library that exports a symbol in the form you're using and then 
verify correctness of the bridge that way.

   - Andreas

> -----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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