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FFI - can I access external constants?

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Jim McLoughlin

FFI - can I access external constants?

Hello,

I have gotten pretty comfortable using FFI to access external
libraries from squeak, but have hit a seemingly simple snag:

I want to access some constant structures defined in my external lib.
If there was a function that returned them, it would be no problem,
but I don't see any obvious way to directly access them.

Are my only options 1) enhance the external lib to make constants
accessible via functions; 2) abandon FFI for a plugin approach ?

The constants are defined in the library like:

extern const stuctType *constantName;

best,

Jim M

Andreas.Raab

Re: FFI - can I access external constants?

Jim McLoughlin wrote:
> I want to access some constant structures defined in my external lib.
> If there was a function that returned them, it would be no problem,
> but I don't see any obvious way to directly access them.

Unfortunately, there isn't. The FFI doesn't expose the direct equivalent
of dlsym() but only uses it implicitly when invoking an FFI call. I
guess this should be considered a bug...

Cheers,
- Andreas

johnmci

Re: FFI - can I access external constants?

Ah, of course obviously if there is some way to expose the value via
a library call, you could do that in FFI.
Like use dlsym() to get the address of the constant, then memcpy to a
squeak external data object....

On Feb 13, 2007, at 1:34 PM, Andreas Raab wrote:

> Jim McLoughlin wrote:
>> I want to access some constant structures defined in my external lib.
>> If there was a function that returned them, it would be no problem,
>> but I don't see any obvious way to directly access them.
>
> Unfortunately, there isn't. The FFI doesn't expose the direct
> equivalent of dlsym() but only uses it implicitly when invoking an
> FFI call. I guess this should be considered a bug...
>
> Cheers,
> - Andreas
>

--
========================================================================
===
John M. McIntosh <[hidden email]>
Corporate Smalltalk Consulting Ltd. http://www.smalltalkconsulting.com
========================================================================
===

Andreas.Raab

Re: FFI - can I access external constants?

John M McIntosh wrote:
> Ah, of course obviously if there is some way to expose the value via a
> library call, you could do that in FFI.
> Like use dlsym() to get the address of the constant, then memcpy to a
> squeak external data object....

Good point! Of course, you can just use FFI to access dlopen(), dlsym()
and friends and then construct the appropriate handle from it. It's a
little bit awkward and I don't know if that works correctly on all
platforms (e.g. whether the system will use the already mapped shared
library or actually map a new one) but if you're really desperate this
is definitely worth trying.

Cheers,
- Andreas

>
>
> On Feb 13, 2007, at 1:34 PM, Andreas Raab wrote:
>
>> Jim McLoughlin wrote:
>>> I want to access some constant structures defined in my external lib.
>>> If there was a function that returned them, it would be no problem,
>>> but I don't see any obvious way to directly access them.
>>
>> Unfortunately, there isn't. The FFI doesn't expose the direct
>> equivalent of dlsym() but only uses it implicitly when invoking an FFI
>> call. I guess this should be considered a bug...
>>
>> Cheers,
>> - Andreas
>>
>
> --
> ===========================================================================
> John M. McIntosh <[hidden email]>
> Corporate Smalltalk Consulting Ltd. http://www.smalltalkconsulting.com
> ===========================================================================
>
>
>
>

Jim McLoughlin

Re: FFI - can I access external constants?

I am on OSX - I used ffi calls to dlopen(), dlsym() in
module:System.framework and it worked like a charm. Many thanks.

Jim M

On 2/13/07, Andreas Raab <[hidden email]> wrote:

> John M McIntosh wrote:
> > Ah, of course obviously if there is some way to expose the value via a
> > library call, you could do that in FFI.
> > Like use dlsym() to get the address of the constant, then memcpy to a
> > squeak external data object....
>
> Good point! Of course, you can just use FFI to access dlopen(), dlsym()
> and friends and then construct the appropriate handle from it. It's a
> little bit awkward and I don't know if that works correctly on all
> platforms (e.g. whether the system will use the already mapped shared
> library or actually map a new one) but if you're really desperate this
> is definitely worth trying.
>
> Cheers,
> - Andreas