[geos-devel] Ruby, FFI and ERROR_/NOTICE_MESSAGE

J Smith dark.panda+lists at gmail.com
Thu Dec 9 00:34:22 EST 2010

On Thu, Dec 9, 2010 at 12:09 AM, Charlie Savage <cfis at savagexi.com> wrote:
>> I believe that the crux of the problem may be due to GEOS's error
>> handlers using varargs, which libffi cannot handle in callbacks
>> according to its documentation.
> Just did a quick read of https://github.com/ffi/ffi/wiki/examples and the
> google group. Seems like in general FFI does support varargs.  Are callbacks
> a special case that isn't handled?  How hard do you suppose to get them to
> work?

I think callbacks are a special case, although the documentation is
somewhat vague. In the Missing Features section of the info
documentation, it says:

"There is no support for calling varargs functions.  This may work on
some platforms, depending on how the ABI is defined, but it is not

I believe this is referring to callbacks. When setting up a callback
along these lines...

attach_function(:initGEOS_r, callback([ :string, :varargs ], :void), :void)
def error_handler(*args)

All you get for the varargs argument is nil and the segfaults and
weirdness occurs as before. It appears that this is a special case, I

Another option beyond modifying GEOS directly might be to write a
small native C shim between the CAPI and Ruby and access the error
messages through that. It might not be ideal, as I think it would be
preferable to keep the library purely Ruby, but it would be an option
that wouldn't affect the CAPI or cause any binary compatibility breaks
should the context handler need to be changed.

I'll ask around the FFI groups and see what turns up.


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