R devel - Dec 2011 - external pointers

I have an external pointer object that I'd like to pass from my R code to
some C code.  Per Section 5.13 of "Writing R Extensions", I've
noted that
"external pointers should only be used as part of an object with normal
semantics, for example an attribute or an element of a list."  So I've
written up a workable C function as such:

SEXP my_c_function(SEXP param)    {
     SEXP temp = getAttrib(param, install("ptr_attribute"));
     void * ptr = R_ExternalPtrAddr(temp);
     ...   // do useful things with ptr
}

I can pass my_c_function() an object with "normal" semantics (such as
an
integer vector), where the external pointer object is a parameter named
"ptr_attribute".  The function extracts the pointer object from param;
i.e.
this function does what I want it to do.

My question is:  is there a way to do this using the .C() interface, rather
than .Call()?   Using call_R seems a possibility - has anyone had luck
going this route?

Any suggestions appreciated.

Thanks, Patrick

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Patrick,

On Dec 28, 2011, at 12:07 AM, Patrick Leyshock wrote:
> I have an external pointer object that I'd like to pass from my R code
to
> some C code.  Per Section 5.13 of "Writing R Extensions",
I've noted that
> "external pointers should only be used as part of an object with
normal
> semantics, for example an attribute or an element of a list."  So
I've
> written up a workable C function as such:
> 
> SEXP my_c_function(SEXP param)    {
>     SEXP temp = getAttrib(param, install("ptr_attribute"));
>     void * ptr = R_ExternalPtrAddr(temp);
>     ...   // do useful things with ptr
> }
> 
> I can pass my_c_function() an object with "normal" semantics
(such as an
> integer vector), where the external pointer object is a parameter named
> "ptr_attribute".  The function extracts the pointer object from
param; i.e.
> this function does what I want it to do.
> 
> My question is:  is there a way to do this using the .C() interface, rather
> than .Call()?
No. Why would you want to do that? .C is slower and mostly for
backwards-compatibility. You can't pass anything complex with .C and the
docs recommend .Call for this. The whole point of .C was to bypass R API but in
your case you need it to get the pointer.

>   Using call_R seems a possibility - has anyone had luck
> going this route?
> 
call_R has nothing to do with this, it is sort of the equivalent of using the
restricted set of objects to evaluate R function (so you still can't pass
your pointer). As the docs say there is no point in using it in modern code.

Cheers,
Simon

> Any suggestions appreciated.
> 
> Thanks, Patrick
> 
> 	[[alternative HTML version deleted]]
> 
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