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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