[Rd] Passing externalptr to .C()

[Duncan Murdoch]

On 12-05-11 5:20 PM, Rick Sayre wrote:
> Greetings.
>
> 2.15.0 added this behavior
> http://developer.r-project.org/blosxom.cgi/R-devel/NEWS/2012/03/29#n2012-03-29
>
>       o Passing R objects other than atomic vectors, functions, lists and
>         environments to .C() is now deprecated and will give a warning.
>         Most cases (especially NULL) are actually coding errors.  NULL
>         will be disallowed in future.
>
>
> This seems to make sense, except that this case includes externalptrs.
>
> I have quite a bit of code, designed to interface with various
> external hardware devices, which uses this sort of idiom:
>
> # for example
> getDeviceInfo<- function(handle) {
>       .C("groovyDevice_getDeviceInfo", PACKAGE="groovyDevice",
> 	handle,			
> 	status = as.integer(0))[-1]	# skip handle
> }
>
> where "handle" was the result of a .Call to a routine which
> returned a SEXP which was the result of a R_MakeExternalPtr() call
>
> The "c" routine looked like:
> void
> groovyDevice_getDeviceInfo(SEXP handle, int *status)
> {
>       groovyHandle *gh = R_ExternalPtrAddr(handle);
>       *status = GroovyStatus(gh);
> }
>
> This all used to work fine.  As of 2.15.0, I now get this:
> Warning message:
> In getDeviceInfo() :
>     passing an object of type 'externalptr' to .C (arg 1) is deprecated
>
> Passing the same handle to a .Call() does [of course] work fine.
>
> I thought my usage was exactly as designed.  How then should I be
> passing an externalptr to a .C() call?

I think you shouldn't be doing that.  You should be using .Call.

If your code can handle external pointer objects (as in the example), 
this should be an easy transition.  I haven't tested this, but I think 
the conversion to the .Call interface (less error checking) is simply

  SEXP_getDeviceInfo(SEXP handle, SEXP status)
  {
        groovyHandle *gh = R_ExternalPtrAddr(handle);
        INTEGER(Sstatus)[0] = GroovyStatus(gh);
        return R_NilValue;
  }


If your code is using the pointer just as an opaque handle, I'd suggest 
keeping an array of them, and pass an index into that array:  then .C 
will be fine.

Duncan Murdoch