[Rd] Copying objects prior to .Call

[Simon Urbanek]

On Jan 11, 2012, at 12:08 PM, Taylor Arnold wrote:

> R-devel,
> 
> I have noticed that making a copy of an object in R prior to using
> .Call on the original object can
> cause the C code to alter not only the object passed to it but also
> the copy in R.

Please see the docs - .Call does *NOT* have a DUP argument - you are responsible for duplication at all times if you make modifications (e.g. using duplicate()).

Cheers,
Simon


> A simple example
> is:
> 
>> x <- 2
>> y <- x
>> .Call("addOne", x, DUP=TRUE) # Changing DUP does not alter output
> NULL
>> x
> [1] 3
>> y
> [1] 3
> 
> And corresponding simple C code:
> 
> "test.c":
> #include <R.h>
> #include <Rinternals.h>
> #include <Rmath.h>
> 
> SEXP addOne(SEXP input) {
>   REAL(input)[0] = REAL(input)[0] + 1;
>   return R_NilValue;
> }
> 
> I assume that this is simply a result of lazy loading in R, and well
> documented. My question is, do
> there exist functions to (1) force R to make a copy of an object
> (force() does not work), and (2) to check
> whether two objects are actually pointing to the same memory address.
> For question 1, I have
> found specific operations which force a copy of a given datatype, but
> would prefer a more general
> solution.
> 
> Thank you,
> 
> Taylor
> 
>> sessionInfo()
> R version 2.14.1 (2011-12-22)
> Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)
> 
> locale:
> [1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8
> 
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
> 
> loaded via a namespace (and not attached):
> [1] tools_2.14.1
> 
> 
> --
> Taylor B. Arnold
> Department of Statistics
> Yale University
> 24 Hillhouse Avenue
> New Haven, CT 06520
> 
> e-mail: taylor.arnold at yale.edu
> 
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