R devel - Oct 2007 - documentation of .C (PR#9948)

Full_Name: Martin Schlather
Version: R version 2.7.0 Under development (unstable) (2007-10-01 r43043)
OS: Linux
Submission from: (NULL) (91.3.209.203)


Hi,

There are 2 dangers with using 'DUP=FALSE' mentioned:
  * formal arguments
  * lists

Would you also mention a third one, namely
that values in R are now only referenced  whenever 
possible and not always copied; hence .C(..., DUP=FALSE) 
may change the values of other local variables.

E.g., with C code 
   void addone(double *x) {  *x = *x + 1; }

you get 
  
  x <- as.double(1)
  y <- x
  .C("addone", x, PACKAGE="test", DUP=FALSE)
  print(c(x,y))
#[1] 2 2

  
  x <- as.double(1)
  y <- as.double(x)
 .C("addone", x, PACKAGE="test", DUP=FALSE)
  print(c(x,y))
#[1] 2 2 

  x <- as.double(1)
  y <- as.integer(x)
 .C("addone", x, PACKAGE="test", DUP=FALSE)
  print(c(x,y))
#[1] 2 1

Many thanks and kind regards,
Martin

On Wed, 10 Oct 2007, schlather at math.uni-goettingen.de wrote:
> Full_Name: Martin Schlather
> Version: R version 2.7.0 Under development (unstable) (2007-10-01 r43043)
> OS: Linux
> Submission from: (NULL) (91.3.209.203)
>
>
> Hi,
>
> There are 2 dangers with using 'DUP=FALSE' mentioned:
>  * formal arguments
>  * lists
>
> Would you also mention a third one, namely
> that values in R are now only referenced  whenever
> possible and not always copied; hence .C(..., DUP=FALSE)
> may change the values of other local variables.
>
How about a warning like

"if you pass a local variable to .C/.Fortran with 
DUP=FALSE, your compiled code can alter the local variable and not just 
the copy in the return list. "

 	-thomas

(copied from ?.C, of course)

On Wed, 10 Oct 2007, schlather at math.uni-goettingen.de wrote:
> Full_Name: Martin Schlather
> Version: R version 2.7.0 Under development (unstable) (2007-10-01 r43043)
> OS: Linux
> Submission from: (NULL) (91.3.209.203)
>
>
> Hi,
>
> There are 2 dangers with using 'DUP=FALSE' mentioned:
>  * formal arguments
>  * lists
>
> Would you also mention a third one, namely
> that values in R are now only referenced  whenever
> possible and not always copied; hence .C(..., DUP=FALSE)
> may change the values of other local variables.
That has always been the case (depending on the meaning of 'possible'), 
and is part of the first point made.  It *does* give a circumstance in 
which other variables can be changed.  Spelling out all of those (and 
'local' is not really relevant) would be a mammoth task.
> E.g., with C code
>   void addone(double *x) {  *x = *x + 1; }
>
> you get
>
>  x <- as.double(1)
>  y <- x
>  .C("addone", x, PACKAGE="test", DUP=FALSE)
>  print(c(x,y))
> #[1] 2 2
>
>
>  x <- as.double(1)
>  y <- as.double(x)
> .C("addone", x, PACKAGE="test", DUP=FALSE)
>  print(c(x,y))
> #[1] 2 2
>
>  x <- as.double(1)
>  y <- as.integer(x)
> .C("addone", x, PACKAGE="test", DUP=FALSE)
>  print(c(x,y))
> #[1] 2 1
These are the result of changing an actual argument.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595