Ok, that is why i have suspected.

Thanks for the clear explanation.

[]s
Cassiano




2014-04-09 18:37 GMT-03:00 Peter Langfelder <peter.langfelder@gmail.com>:

> On Wed, Apr 9, 2014 at 11:27 AM, Cassiano dos Santos <crns13@gmail.com>
> wrote:
> > I am testing a call to a C function from R, using .C interface. The test
> > consists in passing a numeric vector to the C function with no entries,
> > dynamically allocates n positions, makes attributions and return the
> vector
> > to R.
>
> When execution enters your C function, the pointer x points to the
> content (numerical values) of the R object known as 'x' to R code.
> However, the content has length 0 and the value of the pointer may be
> undefined (not sure about how R handles empty vectors).
>
> You then change the C pointer x to point to the memory you allocated.
> This memory has no relation to the R object 'x', so any changes you
> make cannot be reflected in the R object x.
>
> Further, when execution exits your function, the pointer to your
> allocated memory is lost and your memory is not de-allocated (that is,
> returned to the system). You should call the Free function on exit
> from your function.
>
> So the answer is that you cannot use the .C interface for this. You
> could achieve your goal via the .Call interface but you have to read
> up about how to work with R objects in C code.
>
> HTH,
>
> Peter
>
> >
> > I'm using Calloc from R.h. The prototype of the function is
> >
> > type* Calloc(size_t n, type)
> >
> > as noted in Writing R Extensions.
> >
> > The problem is that I don't get the new vector with the allocated
> positions
> > in R. The vector continues to have no entries.
> >
> > *The code in R*
> >
> > fooR <- function(x) {
> >   if (!is.numeric(x))
> >     stop("argument x must be numeric")
> >   out <- .C("foo",
> >             x=as.double(x))
> >   return(out$x)}
> >
> > x <- numeric()
> >
> > result <- myfooR(x)
> >
> > *The function in C*
> >
> > #include <R.h>
> > void myfooRealloc(double *x){
> >   int i, n;
> >
> >   n = 4;
> >   x = Calloc(n, double);
> >
> >   for (i = 0; i < n; i++) {
> >     x[i] = i;
> >     printf("%f\n", x[i]); //just to check
> >   }}
> >
> > The question is: Can .C inteface handle with such memory allocation?
> >
> >         [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>

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