Torsten,
Of course, R itself is a C program so the principles apply to interpreted
calls too, with your code in bar returning control to do_dotcall in your
first example.
There can be no R memory allocation between returning from bar and
assigning to a in foo, and it is memory allocation (and invoked gc's) one
is protecting against.
Brian
On Fri, 2 Apr 2004, Torsten Hothorn wrote:
>
> Good morning!
>
> The descriptions of memory protection all assume that one
> is calling a C-function directly from R. I'm not sure if my
> understanding of calling a C-function from another C-function
> is correct:
>
> Suppose there are two functions
>
> SEXP bar(SEXP y) {
> SEXP b;
> PROTECT(b = allocVector(...));
> ... computations on b
> UNPROTECT(1);
> return b
> }
>
> and
>
> SEXP foo(SEXP x) {
> SEXP a;
>
> PROTECT(a = bar(x)); /* use bar to do lowlevel computations */
> ... futher computations on a
> UNPROTECT(1);
> return a;
> }
>
> Of course,
>
> R> .Call("bar", x)
>
> is safe but is
>
> R> .Call("foo", x)
>
> too? May it happen that the object `b' points to is destroyed before it is
> protected by `PROTECT(a = bar(x))'?
>
> I searched for examples of that in R and some packages but did only find
> some where `bar' is defined by
>
> double* bar(double *y)
>
> and the problems do not occur. Anyway, I want to be able to call both
> `foo' and `bar' directly from R and C-level (say for the sake of writing
> tests in R for the lowlevel functions).
>
> Any clarification is very welcome && Thanks in advance,
--
Brian D. Ripley, [EMAIL PROTECTED]
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
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