On Wed, 16 Jan 2008, Peter Dalgaard wrote:
Herve Pages wrote:
Hi Peter,
Peter Dalgaard wrote:
Herve Pages wrote:
Hi,
I'm wondering if this code from the "Writing R Extensions" manual
is really safe:
SEXP mkans(double x)
{
SEXP ans;
PROTECT(ans = allocVector(REALSXP, 1));
REAL(ans)[0] = x;
UNPROTECT(1);
return ans;
}
double feval(double x, SEXP f, SEXP rho)
{
defineVar(install("x"), mkans(x), rho);
return(REAL(eval(f, rho))[0]);
}
In C, the order in which function arguments are evaluated before the
function itself is called is undefined. Hence there is no guarantee
that install("x") will be evaluated before mkans(x). What happens if
mkans(x) is evaluated first? Then install("x") will be called and
eventually trigger garbage collection while the SEXP returned by
mkans(x) is still unprotected.
I'm asking because I'm getting all sorts of problems with
defineVar(install(somekey), mkans(x), rho);
In my code this line is inside a big loop (hundred of thousands of
iterations) so I end up with a lot of symbols in the rho environment.
The problems I've seen are hard to reproduce: sometimes it's a segfault,
sometimes a "cons memory exhausted" error, or sometimes everything looks
fine except that, later, when I retrieve values from the rho environment
with findVar(), some of them are altered!
But if I replace the above line by:
PROTECT(ans = mkans(x));
defineVar(install(somekey), ans, rho);
UNPROTECT(1);
then everything works fine :-)
Sounds like you are right. You don't really have the "smoking gun", but
it doesn't seem to be worth trying to catch the actual bug in action
with hardware watchpoints and whatnot.
The opposite fix should work too (does it?):
{ SEXP sym = install(somekey) ; defineVar(sym, mkans(x), rho);}
So now you are protected against install(somekey) eventually triggering
garbage collection but you are still not protected against defineVar() itself
triggering garbage collection. Maybe defineVar() does not do that, and will
never do it, but isn't it risky to rely on this kind of assumption?
Thanks!
H.
That's not the problem you raised (argument evaluation order), but
there's a CONS inside defineVar, and as far as I can see, it doesn't
protect its arguments, so you could well be right.
CONS itself protects its arguments though.
luke
(I don't think you need to PROTECT elements in the symbol table)
--
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--
Luke Tierney
Chair, Statistics and Actuarial Science
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa Phone: 319-335-3386
Department of Statistics and Fax: 319-335-3017
Actuarial Science
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