[Rcpp-devel] Problem passing Armadillo objects back to R

Hmmmmmmmm. I'm afraid I can't fix this.

Here is the story: both wrap(X) and wrap(X) create a new SEXP and allocate memory. When wrap(Y) is called, since the created object is large, R calls the GC to collect unprotected SEXP to free some space. So my guess is that at that point the SEXP that is generated by wrap(X) is collected since it is not protected.

I can not deal with this inside ::create, because it is too late.


A Workaround is to use this :

return Rcpp::List::create(
Rcpp::Named("X") = X,
Rcpp::Named("Y") = Y
);

i.e. use the implicit wrap. here wrap will be called much later and things are fine.


The issue is that when calling wrap it creates an unprotected SEXP, and Rcpp is not responsible for these. If the programmer passes an unprotected SEXP to ::create, it is his responsability to protect it. So another alternative would be:

SEXP xx = PROTECT( wrap( X ) ) ;
SEXP yy = PROTECT( wrap( Y ) ) ;
Rcpp::List result =  Rcpp::List::create(
Rcpp::Named("X") = xx,
Rcpp::Named("Y") = yy
);
UNPROTECT(2) ;
return res ;

yes, this is ugly.

Romain

Le 06/04/11 11:42, Romain Francois a ?crit :

Good news. I can reproduce it.
Bas news. This is likely a PROTECT problem with the Rcpp::List constructor.

Wearing my bugbuster costume, and getting ready for a fight.

Romain

Le 07/03/11 23:03, "G?nter J. Hitsch" a ?crit :

I'm new to Rcpp and RcppArmadillo---so far I like it a lot! Thanks to
the developers for their good work.

I run into a peculiar kind of problem when I try to pass a "large"
Armadillo object back to R. Here's some code to replicate the problem in
stylized form:


extern "C" SEXP testFun(SEXP L_)
{
const long L = Rcpp::as<long>(L_);
arma::mat X(L,1);
arma::mat Y(L,1);
X.fill(1);
Y.fill(2);
return Rcpp::List::create(
Rcpp::Named("X") = Rcpp::wrap(X),
Rcpp::Named("Y") = Rcpp::wrap(Y)
);
}


I compile (both using g++ and the Intel Compiler --- choice of compiler
makes no difference) and then call from R:


L = 1000000
ret = .Call("testFun", as.integer(L))
print(ret$X[1:5,])
print(ret$Y[1:5,])


Here's what I often get as output:

[1] 1 1 1 1 1
[1] 1 1 1 1 1

However, the second rows should be all 2's!

When I try to pass smaller matrices, for example by setting L=100000,
the problem goes away:

[1] 1 1 1 1 1
[1] 2 2 2 2 2


Also, the problem does not arise when I create and pass pack objects of
type Rcpp::NumericMatrix; so far I've see the problem only with
Armadillo objects. I've encountered this on two Macs running OS X
10.6.6, R 2.12.2, and I'm using the latest versions of Rcpp and
RcppArmadillo.

Help on this matter is appreciated!

G?nter

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