[Rcpp-devel] How to free memory in Rcpp

[Romain Francois]

Le 14/12/12 03:15, Honglang Wang a écrit :
> Hi,
> Here is the sample code:
> // [[Rcpp::export]]
> List betahat(Function ker, double t0, NumericMatrix Xr, NumericMatrix
> yr, NumericVector tr, double h, int m) {
>    int n = Xr.nrow(), p = Xr.ncol();
>    arma::mat X(Xr.begin(), n, p, false);
>    arma::mat y(yr.begin(), n, 1, false);
>    arma::colvec t(tr.begin(), tr.size(), false);
>    arma::mat T = X;
>    T.each_col() %= (t-t0)/h;
>    arma::vec K =as<arma::vec>(ker(tr-t0,h))/
> m;
>    double L1 = arma::accu(K%X.col(0)%X.col(0));
>    double L2 = arma::accu(K%X.col(0)%X.col(1));
>    double L3 = arma::accu(K%X.col(1)%X.col(1));
>    double L4 = arma::accu(K%X.col(0)%T.col(0));
>    double L5 = arma::accu(K%X.col(1)%T.col(0));
>    double L6 = arma::accu(K%X.col(1)%T.col(1));
>    double L7 = arma::accu(K%T.col(0)%T.col(0));
>    double L8 = arma::accu(K%T.col(0)%T.col(1));
>    double L9 = arma::accu(K%T.col(1)%T.col(1));
>    double R1 = arma::accu(K%X.col(0)%y);
>    double R2 = arma::accu(K%X.col(1)%y);
>    double R3 = arma::accu(K%T.col(0)%y);
>    double R4 = arma::accu(K%T.col(1)%y);
>    arma::mat L(2*p,2*p);
>    L(0,0)=L1;L(0,1)=L2;L(0,2)=L4;L(0,3)=L5;
>    L(1,0)=L2;L(1,1)=L3;L(1,2)=L5;L(1,3)=L6;
>    L(2,0)=L4;L(2,1)=L5;L(2,2)=L7;L(2,3)=L8;
>    L(3,0)=L5;L(3,1)=L6;L(3,2)=L8;L(3,3)=L9;
>    arma::mat R(2*p,1);
>    R(0,0)=R1;R(1,0)=R2;R(2,0)=R3;R(3,0)=R4;
>    arma::vec betahat = arma::solve(L,R);
>
>    arma::colvec betahat0(betahat.begin(),betahat.size()/2,false);
>    return List::create(Named("betahat") = betahat0);
> }
>
> I will call this function repeatedly, and at some point, it went wrong
> with the following error:
> Error in betahat(ker, x, X, y, t, h, m) :
>    promise already under evaluation: recursive default argument
> reference or earlier problems?
> Calls: system.time ... apply -> FUN -> betahat -> .External -> cpp_exception
> Execution halted
>
> I have no idea what's this error. I am just wondering whether I need to
> free some memory in this code. Thanks.

You don't. The explicit constructor/destructor scheme of C++ takes care 
of this automatically for you.

So there is a chance you are victim of a well hidden bug. These are 
usually related to garbage collection and us not making enough 
precaution somewhere. This is very hard to track.

As a wild guess, I think it is because Function return you an 
unprotected SEXP. It does not matter most of the time because you 
consume it, but sometimes the GC will run and collect your object before 
as<mat> has a chance to process it.

If my guess is correct, then doing yourself some protection might do the 
trick, something like:

NumericVector result = ker(tr-t0,h) ;
arma::vec K =as<arma::vec>(result)/m;

will do the trick since the result of the function call then gets 
protected by the "result" object.

If this does not work, then you can do some debugging as indicated by 
others.

Romain

> Best wishes!
> Honglang Wang
> Office C402 Wells Hall
> Department of Statistics and Probability
> Michigan State University
> 1579 I Spartan Village, East Lansing, MI 48823
> wangho16 at msu.edu <mailto:wangho16 at msu.edu>
>
>
>
> On Thu, Dec 13, 2012 at 4:57 PM, <romain at r-enthusiasts.com
> <mailto:romain at r-enthusiasts.com>> wrote:
>
>
>     Hello,
>
>     What memory do you want to free ? Can you give an example of code
>     where there is memory you'd like to "free" ?
>
>     Romain
>
>     Le 2012-12-13 22:52, Honglang Wang a écrit :
>
>         Dear All,
>         How to free memory in Rcpp? What's the command? Thanks.
>
>         Best wishes!
>
>         Honglang Wang
>
>         Office C402 Wells Hall
>         Department of Statistics and Probability
>         Michigan State University
>         1579 I Spartan Village, East Lansing, MI 48823
>         wangho16 at msu.edu <mailto:wangho16 at msu.edu> [1]
>
>
>         Links:
>         ------
>         [1] mailto:wangho16 at msu.edu <mailto:wangho16 at msu.edu>


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Romain Francois
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