[Rcpp-devel] returning array from C

[Steve Jaffe]

Thanks.

It seems from my quick reading that there are ways around the GC issue, this is done for the "External Pointer" type as I understand it, so that might not be insurmountable.

On the other hand, it might not be possible to separate GC from R lifetime for the vector type (ie REALSXP). That's where more knowledge of R internals is needed than I at the moment possess.

Steve

From: dmbates at gmail.com [mailto:dmbates at gmail.com] On Behalf Of Douglas Bates
Sent: Tuesday, June 11, 2013 11:58 AM
To: Steve Jaffe
Cc: rcpp-devel at lists.r-forge.r-project.org
Subject: Re: [Rcpp-devel] returning array from C

On Tue, Jun 11, 2013 at 10:51 AM, Steve Jaffe <sjaffe at riskspan.com<mailto:sjaffe at riskspan.com>> wrote:
Is there a way to 'wrap', say, an array of double allocated on the heap in C/C++ and return it to R without copying, ie as the data inside a REALSXP?

I don't know of any way of doing that.  R is very possessive about memory and expects to be able to garbage-collect any memory allocated in an SEXPREC.

(Let's say for example that this array allocation is in a 3rd-party library which can't be modified)

Looking at the 'wrap' code it appears that copying is done in all cases (except for the 'external pointer').

Yes.  The only way to avoid copying is if the external library allows objects to be constructed from pre-allocated memory.  This is used in the RcppArmadillo and RcppEigen packages.


Clearly the memory ownership issue is one that would need to be handled. Handing over ownership to R would seem the simplest approach. But it would be more interesting to be able to integrate with something like boost::shared_array so that the lifetime would be correctly managed across both the C and R environments.

I've read through Rcpp documentation and "Writing R Extensions" without coming up with a good answer, but I'm new to this and I suspect someone familiar with R internals (unlike me) would immediately see what was involved.

Thanks for your help.

Steve


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