invoking R function in C++ in parallel

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- either work in something like OpenMP and run in multiple threads that
   remain /completely/ shielded from R, ie no R calls, and not even R data
   types as you cannot trigger gc() calls from different threads

I think you can use R objects, as long as you don't call R functions on
them (not even from R's C api, although some of them are currently fine)
and consider them as read-only. E.g. if you have a numeric vector, you can
do
double *cvec = REAL(vec);
and then use cvec in your thread(s).

This is pretty restrictive, but could be enough in some cases.

Gabor

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