Single-threaded aspect

As others said XPtr is not something in R so Rcpp mailing list would be the right place for that aspect.

However, it you forget Rcpp and phrase it as an R question, you also get much closer to the reason and answer. SEXP type is the internal representation of all objects in R. I assume your question is which operations in the R API on those are thread-safe. The answer is that most of them are not, the main reason being that the memory management is not thread-safe, i.e. you cannot allocate anything without synchronization. Since almost all API calls involve some memory allocations, they are not thread-safe. You can, however, allocate objects and the operate on their payload, e.g., you can get numerical input vectors, allocate the result vector and then perform your threaded computation in C on those, synchronize and get back - that's how most implicit parallel operations in R work (leveraging BLAS, OpenMP, etc.). That is also what Dirk replied in your SO answer (quote: "Packages like RcppParallel are very careful about using non-R data structures for multithreaded work."). Note that the payload of most native vectors (integer, real, complex) is technically non-R data structure in the sense so you can operate on those directly (some read-only operations are also thread-safe in the API as long as they can't trigger errors/warning/side-effects).

For completeness, memory allocation is not the only reason or obstacle for thread-safe R API calls, but a main one. Other issues involve error handling (you may long-jump out of your thread stack) and global state (devices, connections etc.). In short, it's not something that can be really solved without complete re-design and re-write.

Cheers,
Simon

Single-threaded aspect

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