Windows/R issue with FFI

[Dominick Samperi]

Hello,
FFI code that used to work now fails under Windows (still seems to work
under Ubuntu), and I wonder if anybody has seen anything like this and
can provide some pointers...

The code uses FFI to fetch information from the R side like R_NilValue,
using something like this;

-- Fetch R's R_NilValue...
foreign import ccall unsafe "&R_NilValue" r_NilValue_ptr :: Ptr R_EXP
r_NilValue :: IO R_EXP
r_NilValue = peek r_NilValue_ptr
rNilValue1 :: IO REXP
rNilValue1 = do
    x <- r_NilValue
    traceShow("addr=",x) extREXP x

Under Windows the address displayed is obviously bad, and this causes
the app to crash. This does not happen under Linux (Ubuntu).

Now, replace the line containing peek with

r_NilValue = trace "PEEK" peek r_NilValue_ptr

The address is now valid! It seems that adding the trace "PEEK" adds
some delay and somehow resolves the problem.

This problem is intermittent, so it is hard to come up with a
simple example that fails every time.

A little background: R_NilValue is a pointer to a SEXP that is not
initialized until an embedded instance of R is initialized, and the
code above is not triggered until this happens. Perhaps there is
a race condition between the time R initializes itself and Haskell
performs the peek? I don't think R_NilValue is garbage collected
once initialized.

Any tips would be appreciated.
Dominick
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