Re: Windows/R issue with FFI

Thanks, that does indeed look dynamically linked. Could you also paste on the ticket the contents of hR.buildinfo? Cheers, Tamar Sent from my Mobile On Mon, Mar 27, 2023, 15:18 Dominick Samperi wrote:

Yes, everything else stays the same, including x <- r_NilValue.

I opened a ticket here where more details are provided https://gitlab.haskell.org/ghc/ghc/-/issues/23183

After initializing an R instance, if you fetch R_NilValue and peek at its value (using FFI peek) you get a bad address. But if you add a trace statement before the peek the address is valid.

A "race condition" should not be possible in a single-threaded application, so I am not sure what is going on. I tried to come up with a simple reproducible example where a library module does nothing but fetch R_NilValue, and the client also uses FFI to fetch R_NilValue, but in this example both addresses are valid and equal.

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On Mon, Mar 27, 2023 at 9:48 AM Phyx wrote:

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Hi,

I'm missing some details here here as I'm having trouble following the flow.

What provides the symbol for that import? As in where does R_NilValue come from? As in, how is it defined. Are you linking against a library or C sources?

When you say you replace the trace statement, do you keep the x <- r_NilValue?

The address to R_NilValue should never change during initialization so I'm more suspicious of how it's declared. Unless you're linking to a symbol in a shared library, in which case that could be possible due to ASLR.

Kind regards, Tamar

Sent from my Mobile

On Sun, Mar 26, 2023, 14:15 Dominick Samperi wrote:

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Thanks Ben, I'll see what I can do to reliably reproduce and open a ticket.

One theory I'm investigating is that this might have something to do with my anti-virus software (AVG), since it sometimes interacts with Windows in strange ways (for example, an extra instance of a terminal app pops up, then disappears after a few seconds). But disabling this software does not seem to solve the problem.

On Sat, Mar 25, 2023 at 11:18 PM Ben Gamari wrote:

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This sounds like a bug. Could you open a ticket, ideally with a fairly standalone reproducer?

Cheer,

- Ben

On March 25, 2023 6:49:09 PM EDT, Dominick Samperi wrote:

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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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