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POSchedule error when calling multi-threaded C FFI with Haskell Callback function
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copied!<p>Below is a Haskell/C FFI code that is throwing schedule error at runtime (GHC 7.0.3, Mac OS 10.7, x86_64). I searched for explanation of the error but didn't find anything relevant.</p> <p>C Code (<code>mt.c</code>):</p> <pre><code>#include <pthread.h> #include <stdio.h> typedef void(*FunctionPtr)(int); /* This is our thread function. It is like main(), but for a thread*/ void *threadFunc(void *arg) { FunctionPtr fn; fn = (FunctionPtr) arg; fn(1); //call haskell function with a CInt argument to see if it works } void create_threads(FunctionPtr* fp, int numThreads ) { pthread_t pth[numThreads]; // array of pthreads int t; for (t=0; t < numThreads;){ pthread_create(&pth[t],NULL,threadFunc,*(fp + t)); t++; } printf("main waiting for all threads to terminate...\n"); for (t=0; t < numThreads;t++){ pthread_join(pth[t],NULL); } } </code></pre> <p>Haskell code (<code>t.hs</code>) - it calls <code>create_threads</code> in <code>mt.c</code> above with <code>Storable Vector</code> of <code>FunPtr</code> to Haskell function <code>f</code> (after applying first three arguments to <code>f</code>):</p> <pre><code>{-# LANGUAGE BangPatterns #-} import Control.Concurrent (forkIO, threadDelay, MVar, newEmptyMVar, putMVar, takeMVar) import qualified Data.Vector.Storable.Mutable as MSV import qualified Data.Vector.Storable as SV import Control.Monad.Primitive (PrimState) import Control.Monad (mapM, forM_) import Foreign.Ptr (Ptr, FunPtr) import Foreign.C.Types (CInt) type Length = CInt -- | f is a function that is called back by create_threads in mt.c f :: MVar Int -> MSV.MVector (PrimState IO) CInt -> Length -> CInt -> IO () f m v l x = do !i <- takeMVar m case (i< fromIntegral l) of True -> MSV.unsafeWrite v i x >> print x >> putMVar m (i+1) False -> return () -- overflow -- a "wrapper" import gives us a converter for converting a Haskell function to a foreign function pointer foreign import ccall "wrapper" wrap :: (CInt -> IO()) -> IO (FunPtr (CInt -> IO())) foreign import ccall safe "create_threads" createThreads :: Ptr (FunPtr (CInt -> IO())) -> CInt -> IO() main = do let threads = [1..4] m <- mapM (\x -> newEmptyMVar) $ threads -- intialize mvars with 0 forM_ m $ \x -> putMVar x 0 let l = 10 -- intialize vectors of length 10 that will be filled by function f v <- mapM (\x -> MSV.new l) threads -- create a list of function pointers to partial function - the partial function is obtained by applying first three arguments to function f lf <- mapM (\(x,y) -> wrap (f x y (fromIntegral l))) $ zip m v -- convert above function list to a storable vector of function pointers let fv = SV.fromList lf -- call createThreads with storable vector of function pointers, and number of threads - createThreads will spawn threads which will use function pointers for callback SV.unsafeWith fv $ \x -> createThreads x (fromIntegral $ length threads) </code></pre> <p>Please ignore unsafe parts in the code - my objective here is to test callback using Haskell FFI with multi-threaded C code. When I compile it, and run it, I get the error below:</p> <pre><code>$ ghc -O2 t.hs mt.c -lpthread [1 of 1] Compiling Main ( t.hs, t.o ) Linking t ... $ ./t main waiting for all threads to terminate... t: schedule: re-entered unsafely. Perhaps a 'foreign import unsafe' should be 'safe'? $ uname -a Darwin desktop.local 11.2.0 Darwin Kernel Version 11.2.0: Tue Aug 9 20:54:00 PDT 2011; root:xnu-1699.24.8~1/RELEASE_X86_64 x86_64 $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.3 </code></pre> <p>The schedule error happens only if I have the C threads call back haskell function f. I guess it is more likely there is a bug in my code, than there is a bug in one of the libraries or GHC. So, I will like to check here first for pointers on cause of the error.</p>

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