Concurrency and the FFI

cbit.c

#include <stdio.h>
int bottom(int a) {
    int i = 0;
    for(i = 0; i < 5; ++i) {printf("%d\n", a);sleep(1);}
    return a;
}

cbit.h

int bottom(int a);

Compiling

$ gcc -c -o cbit.o cbit.c
$ ghc -threaded --make ffi.hs cbit.o -rtsopts

ffi.hs

{-# LANGUAGE ForeignFunctionInterface #-}

import Foreign.C
import Control.Concurrent

main = runInUnboundThread $ do
    forkIO $ do
      safeBottom 1
      return ()
    forkIO $ do
      unsafeBottom 2
      return ()
    print "Pass (not expected)"

foreign import ccall "cbit.h bottom" safeBottom :: CInt -> IO CInt
foreign import ccall unsafe "cbit.h bottom" unsafeBottom :: CInt -> IO CInt

Running.md

$ ./ffi +RTS -N1
1
2
1
2
1
2
1
2
1
2
"Pass (not expected)"

---

$ ./ffi +RTS -N2
1
2
1
2
1
2
1
2
1
2
"Pass (not expected)"

---

$ ./ffi +RTS -N3
1
2
"Pass (not expected)"
1
2
1
2
1
2
1
2