Howdy, I've tested the previous program with all versions down to 6.12.3 and I've got the same results. Then I tried something different: main = do let lock = (WriteLock, AbsoluteSeek, 0, 0) fd <- openFd "/tmp/foobar" ReadWrite (Just stdFileMode) defaultFileFlags {trunc=True} setLock fd lock >> putStrLn "parent: locked!" pid <- forkProcess $ do setLock fd lock >> putStrLn "child: locked!" executeFile "/usr/bin/sleep" False ["5"] Nothing void $ getProcessStatus True False pid Which, always works as it supposes to: child process always fail to acquire the lock. The following one is quite interesting, though. The moment I insert the threadDelay function (like in the previous example), it fails some times (it seems to have something to do with cpu idleness). I guess this this explains why the previous version didn't work properly for me: main = do let lock = (WriteLock, AbsoluteSeek, 0, 0) fd <- openFd "/tmp/foobar" ReadWrite (Just stdFileMode) defaultFileFlags {trunc=True} pid0 <- forkProcess $ do setLock fd lock >> putStrLn "child0: locked!" executeFile "/usr/bin/sleep" False ["5"] Nothing pid1 <- forkProcess $ do setLock fd lock >> putStrLn "child1: locked!" executeFile "/usr/bin/sleep" False ["5"] Nothing threadDelay $ 1 * 1000 * 1000 -- take out this line and everything works mapM_ (getProcessStatus True False) [pid1, pid2] $ ghc -threaded -fforce-recomp --make -O2 ~/test; for _ in `seq 1 10`; do ~/test; echo; done; [1 of 1] Compiling Main ( /home/dsouza/test.hs, /home/dsouza/test.o ) Linking /home/dsouza/test ... parent: locked! test: setLock: resource exhausted (Resource temporarily unavailable) parent: locked! child: locked! child: locked! test: setLock: resource exhausted (Resource temporarily unavailable) child: locked! test: setLock: resource exhausted (Resource temporarily unavailable) parent: locked! test: setLock: resource exhausted (Resource temporarily unavailable) parent: locked! child: locked! child: locked! test: setLock: resource exhausted (Resource temporarily unavailable) parent: locked! child: locked! parent: locked! child: locked! child: locked! parent: locked! Am I doing something wrong? Thanks! ~dsouza