I have some code that reads (infrequently) small amounts of data from /dev/urandom, and because this is pretty infrequent, I simply open the handle and close it every time I need some random bytes. The problem is that I recently discovered that, thanks to buffering within GHC, I was actually reading 8096 bytes when I only need 16 bytes, and thus wasting entropy. Moreover calling hSetBuffering handle NoBuffering did not change this behavior. I'm not sure if this behavior is a bug or a feature, but in any case it's unacceptable for dealing with /dev/urandom. Probably the simplest way to fix this is to write a little C helper function that will read from /dev/urandom for me, so that I have precise control over the system calls involved. But I'm curious if GHC can manage this use case correctly; I've just started digging into the GHC.IO code myself. Best, Leon {-# LANGUAGE BangPatterns, ViewPatterns #-} import Control.Applicativeimport Data.Bitsimport Data.Word(Word64)import qualified Data.ByteString as Simport qualified Data.ByteString.Lazy as Limport Data.ByteString.Internal (c2w)import qualified System.IO as IOimport qualified Data.Binary.Get as Get showHex :: Word64 -> S.ByteStringshowHex n = s where (!s,_) = S.unfoldrN 16 f n f n = Just (char (n `shiftR` 60), n `shiftL` 4) char (fromIntegral -> i) | i < 10 = (c2w '0' - 0) + i | otherwise = (c2w 'a' - 10) + i twoRandomWord64s :: IO (Word64,Word64)twoRandomWord64s = IO.withBinaryFile "/dev/urandom" IO.ReadMode $ \handle -> do IO.hSetBuffering handle IO.NoBuffering Get.runGet ((,) <$> Get.getWord64host <*> Get.getWord64host) <$> L.hGet handle 16 main = do (x,y) <- twoRandomWord64s S.hPutStrLn IO.stdout (S.append (showHex x) (showHex y)) {- Relevant part of strace: open("/dev/urandom", O_RDONLY|O_NOCTTY|O_NONBLOCK) = 3 fstat(3, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 9), ...}) = 0 ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffff367e528) = -1 EINVAL (Invalid argument) ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffff367e528) = -1 EINVAL (Invalid argument) read(3, "N\304\4\367/\26c\"\3218\237f\214yKg~i\310\r\262\"\224H\340y\n\376V?\265\344"..., 8096) = 8096 close(3) = 0 -}