[Haskell-cafe] How can I avoid buffered reads?

[Thomas DuBuisson]

As an alternative, If there existed a Haskell package to give you fast
cryptographically secure random numbers or use the new Intel RDRAND
instruction (when available) would that interest you?

Also, what you are doing is identical to the "entropy" package on
hackage, which probably suffers from the same bug/performance issue.

Cheers,
Thomas

On Wed, Nov 28, 2012 at 11:38 AM, Leon Smith <leon.p.smith at gmail.com> wrote:
> 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.Applicative
> import           Data.Bits
> import           Data.Word(Word64)
> import qualified Data.ByteString as S
> import qualified Data.ByteString.Lazy as L
> import           Data.ByteString.Internal (c2w)
> import qualified System.IO        as IO
> import qualified Data.Binary.Get        as Get
>
> showHex :: Word64 -> S.ByteString
> showHex 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
>
> -}
>
>
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