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

[Leon Smith]

Quite possibly,  entropy does seem to be a pretty lightweight dependency...

Though doesn't recent kernels use rdrand to seed /dev/urandom if it's
available?   So /dev/urandom is the most portable source of random numbers
on unix systems,  though rdrand does have the advantage of avoiding system
calls,  so it certainly would be preferable, especially if you need large
numbers of random numbers.

Best,
Leon

On Wed, Nov 28, 2012 at 2:45 PM, Thomas DuBuisson <
thomas.dubuisson at gmail.com> wrote:

> 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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> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
>
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