On 11/28/2012 08:38 PM, Leon Smith 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.
Use openFd, fdReadBuf and closeFd from the System.Posix.IO.ByteString module in the 'unix' package. Those correspond directly to system calls and are thus unbuffered.