On 27 May 2005 03:33, Nobuo Yamashita wrote:
I am curious about reason why in GHC hIsEOF blocks in BlockBuffering mode while hGetChar doesn't.
I wrote following three programs for an experiment.
-- echo0.hs module Main where import System.IO main = hSetBuffering stdin (BlockBuffering (Just 5)) >> echo echo = getChar >>= putChar >> hFlush stdout >> echo
-- echo1.hs module Main where import System.IO main = hSetBuffering stdin (BlockBuffering (Just 5)) >> echo echo = do eof <- isEOF if eof then return () else getChar >>= putChar >> hFlush stdout >> echo
-- as.hs module Main where import System.IO import Control.Concurrent main = putChar 'a' >> hFlush stdour >> threadDelay 1000000 >> main
And run next two commandlines
% runghc as.hs | runghc echo0.hs % runghc as.hs | runghc echo1.hs
I had expected that the both behaviors were same; a character was output a second. But the former output a character a second, and the latter output 5 characters a time every 5 seconds.
Quite right, that's a bug, or at least an inconsistency. hGetChar used to wait for a completely full buffer before returning a character, but we changed it to return as soon as any data at all was available. However, we didn't change hIsEOF to match: it still waits for a completely full buffer. I've now fixed this. As it turns out, BlockBuffering doesn't really do full block buffering on an input Handle. The BlockBuffering size does specify the maximum amount of buffering performed, but all read operations return as soon as the required amount of data is available, without waiting for the buffer to completely fill up. This seems a lot more useful than strict block buffering. Thanks for a well-researched bug report! Cheers, Simon