Here's the code from hGetContents (base/GHC/IO.lhs):
-- we never want to block during the read, so we call fillReadBuffer
with
-- is_line==True, which tells it to "just read what there is".
lazyReadBuffered h handle_ fd ref buf = do
catch
(do buf <- fillReadBuffer fd True{-is_line-} (haIsStream
handle_) buf
lazyReadHaveBuffer h handle_ fd ref buf
)
-- all I/O errors are discarded. Additionally, we close the
handle.
(\e -> do handle_ <- hClose_help handle_
return (handle_, "")
)
So, it reads whatever is available, further description is available
from the definition
of fillReadBuffered:
-- For a line buffer, we just get the first chunk of data to arrive,
-- and don't wait for the whole buffer to be full (but we *do* wait
-- until some data arrives). This isn't really line buffering, but it
-- appears to be what GHC has done for a long time, and I suspect it
-- is more useful than line buffering in most cases.
So for a disc buffer I would expect 1 complete buffer to be returned
most of the time, for
a network read, I guess one packet (MTUs) worth should be expected...
Regards,
Keean.
Keean Schupke wrote:
John Goerzen wrote:
On Tue, Sep 20, 2005 at 02:29:12PM +0100, Keean Schupke wrote:
It's unclear to me exactly how to mix the IO monad with Parsec. It
doesn't really seem to be doable.
Not to mention that if hGetContents is used, the Handle has to be put
into non-buffering mode, which means one syscall per character read.
Terribly slow.
Does it? I didn't think so ...
strace seems to say yes.
Thats odd, the source code seems to suggest that when you read past
the end of the buffer
it reads the next entire buffer (it has cases for each possible buffer
configuration, line, block and none) - and I can think of no reason
_why_ it cannot use buffering... I would think that it's a bug if it
is the case.
Regards,
Keean.
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