[Haskell-cafe] Re: standard poll/select interface

Simon Marlow simonmarhaskell at gmail.com
Fri Feb 10 07:26:30 EST 2006

Bulat Ziganshin wrote:
> Hello Einar,
> Friday, February 10, 2006, 2:09:08 AM, you wrote:
>>>as i understand this idea, transformer implementing async i/o should
>>>intercept vGetBuf/vPutBuf calls for the FDs, start the appropriate
>>>type FD = Int
>>>vGetBuf_async :: FD -> Ptr a -> Int -> IO Int
>>>vPutBuf_async :: FD -> Ptr a -> Int -> IO Int
> EK> Please don't fix FD = Int, this is not true on some systems,
> EK> and when implementing efficient sockets one usually wants
> EK> to hold more complex state.
> the heart of the library is class Stream. both "File" and "Socket"
> should implement this interface. just now i use plain "FD" to
> represent files, but that is temporary solution - really file also
> must carry additional information: filename, open mode, open/closed
> state. This "File" will be an abstract datatype, what can be based not
> on FD in other operating systems.
> The same applies to the "Socket". it can be any type what carry enough
> information to work with network i/o.
> implementation of async i/o should have a form of Stream Transformer,
> which intercepts only the vGetBuf/vPutBuf operations and pass other
> operations as is:

I don't think async I/O is a stream transformer, fitting it into the 
stream hierarchy seems artificial to me.

It is just another way of doing I/O directly to/from file descriptors. 
If your basic operation to read from an FD is

   readFD :: FD -> Int -> Ptr Word8 -> IO Int

then an async I/O layer simply provides you with the exact same 
interface, but with an implementation that doesn't block other threads. 
  It is part of the file descriptor interface, not a stream transformer. 
  Also, you probably need

   readNonBlockingFD :: FD -> Int -> Ptr Word8 -> IO Int
   isReadyFD :: FD -> IO Bool

in fact, I think this should be the basic API, since you can implement 
readFD in terms of it.  (readNonBlockingFD always reads at least one 
byte, blocking until some data is available).  This is used to partially 
fill an input buffer with the available data, for example.

One problem here is that in order to implement readNonBlockingFD on Unix 
you have to put the FD into O_NONBLOCK mode, which due to misdesign of 
the Unix API affects other users of the same file descriptor, including 
other programs.  GHC suffers from this problem.


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