Re: [Haskell-cafe] non-blocking recv from UDP socket

2010/8/26 Johan Tibell :

Hi Thu,

On Thu, Aug 26, 2010 at 11:13 AM, Vo Minh Thu wrote:

...

Is is possible to get Network.Socket.ByteString.recv to be non-blocking (i.e. return directly even if no data is available) ?

Unfortunately not.

...

I have tried ti use

 setSocketOption sock NoDelay 1

but then I get the following error:

 setSocketOption: unsupported operation (Protocol not available)

Sockets are already set to be non-blocking and the blocking semantics are implemented on top of non-blocking sockets using e.g. the select system call. It's not the recv syscall that blocks but the I/O manager who blocks you're thread because recv returned WOULD_BLOCK. The I/O manager calls threadWaitRead upon receiving this error. threadWaitRead puts the thread to sleep until the socket is readable (as indicated by select).

Ok, that explains also why using fcntl directly on the fd didn't work either. So, I think I will go the FFI road and create my socket the way I want. Do you see another way?

We could perhaps implement a truly non-blocking API.

If it is widely useful, I guess it would be great. I don't mind writing some C and using the FFI though. Thanks for your answer, Thu