I assume that you're binding the libc function directly here:
I'm using Network.Socket. Sory if it's not clear from my previous posts.
In that case, you need to have the RTS manage sleeping your thread for you. You should make the socket non-blocking and handle the EAGAIN and EWOULDBLOCK cases by calling threadWaitRead[1] to block the current Haskell thread only, until the fd is readable. Note that threadWaitRead is GHC only. If you download the source to network-bytestring[2], you can see a very similar pattern in the code for send and recv in there.
Thanks, but I haven't managed to find a way of setting a socket into non blocking mode without using FFI directly (and I haven't found solution in network-bytestring too). How can I make this? The only way I've found is making handle by socketToHandle then reading by hGetBufNonBlocking. But this way seems not suited for non-connection based sockets.