This is another one of those posts that I make mostly for myself, you know for organising and help my memory :-)

There are as far as I can see three ways to deal with sockets in Haskell. There’s the type Socket which is used throughout Network.Socket. From that it’s possible to get to the underlying filedescriptor, and it in turn can be converted to a Handle.

When coupled with fork+exec it’s crucial to make sure the child process can find the socket Leaving it in a predictable place seems to be the easiest way to do that, and as far as I can see that requires using dupTo from System.Posix.IO. So, on the child-side it’s necessary to find a way to turn an integer (CInt) into something that can be treated as a socket (i.e. a Socket, a Handle, or a filedescriptor).

A basic parent-child which obviously won’t work since the child’s socket is represented as a Socket:

import Control.Concurrent
import System.Posix.Process
import Network.Socket

childFunc s = send s "Ping from child" >> return ()

main = do
    (childSock, parentSock) <- socketPair AF_UNIX Stream defaultProtocol
    print (childSock, parentSock)
    child <- forkProcess $ childFunc childSock
    recv parentSock 10 >>= print

Let the child take a CInt and turn it into a filedescriptor:

import Control.Concurrent
import Control.Concurrent.MVar
import System.Posix.Process
import System.Posix.IO
import System.Posix.Types
import Network.Socket

childFunc sInt = do
    let fd = Fd sInt
    fdWrite fd "Ping from child" >> return ()

main = do
    (childSock, parentSock) <- socketPair AF_UNIX Stream defaultProtocol
    let childInt = fdSocket childSock
    print (childInt, parentSock)
    child <- forkProcess $ childFunc childInt
    recv parentSock 10 >>= print

Let the child take a CInt and turn it into a Handle:

import Control.Concurrent
import System.Posix.Process
import System.Posix.IO
import System.Posix.Types
import Network.Socket
import System.IO

childFunc sInt = do
    h <- fdToHandle $ Fd sInt
    hPutStr h "Ping from child"
    hFlush h

main = do
    (childSock, parentSock) <- socketPair AF_UNIX Stream defaultProtocol
    let childInt = fdSocket childSock
    print (childSock, parentSock)
    child <- forkProcess $ childFunc childInt
    recv parentSock 10 >>= print

Let the child take a CInt and turn it into a Socket:¹

import Control.Concurrent
import Control.Concurrent.MVar
import System.Posix.Process
import System.Posix.IO
import System.Posix.Types
import Network.Socket

childFunc sInt = do
    s <- mkSocket sInt AF_UNIX Stream defaultProtocol Connected
    send s "Ping from child" >> return ()

main = do
    (childSock, parentSock) <- socketPair AF_UNIX Stream defaultProtocol
    let childInt = fdSocket childSock
    print (childInt, parentSock)
    child <- forkProcess $ childFunc childInt
    recv parentSock 10 >>= print