[Haskell-cafe] How to daemonize a threaded Haskell program?

Bas van Dijk v.dijk.bas at gmail.com
Sat Mar 5 20:51:59 CET 2011


I like to turn my Haskell program into a unix daemon. One of the steps
in "daemonizing" a process is to fork it then exit the parent and
continue with the child. All this is nicely abstracted in
hdaemonize[1] which internally calls forkProcess[2].

I would also like to use multiple simultaneous threads in my program.
Unfortunately forkProcess is not supported when running with +RTS -N
so I can't use hdaemonize.

I understand why it's problematic to fork a process which is in the
middle of running multiple simultaneous threads. However, in the case
of a daemon the fork happens in the beginning of the program. So if I
can manage to create a program that first daemonizes my process then
starts the Haskell program, all is good.

My current plan is to have a custom Haskell main function which is
exported using the FFI:

{-# LANGUAGE ForeignFunctionInterface #-}

module MyMain where

import Control.Monad       ( forM_ )
import Control.Concurrent  ( threadDelay )

-- from hsyslog:
import System.Posix.Syslog ( Priority(Debug), syslog )

foreign export ccall myMain :: IO ()

myMain :: IO ()
myMain = forM_ [1..10 :: Int] $ \n -> do
           syslog Debug $ "test " ++ show n
           threadDelay 1000000

Then create a C program that first daemonizes my process (using the
'daemon'[3] function from unistd) then start up my custom Haskell main

#include <unistd.h>
#include "HsFFI.h"
#include "MyMain_stub.h"

extern void __stginit_Main ( void );

int main(int argc, char *argv[])
  int r;
  r = daemon(0,0);
  if (r < 0)
    return r;

  hs_init(&argc, &argv);
  return 0;

My question is: how can I combine these two into a single program?

I very much prefer to do this using Cabal since my actual program
contains lots of dependencies.



[1] http://hackage.haskell.org/package/hdaemonize
[2] http://hackage.haskell.org/packages/archive/unix/latest/doc/html/System-Posix-Process.html#v:forkProcess
[3] http://www.kernel.org/doc/man-pages/online/pages/man3/daemon.3.html

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