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

[Bas van Dijk]

On 5 March 2011 21:43, Donn Cave <donn at avvanta.com> wrote:
> Quoth Bas van Dijk <v.dijk.bas at gmail.com>,
> ...
>> 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.
>
> type ProcessID = CInt
> type Fd = CInt
>
> foreign import ccall "fork" c_fork :: IO CInt
> foreign import ccall "_exit" _exit :: CInt -> IO ()
>
> fork :: IO Int -> IO ProcessID
> fork fn = do
>        pid <- c_fork
>        if pid == 0
>                then do
>                        fn >>= _exit . fromIntegral
>                        return 0 -- unused, I reckon
>                else if pid > 0
>                        then return pid
>                        else throwErrno "fork"
>
> System.PosixProcess (exitImmediately) is supposed to be "_exit".
>
> I would not care to hazard a guess as to whether this will work
> reliably for you.

Thanks, I thought about this too, however I consider myself too
unfamiliar with the RTS to know if this is safe.

Bas