fork question...

fork question...

[Micha]

hi,

below is a code piece of the ocaml-book. What I don't understand is why is 
there the second fork() ? Maybe that's a Unix question, but I saw it the 
first time in the ocaml book :-)


Michael



let establish_server server_fun sockaddr =
   let domain = domain_of sockaddr in
   let sock = Unix.socket domain Unix.SOCK_STREAM 0 
   in Unix.bind sock sockaddr ;
      Unix.listen sock 3;
      while true do
        let (s, caller) = Unix.accept sock 
        in match Unix.fork() with
               0 -> if Unix.fork() <> 0 then exit 0 ;          (* <<<< ???? *)
                    let inchan = Unix.in_channel_of_descr s 
                    and outchan = Unix.out_channel_of_descr s 
                    in server_fun inchan outchan ;
                       close_in inchan ;
                       close_out outchan ;
                       exit 0
             | id -> Unix.close s; ignore(Unix.waitpid [] id)
      done ;;

Re: [Caml-list] fork question...

[Keith Wansbrough]

> hi,
> 
> below is a code piece of the ocaml-book. What I don't understand is why is 
> there the second fork() ? Maybe that's a Unix question, but I saw it the 
> first time in the ocaml book :-)

It's a Unix question, not an OCaml one.  The reason is to completely
dissolve the connection between the parent process and the grandchild.
Normally, the parent is responsible for cleaning up after the child -
in particular, wait()ing on it to obtain its return status.  In
situations like this, where establish_server doesn't want to wait for
server_fun to complete, you do the following:

* fork()
* in parent, wait for child to exit
* in child, fork() again
* in grandchild, do your work
* in child, exit.  This means the grandchild is orphaned, and "init"
  (PID 1) adopts it and takes responsibility for cleaning up after it.
* in parent, clean up after child
* in parent, continue

If you don't do this, your process table fills up with zombie
processes (processes that have completed, but haven't yet been
wait()ed for.

See any standard Unix text (eg Stevens, Unix Network Programming).

HTH.

--KW 8-)

Re: [Caml-list] fork question...

[Christophe TROESTLER]

On Fri, 25 Feb 2005, Keith Wansbrough <Keith.Wansbrough@cl.cam.ac.uk> wrote:
> 
> It's a Unix question, not an OCaml one.  The reason is to completely

The following may is really nice (in French only unfortunately):

  "Principes et Programmation des Systèmes d'exploitation"
  http://cristal.inria.fr/~remy/poly/system/

Maybe -- if the authors agree -- some people may be interested to
translate it to english ?

Cheers,
ChriS