printf and fork question

printf and fork question

[Michael]

Hi,

I don't understand why this:

open Unix;;
let _ = 
	Printf.printf "Hi! %d\n" (getpid());
	match fork() with
	| 0 -> if fork() <> 0 then exit 0;
	       ()
	| id -> ignore (waitpid [] id)

prints out that:
Hi! 12215
Hi! 12215
Hi! 12215


with ocaml 3.08.2. Any hints?

 Michael

Re: [Caml-list] printf and fork question

[Gerd Stolpmann]

Am Montag, den 07.03.2005, 14:44 +0100 schrieb Michael:
> Hi,
> 
> I don't understand why this:
> 
> open Unix;;
> let _ = 
> 	Printf.printf "Hi! %d\n" (getpid());
> 	match fork() with
> 	| 0 -> if fork() <> 0 then exit 0;
> 	       ()
> 	| id -> ignore (waitpid [] id)
> 
> prints out that:
> Hi! 12215
> Hi! 12215
> Hi! 12215

You have three copies of the stdout buffer. Call "flush stdout" after
printf.

Gerd
-- 
------------------------------------------------------------
Gerd Stolpmann * Viktoriastr. 45 * 64293 Darmstadt * Germany 
gerd@gerd-stolpmann.de          http://www.gerd-stolpmann.de
------------------------------------------------------------

Re: [Caml-list] printf and fork question

[Basile STARYNKEVITCH]

Le Mon, Mar 07, 2005 at 02:44:22PM +0100, Michael écrivait/wrote:
> Hi,
> 
> I don't understand why this:
> 
> open Unix;;
> let _ = 
> 	Printf.printf "Hi! %d\n" (getpid());

I suggest adding a 
        flush stdout;
at this point. Flushing buffers is always good (and not language
related) before forking under Unix.

> 	match fork() with
> 	| 0 -> if fork() <> 0 then exit 0;
> 	       ()
> 	| id -> ignore (waitpid [] id)
> 
> prints out that:
> Hi! 12215
> Hi! 12215

I don't know what is happening, but if the the flushing occurs after
the forks - eg at program exit - the behavior is understandable.

Regards.
-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/ 
email: basile<at>starynkevitch<dot>net 
aliases: basile<at>tunes<dot>org = bstarynk<at>nerim<dot>net
8, rue de la Faïencerie, 92340 Bourg La Reine, France

Re: [Caml-list] printf and fork question

[David Fox]

It looks like a side effect of Printf.printf.  Try prerr_endline 
(Printf.sprintf "Hi! %d\n" (getpid ())) instead.

Michael wrote:

>Hi,
>
>I don't understand why this:
>
>open Unix;;
>let _ = 
>	Printf.printf "Hi! %d\n" (getpid());
>	match fork() with
>	| 0 -> if fork() <> 0 then exit 0;
>	       ()
>	| id -> ignore (waitpid [] id)
>
>prints out that:
>Hi! 12215
>Hi! 12215
>Hi! 12215
>
>
>with ocaml 3.08.2. Any hints?
>
> Michael
>
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>  
>
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Re: [Caml-list] printf and fork question

[Olivier Andrieu]

 > Michael [Mon, 7 Mar 2005]:
 > Hi,
 > 
 > I don't understand why this:
 > 
 > open Unix;;
 > let _ = 
 > 	Printf.printf "Hi! %d\n" (getpid());
 > 	match fork() with
 > 	| 0 -> if fork() <> 0 then exit 0;
 > 	       ()
 > 	| id -> ignore (waitpid [] id)
 > 
 > prints out that:
 > Hi! 12215
 > Hi! 12215
 > Hi! 12215
 > 
 > 
 > with ocaml 3.08.2. Any hints?

You haven't flushed stdout : so the Hi! stays in the buffer and the
children inherit it.

-- 
   Olivier

Re: [Caml-list] printf and fork question

[Michael]

thanks for the answers - I realized it at once after reading the word "buffered" in the replys :-)

 Michael