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References: <20060111123745.GB30219@lgb.hu> <1136983910.2929.39.camel@laptopd505.fenrus.org>
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From: Roland Kuhn <rkuhn@e18.physik.tu-muenchen.de>
Subject: Re: OT: fork(): parent or child should run first?
Date: Wed, 11 Jan 2006 14:34:19 +0100
To: Arjan van de Ven <arjan@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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Hi Arjan!

On 11 Jan 2006, at 13:51, Arjan van de Ven wrote:

> you just cannot depend on which would run first, child or parent. Even
> if linux would do it the other way around, you have no guarantee.  
> Think
> SMP or Dual Core processors and time slices and cache misses... your
> code just HAS to be able to cope with it. Even on solaris ;)

That means that the starting of the child process needs to be  
synchronized by the application itself. I tried it once but then I  
discovered that my case was easily solved in a completely different  
way (it was a very small project). However, which one is the easiest/ 
fastest way to do this synchronization? Using SysV-Semaphores? Pipes?  
Would something like this work?

int fd[2], pid;

pipe(fd);
pid = fork();
if (pid < 0) {
   error();
} else if (pid == 0) {
   close(fd[1]);
   read(fd[0]);
   close(fd[0]);
   child_code();
} else {
   store_pid();
   close(fd[0]);
   close(fd[1]); // this should wake up the child, right?
}

This should ensure that store_pid() is executed before child_code()...

Ciao,
                     Roland

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