linux-kernel - Re: the different effect of system call fork()

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Message-ID: <20090409123644.GA10495@mail.local.tull.net>
Date:	Thu, 9 Apr 2009 22:36:44 +1000
From:	Nick Andrew <nick@...k-andrew.net>
To:	Bernd Petrovitsch <bernd@...mix.at>
Cc:	Sino <szj087@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: the different effect of system call fork()

On Wed, Apr 08, 2009 at 03:08:48PM +0200, Bernd Petrovitsch wrote:
> >    if (pid_t pid = fork() < 0)
> 
> I don't think that this line does what you want it to do.
> Hint: Using a "pid_t" for the boolean result for a comparison seems
> strange at best.

The "problem" is due to a misunderstanding of C/C++ operator precedence.

"(pid = fork() < 0)" binds like "pid = (fork() < 0)" and so pid always
has a value of zero and so both parent and child process run the "first
level child process" code ... soon there are 4 processes all confused
about who they are.

Nick.
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