linux-kernel - Re: PROBLEM: getpid() returning same value as getppid()

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Message-ID: <20100602150235.GD12784@tiehlicka.suse.cz>
Date:	Wed, 2 Jun 2010 17:02:35 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	S?bastien Paumier <sebastien.paumier@...v-mlv.fr>,
	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: getpid() returning same value as getppid()

On Wed 26-05-10 11:16:51, Nick Bowler wrote:
> On 16:45 Wed 26 May     , S?bastien Paumier wrote:
> > Hi,
> > here is a bug that occurs on my kernel 2.6.31-21, maybe with older ones. 
> >   If a C program contains a function with the constructor attribute that 
> > calls getpid(), then, a call to syscall(SYS_fork) produces a son that 
> > obtains the same value calling getpid() or getppid().
> 
> The GNU C library caches the results of getpid, which is probably the
> cause of your grief.  This cache relies on the glibc wrappers for fork
> and friends, which you have bypassed by using syscall directly.

man page for getpid states that explicitly:
NOTES
       Since  glibc  version  2.3.4, the glibc wrapper function for
       getpid() caches PIDs, so as to avoid additional system calls when
       a process calls getpid() repeatedly.  Normally this caching is
       invisible, but its correct operation relies on support in the
       wrapper  functions for  fork(2), vfork(2),  and  clone(2):
       if an application bypasses the glibc wrappers for these system
       calls by using syscall(2), then a call to getpid() in the child
       will return the wrong value (to be precise: it will return the
       PID of the parent process).  See also clone(2) for  discussion
       of  a case where getpid() may return the wrong value even when
       invoking clone(2) via the glibc wrapper function.

> 
> -- 
> Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)
> --
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-- 
Michal Hocko
L3 team 
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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