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Date: Mon, 5 Nov 2007 11:21:47 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: cygwin stable and cvs snapshot - fork() bug
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On Nov  1 10:58, Corinna Vinschen wrote:
> On Oct 31 14:26, Lev Bishop wrote:
> > $ cat lev.c && gcc -o lev lev.c -Wall -Wextra && CYGWIN=server ./lev
> > #include <stdio.h>
> > #include <unistd.h>
> > #include <sys/shm.h>
> > 
> > int main(void)
> > {
> >         int shmid;
> >         if ((shmid = shmget(IPC_PRIVATE, 100,IPC_CREAT | 0600 )) < 0 ||
> >                 !shmat(shmid, NULL, 0) ||
> >                 shmctl(shmid, IPC_RMID, NULL) < 0)
> >                 puts("problems with shm!");
> >         fork();
> > }
> > lev.c: In function `main':
> > lev.c:13: warning: control reaches end of non-void function
> >       3 [main] lev 1924 c:\Documents and
> > Settings\Lev\Desktop\mpd-0.13.0\lev.exe: *** fatal error -
> > MapViewOfFileEx (0x3E0000), Win32 error 6.  Terminating.
> >     124 [main] lev 5076 fork: child 1924 - died waiting for dll
> > loading, errno 11
> 
> Thanks for the testcase.  I'm surprised that nobody experienced this
> problem before.  Sorta holiday here, so I'll look into it next week.

Ouch, ouch, ouch.  shmctl(IPC_RMID) closed the handle to the shared
memory, but neglected to remove the actual mappings as well as the
bookkeeping structure.  The result is that after a fork the child thinks
there are still mappings which have to be duplicated into its own
memory.  But the handle has already been closed in the parent, so the
MapViewOfFile call fails with "invalid handle".

Unfortunately not many applications use shmctl(IPC_RMID) before creating
a child process since usually the shared memory is meant to be... well,
shared.  That's why this didn't crop up more often, obviously.

I've applied a fix to CVS.


Thanks again for the testcase,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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