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Date: Mon, 16 Aug 2010 11:05:52 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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Subject: Re: Bug in fork() while in a thread
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On Aug 15 14:53, Christopher Faylor wrote:
> On Sun, Aug 15, 2010 at 07:42:01PM +0200, Jason Curl wrote:
> >Is it allowed to issue the fork() system call while not in the main 
> >thread? When I read the OpenGroup specifications I don't seem to find 
> >anything against allowing this.
> >
> >In particular, if I create a thread, then issue a fork(), data that 
> >exists on the stack is corrupted after the fork() is in the child. Using 
> >data on the heap doesn't show any issues (and is currently my 
> >workaround, in case this is a bug).
> 
> If I'm reading this correctly then "the stack" in this case is the stack
> associated with the main thread.  Cygwin only duplicates the stack in
> the executing thread.  In your example, env (or presumably env2) from
> the main thread is passed to another thread which then calls fork.  In
> that scenario, the forked process is going to see garbage in env since
> the array has never been initialized.
> 
> It is theoretically possible to duplicate the stack of the main thread
> and other threads in the forked process [...]

I guess I'm missing something here.  Here's an excerpt from the SUSv4
fork man page:

  The fork() function shall create a new process. The new process (child
  process) shall be an exact copy of the calling process (parent process)
  except as detailed below:

  [...]

  o A process shall be created with a single thread. If a
    multi-threaded process calls fork(), the new process shall contain
    a replica of the calling thread and its entire address space,
    possibly including the states of mutexes and other resources.
    [...]

Is the stack of another thread, which is not executed in the forked
process anymore, a part of the calling's thread "entire address space"?


Corinna

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

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