Pass data to a subprocess

[Grant Edwards]

On 2012-08-02, Laszlo Nagy <gandalf at shopzeus.com> wrote:
>
>> I still don't get it.  shm_unlink() works the same way unlink() does.
>> The resource itself doesn't cease to exist until all open file
>> handles are closed. From the shm_unlink() man page on Linux:
>>
>>         The operation of shm_unlink() is analogous to unlink(2): it
>>         removes a shared memory object name, and, once all processes
>>         have unmapped the object, de-allocates and destroys the
>>         contents of the associated memory region. After a successful
>>         shm_unlink(), attempts to shm_open() an object with the same
>>         name will fail (unless O_CREAT was specified, in which case a
>>         new, distinct object is created).
>>         
>> Even if the parent calls shm_unlink(), the shared-memory resource
>> will continue to exist (and be usable) until all processes that are
>> holding open file handles unmap/close them.  So not only will
>> detached children not crash, they'll still be able to use the shared
>> memory objects to talk to each other.

Note that when I say the detached children will still be able to talk
to each other using shared memory after the parent calls shm_unlink()
and exit(), I'm talking about the general case -- not specifically
about the multiprocessing module. There may be something else going on
with the multiprocessing module.

> I stand corrected. It should still be examined, what kind shared
> memory is used under non-linux systems. System V on AIX? And what
> about Windows? So maybe the general answer is still no. But I guess
> that the OP wanted this to work on a specific system.
>
> Dear Andrea Crotti! Please try to detach two child processes, exit
> from the main process, and communicate over a multiprocessing queue.
> It will possibly work. Sorry for my bad advice.

I'm not claiming it will work, since I don't know how the IPC in the
multiprocessing module works.  It may indeed break when a child
process is detatched (which I'm assuming means being removed from the
process group and/or detached from the controlling tty).

But, I'm not aware of any underlying Unix IPC mechanism that breaks
when a child is detached, so I was curious about what would cause
multiprocessing's IPC to break.

-- 
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