Threading plus multiprocessing plus cv2 error

[Barry]

> On 30 Aug 2020, at 11:03, Stephane Tougard via Python-list <python-list at python.org> wrote:
> 
> On 2020-08-30, Chris Angelico <rosuav at gmail.com> wrote:
>>> I'm not even that makes sense, how 2 processes can share a thread ?
>>> 
>> They can't. However, they can share a Thread object, which is the
>> Python representation of a thread. That can lead to confusion, and
>> possibly the OP's error (I don't know for sure, I'm just positing).
> 
> A fork() is a copy of a process in a new process. If this process has a
> thread (or several), they are part of the copy and the new process has
> those threads as well.

No. See https://www.man7.org/linux/man-pages/man2/fork.2.html which says:

“ Note the following further points:

       *  The child process is created with a single thread—the one that
          called fork().  The entire virtual address space of the parent is
          replicated in the child, including the states of mutexes,
          condition variables, and other pthreads objects; the use of
          pthread_atfork(3) may be helpful for dealing with problems that
          this can cause.”

Barry
> 
> Unless there is a memory sharing between those processes, what happens
> on one thread in the first process is totally independant of what
> happens in the copy of this thread in the other process.
> 
> I'm not specialist on multi-threading in Python, but it should not
> change anything. Both processes (father and child) don't share the same
> thread, each one has its own copy of the thread.
> 
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