Python child process in while True loop blocks parent

[Peter J. Holzer]

On 2021-12-08 18:11:48 +0100, Jen Kris via Python-list wrote:
> To recap, I'm using a pair of named pipes for IPC between C and
> Python.  Python runs as a child process after fork-execv.  The Python
> program continues to run concurrently in a while True loop, and
> responds to requests from C at intervals, and continues to run until
> it receives a signal from C to exit.  C sends signals to Python, then
> waits to receive data back from Python.  My problem was that C was
> blocked when Python started. 
> 
> The solution was twofold:  (1) for Python to run concurrently it must
> be a multiprocessing loop (from the multiprocessing module),

I don't see how this could achieve anything. It starts another (third)
process, but then it just does all the work in that process and just
waits for it. Doing the same work in the original Python process should
have exactly the same effect.

> and (2) Python must terminate its write strings with \n, or read will
> block in C waiting for something that never comes.

That's also strange. You are using os.write in Python and read in C,
both of which shoudn't care about newlines.

> The multiprocessing module sidesteps the GIL; without multiprocessing
> the GIL will block all other threads once Python starts. 

Your Python interpreter runs in a different process than your C code.
There is absolutely no way the GIL could block threads in your C
program. And your Python code doesn't need to use more than one thread
or process.

        hp

-- 
   _  | Peter J. Holzer    | Story must make more sense than reality.
|_|_) |                    |
| |   | hjp at hjp.at         |    -- Charles Stross, "Creative writing
__/   | http://www.hjp.at/ |       challenge!"
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