Hi at all,

Maybe I am using the put function incorrectly, but 
it is not possible to add larger strings there. The programm keeps locked and is doing nothing.

I tested it on latest Debian Buster. 
Python 3.7.3 (default, Jul 25 2020, 13:03:44)

Please check the example script.
python3 test_simple_queue_put.py 2**16-13

On windows 10 it's hanging for me from string length of 8175. 

Stepping through the code, the hang is in the call to _winapi.WaitForMultipleObjects, in PipeConnection._send_bytes, Lib/multiprocessing/connection.py:288

I've now reproduced it on osX, with string length of 65515. It takes a different code path than the windows version, and I was able to see more. 

This seems to be the sequence of events that leads to the hang:

import multiprocessing.reduction
import struct
import os
string_length = 65515
obj = "a"*string_length
obj = multiprocessing.reduction.ForkingPickler.dumps(obj)
m = memoryview(obj)
n = 65533
mm = m[0:n]
_, writer = os.pipe()
header = struct.pack("!i", n)
os.write(writer, header)
os.write(writer, mm)


I think what's happening is that the os.pipe that the queue is writing to has filled up, and then write blocks until any of it has been consumed.  This version of your script seems to confirm that:

import multiprocessing
import sys

q = multiprocessing.SimpleQueue()

string_length = eval(sys.argv[1])
print(string_length)
long_string = "a"*(string_length//2)
q.put(long_string)
q.get()                   <---- comment this out and it hangs again
q.put(long_string)

print("Never reach this line :-(")

You probably need to use Queue instead of SimpleQueue: 
https://docs.python.org/3.8/library/multiprocessing.html#multiprocessing.Queue

I used SimpleQueue on purpose because I only need the functions get and put. 
Of course I could also use Queue but that would be just a workaround for me.

The get and put functions of Queue have the optional 'block' and 'timeout' args that you need, and SimpleQueue doesn't.

Soo... I think I use the functions indeed incorrectly then.

Thank you for your time.

Regards

Raphael