[issue32291] Value error for string shared memory in multiprocessing

[Irit Katriel]

Change by Irit Katriel :


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stage:  -> resolved
status: pending -> closed

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[issue32291] Value error for string shared memory in multiprocessing

[Irit Katriel]

Change by Irit Katriel :


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status: open -> pending

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[issue32291] Value error for string shared memory in multiprocessing

[STINNER Victor]

Change by STINNER Victor :


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nosy:  -vstinner
status: pending -> open

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[issue32291] Value error for string shared memory in multiprocessing

[Irit Katriel]

Irit Katriel  added the comment:

Is there anything left to do here?

It seems hongweipeng's explanation and the link to the documentation pretty 
much cover it.

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nosy: +iritkatriel
resolution:  -> not a bug
status: open -> pending

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[issue32291] Value error for string shared memory in multiprocessing

[hongweipeng]

hongweipeng  added the comment:

I think I know the reason. `c_wchar_p` corresponds to the string pointer 
`wchar_t *`.It's not a good idea to pass pointers between processes. As quoted 
from `multiprocessing` docs:

Note Although it is possible to store a pointer in shared memory remember that 
this will refer to a location in the address space of a specific process. 
However, the pointer is quite likely to be invalid in the context of a second 
process and trying to dereference the pointer from the second process may cause 
a crash.

https://docs.python.org/3.6/library/multiprocessing.html?#module-multiprocessing.sharedctypes

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[issue32291] Value error for string shared memory in multiprocessing

[hongweipeng]

hongweipeng  added the comment:

This problem seems to be support for str.

import multiprocessing
import ctypes

def child_process_fun(share):
share.value = 'bb'

if __name__ == '__main__':
share = multiprocessing.Value(ctypes.c_wchar_p, 'aa')
process = multiprocessing.Process(target=child_process_fun, args=(share, ))
process.start()
process.join()
print(share.value)

It also raises ValueError.When use c_double or c_int, it works well.

Test in python3.7 and 3.8

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[issue32291] Value error for string shared memory in multiprocessing

[hongweipeng]

Change by hongweipeng :


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nosy: +hongweipeng

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[issue32291] Value error for string shared memory in multiprocessing

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +xtreak

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[issue32291] Value error for string shared memory in multiprocessing

[Marc Guetg]

New submission from Marc Guetg :

It seems like sharing a string over processes is not possible.

#!/usr/bin/env python3

import multiprocessing
import threading
import ctypes


def fun(share_c, share_s, set_c, set_s, name):
print(f'{name}: {share_c.value}; {share_s.value}')
share_c.value = set_c
share_s.value = set_s
print(f'{name}: {share_c.value}; {share_s.value}')


if __name__ == '__main__':
share_c = multiprocessing.Value(ctypes.c_wchar, 'a')
share_s = multiprocessing.Value(ctypes.c_wchar_p, 'aa')

print(f'pre_thread: {share_c.value}; {share_s.value}')
thread = threading.Thread(target=fun, args=(share_c, share_s, 
'b', 'bb', 'thread'))
thread.start()
thread.join()

print(f'post_thread: {share_c.value}; {share_s.value}')
process = multiprocessing.Process(target=fun, args=(share_c, 
share_s, 'c', 'cc', 'process'))
process.start()
process.join()

print(f'post_process: {share_c.value}', end='; ')
print(share_s.value)  # <--- Blows here

produces: 

pre_thread: a; aa
thread: a; aa
thread: b; bb
post_thread: b; bb
process: b; bb
process: c; cc
post_process: c; Traceback (most recent call last):
  File "test2.py", line 30, in 
print(share_s.value)  # <--- Blows here
  File "", line 5, in getvalue
ValueError: character U+ff92f210 is not in range [U+; U+10]

Where the character value in the error message is different every time. To me 
this seems like a bug as it is working properly with threads as well as single 
characters. (Maybe relevant question also here: 
https://stackoverflow.com/questions/47763878/how-to-share-string-between-processes?noredirect=1#comment82492062_47763878)

For the case it matters:
Python 3.6.1 (Anaconda 4.4.0) on RHEL 6

--
components: Library (Lib), Unicode
messages: 308144
nosy: ezio.melotti, magu, vstinner
priority: normal
severity: normal
status: open
title: Value error for string shared memory in multiprocessing
type: crash
versions: Python 3.6

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