ctypes incorrect handling of long int on 64bits Linux

[MarcoClemencic]

BPO	24470
Nosy	@terryjreedy, @eryksun
Files	test_py.tar.bz2: minimal test case exposing the problem

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2015-07-19.07:54:25.932>
created_at = <Date 2015-06-19.08:35:40.948>
labels = ['ctypes', 'type-bug', 'invalid']
title = 'ctypes incorrect handling of long int on 64bits Linux'
updated_at = <Date 2015-07-19.08:44:58.149>
user = 'https://bugs.python.org/MarcoClemencic'

bugs.python.org fields:

activity = <Date 2015-07-19.08:44:58.149>
actor = 'terry.reedy'
assignee = 'none'
closed = True
closed_date = <Date 2015-07-19.07:54:25.932>
closer = 'Marco Clemencic'
components = ['ctypes']
creation = <Date 2015-06-19.08:35:40.948>
creator = 'Marco Clemencic'
dependencies = []
files = ['39736']
hgrepos = []
issue_num = 24470
keywords = []
message_count = 5.0
messages = ['245494', '245495', '245525', '246933', '246934']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'eryksun', 'Marco Clemencic']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue24470'
versions = ['Python 2.7']

[MarcoClemencic]

When passing a Python int to a C function expecting long int (or void*) via ctypes, the number gets truncated to 32 bits, even if the args types are correctly declared.
The workaround is to wrap the argument in c_long (or c_void_p), but the automatic unwrapping of c_long (or c_void_p) in the return type lead to undefined behaviour in code like:

myClib.getNumber.restype = c_long
myClib.doSomething.args = [c_long]
l = myClib.getnumber()
myClib.doSomething(l)

[eryksun]

It works for me:

    >>> open('test.c', 'w').write('long test(long x) {return x;}')
    >>> os.system('gcc -shared -fPIC -o test.so test.c')
    0
    >>> test = CDLL('./test.so').test
    >>> test.restype = c_long
    >>> test.argtypes = (c_long,)
    >>> test(2**63-1) == 2**63-1
    True

However, I used the correct attribute name, "argtypes". ctypes objects have dicts to allow setting arbitrary attribute names, so you won't get an AttributeError when setting "args" on a function pointer.

[terryjreedy]

Should this then be closed as not a bug?

[MarcoClemencic]

Hi,

apologies for the very late answer, but I just discovered that the mails got flagged as spam :(

In any case, I do not know where I got this "args" from, but I can confirm that the problem is just a bug on my side.

Thanks
Marco

[terryjreedy]

Thank you for following up on this.