ctypes.Array.from_buffer segmentation fault when trying to create from array.array

[spearsemgmailcom]

BPO	25659
Nosy	@vadmium, @eryksun, @zhangyangyu
PRs	[Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552
Files	from_buffer.patch

^{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 2016-11-21.03:32:14.620>
created_at = <Date 2015-11-18.17:57:16.353>
labels = ['ctypes', '3.7', 'type-crash']
title = 'ctypes.Array.from_buffer segmentation fault when trying to create from array.array'
updated_at = <Date 2017-03-31.16:36:26.989>
user = 'https://bugs.python.org/spearsemgmailcom'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:26.989>
actor = 'dstufft'
assignee = 'none'
closed = True
closed_date = <Date 2016-11-21.03:32:14.620>
closer = 'martin.panter'
components = ['ctypes']
creation = <Date 2015-11-18.17:57:16.353>
creator = 'spearsem@gmail.com'
dependencies = []
files = ['45543']
hgrepos = []
issue_num = 25659
keywords = ['patch']
message_count = 4.0
messages = ['254851', '254853', '281187', '281309']
nosy_count = 5.0
nosy_names = ['python-dev', 'martin.panter', 'eryksun', 'xiang.zhang', 'spearsem@gmail.com']
pr_nums = ['552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue25659'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6', 'Python 3.7']

[spearsemgmailcom]

I'm trying to find a way to create a ctypes array from the underlying memory buffer exposed by an array.array object. The ctypes.Array.from_buffer function isn't documented, but I did find the source code in _ctypes.c around line 497. It's not clear to me where the problem might be.

Code to reproduce it below:

import array, ctypes
a1 = array.array('l')
a1.fromlist(range(10))
ctypes.Array.from_buffer(a1)
#Segfault

[eryksun]

You have to subclass ctypes.Array with a _type_ and _length_. But ctypes types also implement sequence repetition (*) to facilitate creating array types. For example:

    import array, ctypes
    a1 = array.array('l')
    a1.fromlist(range(10))
    c1 = (ctypes.c_long * 10).from_buffer(a1)

    >>> c1[:]
    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    >>> c1[0] = 42
    >>> a1[0]
    42

That said, it's way too easy to segfault this. Why not simply fail the call in this case? e.g.

    StgDictObject *dict = PyType_stgdict(type);
    if (!dict) {
        PyErr_SetString(PyExc_TypeError,
                    "abstract class");
        return NULL;
    }

[vadmium]

Here is a patch implementing Eryk’s suggestion, for both from_buffer() and from_buffer_copy() methods. This is exactly how it is already handled for Array.from_address(). The two buffer methods were added more recently than from_address().

[python-dev]

New changeset 5f061870d49c by Martin Panter in branch '3.5':
Issue bpo-25659: Change assert to TypeError in from_buffer/_copy()
https://hg.python.org/cpython/rev/5f061870d49c

New changeset 1253ef20c947 by Martin Panter in branch '3.6':
Issue bpo-25659: Merge ctypes fix from 3.5
https://hg.python.org/cpython/rev/1253ef20c947

New changeset 821da4891051 by Martin Panter in branch 'default':
Issue bpo-25659: Merge ctypes fix from 3.6
https://hg.python.org/cpython/rev/821da4891051

New changeset 4de956751cf1 by Martin Panter in branch '2.7':
Issue bpo-25659: Change assert to TypeError in from_buffer/_copy()
https://hg.python.org/cpython/rev/4de956751cf1