Currently on Python 3.x:

>>> import ctypes
>>> memoryview(ctypes.c_long()).format
'<l'

This is invalid on 64-bit platforms: the above means 32-bit little-endian float. The '<' endian specification turns on the "standard size" mode (similarly as for the struct module), which makes type character have a platform-independent meaning.

Unfortunately, the struct module format syntax does *not* allow specifying native-size non-native-endian items. So just replacing '<' by '^' cannot be in general be done.

Suggested fix attached. It adds a converter function that maps the platform-dependent ctypes codes to struct module standard-size codes; handling c_long and c_bool specially.

    ***

After this patch (and the one in http://bugs.python.org/issue10744 ):

>>> import numpy as np
>>> from ctypes import *
>>> class Point(Structure):
...     _fields_ = [("x", c_long), ("y", c_long)]
... 
>>> class StructWithArrays(Structure):
...     _fields_ = [("x", c_long * 3 * 2), ("y", Point * 4)]
... 
>>> x = StructWithArrays()
>>> y = np.asarray(x)
>>> y.dtype
dtype([('x', '<i8', (2, 3)), ('y', [('x', '<i8'), ('y', '<i8')], (4,))])
>>> y['x'] = [[1,2,3],[4,5,6]]
>>> y['y']['x'] = np.arange(4) + 10
>>> y['y']['y'] = np.arange(4) + 20
>>> x.x[0][0]
1
>>> x.x[0][1]
2
>>> x.x[0][2]
3
>>> x.y[0].x
10
>>> x.y[1].x
11
>>> x.y[0].y
20
>>> x.y[1].y
21

Could someone do a patch review on this please.

Converted patch to Github PR + some cleanup.

Added the names from the experts list to this ticket.

New changeset 07f1658aa09f6798793c473c72b2951b7fefe220 by Antoine Pitrou (Pauli Virtanen) in branch 'master':
bpo-10746: Fix ctypes PEP 3118 type codes for c_long, c_bool, c_int (#31)
https://github.com/python/cpython/commit/07f1658aa09f6798793c473c72b2951b7fefe220

Pauli, do you want to submit backport PRs? This can be done using the cherry_picker script: https://devguide.python.org/committing/?highlight=cherry_picker#backporting-changes-to-an-older-version

Created backport PR for 3.6: https://github.com/python/cpython/pull/3241
Which versions take backports?

At this point, 2.7 and 3.6 do.

New changeset 2d1653aa43cf02e6b74f9d4f178fac9969a293e2 by Antoine Pitrou (Pauli Virtanen) in branch '3.6':
[3.6] bpo-10746: Fix ctypes PEP 3118 type codes for c_long, c_bool, c_int (GH-31) (#3241)
https://github.com/python/cpython/commit/2d1653aa43cf02e6b74f9d4f178fac9969a293e2

New changeset 990b2d043cdfaafc3378550f7d27259410a6b918 by Antoine Pitrou (Pauli Virtanen) in branch '2.7':
[2.7] bpo-10746: Fix ctypes PEP 3118 type codes for c_long, c_bool, c_int (GH-31) (#3242)
https://github.com/python/cpython/commit/990b2d043cdfaafc3378550f7d27259410a6b918