BUG: Ensure that arr.ctypes works on arrays with overlapping fields

These would previously fail in `from_buffer`, since these arrays cannot be used with the buffer protocol.

2 files changed, 40 insertions, 8 deletions

diff --git a/numpy/core/_internal.py b/numpy/core/_internal.py
index 4b5fda65c..a5742983c 100644
--- a/numpy/core/_internal.py
+++ b/numpy/core/_internal.py
@@ -244,21 +244,44 @@ class _missing_ctypes(object):
             self.value = ptr
 
 
+class _unsafe_first_element_pointer(object):
+    """
+    Helper to allow viewing an array as a ctypes pointer to the first element
+
+    This avoids:
+      * dealing with strides
+      * `.view` rejecting object-containing arrays
+      * `memoryview` not supporting overlapping fields
+    """
+    def __init__(self, arr):
+        self.base = arr
+
+    @property
+    def __array_interface__(self):
+        i = dict(
+            shape=(),
+            typestr='|V0',
+            data=(self.base.__array_interface__['data'][0], False),
+            strides=(),
+            version=3,
+        )
+        return i
+
 
 def _get_void_ptr(arr):
     """
     Get a `ctypes.c_void_p` to arr.data, that keeps a reference to the array
     """
     import numpy as np
-    # don't let subclasses interfere
-    arr = arr.view(ndarray)
-    # collapse the array to point to at most 1 element, so it become contiguous
-    arr = arr[np.s_[:1,] * arr.ndim + np.s_[...,]]
-    # then convert to ctypes now that we've reduced it to a simple, empty, array
-    arr.flags.writeable = True
-    arr = (ctypes.c_char * 0).from_buffer(arr)
+    # convert to a 0d array that has a data pointer referrign to the start
+    # of arr. This holds a reference to arr.
+    simple_arr = np.asarray(_unsafe_first_element_pointer(arr))
+
+    # create a `char[0]` using the same memory.
+    c_arr = (ctypes.c_char * 0).from_buffer(simple_arr)
+
     # finally cast to void*
-    return ctypes.cast(ctypes.pointer(arr), ctypes.c_void_p)
+    return ctypes.cast(ctypes.pointer(c_arr), ctypes.c_void_p)
 
 
 class _ctypes(object):
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index 171b2ad7f..fa21018ec 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -7480,6 +7480,14 @@ class TestCTypes(object):
         np.array([1, 2, 3]),
         np.array([['one', 'two'], ['three', 'four']]),
         np.array((1, 2), dtype='i4,i4'),
+        np.zeros((2,), dtype=
+            np.dtype(dict(
+                formats=['<i4', '<i4'],
+                names=['a', 'b'],
+                offsets=[0, 2],
+                itemsize=6
+            ))
+        ),
         np.array([None], dtype=object),
         np.array([]),
         np.empty((0, 0)),
@@ -7488,6 +7496,7 @@ class TestCTypes(object):
         '1d',
         '2d',
         'structured',
+        'overlapping',
         'object',
         'empty',
         'empty-2d',