BUG: Fix for np.dtype(ctypes.Structure) does not respect _pack_ field

See #10532
Made several changes to offset and size calulation. First pull request
was far from correct.

2 files changed, 33 insertions, 19 deletions

diff --git a/numpy/core/_dtype_ctypes.py b/numpy/core/_dtype_ctypes.py
index e6e675e2c..6829860cc 100644
--- a/numpy/core/_dtype_ctypes.py
+++ b/numpy/core/_dtype_ctypes.py
@@ -34,27 +34,42 @@ def _from_ctypes_array(t):
 
 def _from_ctypes_structure(t):
     # TODO: gh-10533
-    if hasattr(t,"_pack_"):
-        fields = {}
-        for item in t._fields_:
-            if len(item) > 2:
-                raise TypeError(
+    for item in t._fields_:
+        if len(item) > 2:
+            raise TypeError(
                 "ctypes bitfields have no dtype equivalent")
-            fname, ftyp = item
-            fields[fname] = dtype_from_ctypes_type(ftyp), t._pack_
-
-        return np.dtype(fields, align=False)
     else:
-        fields = []
-        for item in t._fields_:
-            if len(item) > 2:
-                raise TypeError(
-                "ctypes bitfields have no dtype equivalent")
-            fname, ftyp = item
-            fields.append((fname, dtype_from_ctypes_type(ftyp)))
+        if hasattr(t,"_pack_"):
+            formats_array = []
+            offsets_array = []
+            names_array = []
+            current_offset = 0
+            current_itemsize = 0
+            for item in t._fields_:
+                fname, ftyp = item
+                names_array.append(fname)
+                formats_array.append(dtype_from_ctypes_type(ftyp))
+                # Each type has a default offset, this is platform dependent for some types.
+                effective_pack = min(t._pack_, ctypes.alignment(ftyp))
+                current_offset = ((current_offset + effective_pack - 1) // effective_pack) * effective_pack
+                offsets_array.append(current_offset)
+                current_offset += ctypes.sizeof(ftyp)
+
+
+            return np.dtype(dict(
+                formats = formats_array,
+                offsets = offsets_array,
+                names = names_array,
+                itemsize = ctypes.sizeof(t)
+            ))
+        else:
+            fields = []
+            for item in t._fields_:
+                fname, ftyp = item
+                fields.append((fname, dtype_from_ctypes_type(ftyp)))
 
-        # by default, ctypes structs are aligned
-        return np.dtype(fields, align=True)
+            # by default, ctypes structs are aligned
+            return np.dtype(fields, align=True)
 
 
 def dtype_from_ctypes_type(t):
diff --git a/numpy/core/tests/test_dtype.py b/numpy/core/tests/test_dtype.py
index ecb51f72d..f4b118bab 100644
--- a/numpy/core/tests/test_dtype.py
+++ b/numpy/core/tests/test_dtype.py
@@ -824,7 +824,6 @@ class TestFromCTypes(object):
         ))
         self.check(Union, expected)
 
-    @pytest.mark.xfail(reason="_pack_ is ignored - see gh-11651")
     def test_packed_structure(self):
         class PackedStructure(ctypes.Structure):
             _pack_ = 1