Fix issue with aligned subclasses

Author: monkeyman192

Lib/test/test_ctypes/test_aligned_structures.py

@@ -1,4 +1,4 @@
-from ctypes import Structure, c_char, c_uint32, c_ubyte, alignment
+from ctypes import Structure, c_char, c_uint32, c_ubyte, alignment, sizeof, Union
 import inspect
 import unittest
 
@@ -22,11 +22,11 @@ class Main(Structure):
                 ('string', Aligned),
             ]
 
-        d = Main.from_buffer(data)
-        self.assertEqual(d.first, 7)
-        self.assertEqual(d.string.value, b'hello world!')
+        main = Main.from_buffer(data)
+        self.assertEqual(main.first, 7)
+        self.assertEqual(main.string.value, b'hello world!')
         self.assertEqual(
-            bytes(d.string.__buffer__(inspect.BufferFlags.SIMPLE)),
+            bytes(main.string.__buffer__(inspect.BufferFlags.SIMPLE)),
             b'\x68\x65\x6c\x6c\x6f\x20\x77\x6f\x72\x6c\x64\x21\x00\x00\x00\x00'
         )
 
@@ -60,12 +60,12 @@ class MainTooBig(Structure):
                 ("x", SomeBoolsTooBig),
                 ("y", c_uint32),
             ]
-        d = Main.from_buffer(data)
-        self.assertEqual(d.y, 7)
+        main = Main.from_buffer(data)
+        self.assertEqual(main.y, 7)
         self.assertEqual(alignment(SomeBools), 4)
-        self.assertEqual(d.x.bool1, True)
-        self.assertEqual(d.x.bool2, False)
-        self.assertEqual(d.x.bool3, True)
+        self.assertEqual(main.x.bool1, True)
+        self.assertEqual(main.x.bool2, False)
+        self.assertEqual(main.x.bool3, True)
 
         with self.assertRaises(ValueError) as ctx:
             MainTooBig.from_buffer(data)
@@ -74,5 +74,92 @@ class MainTooBig(Structure):
                 'Buffer size too small (4 instead of at least 8 bytes)'
             )
 
+    def test_aligned_subclasses(self):
+        data = bytearray(
+            b"\x01\x00\x00\x00\x02\x00\x00\x00\x03\x00\x00\x00\x04\x00\x00\x00"
+        )
+
+        class UnalignedSub(Structure):
+            x: c_uint32
+            _fields_ = [
+                ("x", c_uint32),
+            ]
+
+        class AlignedStruct(UnalignedSub):
+            _align_ = 8
+            y: c_uint32
+            _fields_ = [
+                ("y", c_uint32),
+            ]
+
+        class Main(Structure):
+            _fields_ = [
+                ("a", c_uint32),
+                ("b", AlignedStruct)
+            ]
+
+        main = Main.from_buffer(data)
+        self.assertEqual(alignment(main.b), 8)
+        self.assertEqual(alignment(main), 8)
+        self.assertEqual(sizeof(main.b), 8)
+        self.assertEqual(sizeof(main), 16)
+        self.assertEqual(main.a, 1)
+        self.assertEqual(main.b.x, 3)
+        self.assertEqual(main.b.y, 4)
+
+    def test_aligned_union(self):
+        data = bytearray(
+            b"\x01\x00\x00\x00\x02\x00\x00\x00\x03\x00\x00\x00\x04\x00\x00\x00"
+        )
+
+        class AlignedUnion(Union):
+            _align_ = 8
+            _fields_ = [
+                ("a", c_uint32),
+                ("b", c_ubyte * 8),
+            ]
+
+        class Main(Structure):
+            _fields_ = [
+                ("first", c_uint32),
+                ("union", AlignedUnion),
+            ]
+
+        main = Main.from_buffer(data)
+        self.assertEqual(main.first, 1)
+        self.assertEqual(main.union.a, 3)
+        self.assertEqual(bytes(main.union.b), b"\x03\x00\x00\x00\x04\x00\x00\x00")
+
+    def test_aligned_struct_in_union(self):
+        data = bytearray(
+            b"\x01\x00\x00\x00\x02\x00\x00\x00\x03\x00\x00\x00\x04\x00\x00\x00"
+        )
+
+        class Sub(Structure):
+            _align_ = 8
+            _fields_ = [
+                ("x", c_uint32),
+                ("y", c_uint32),
+            ]
+
+        class MainUnion(Union):
+            _fields_ = [
+                ("a", c_uint32),
+                ("b", Sub),
+            ]
+
+        class Main(Structure):
+            _fields_ = [
+                ("first", c_uint32),
+                ("union", MainUnion),
+            ]
+
+        main = Main.from_buffer(data)
+        self.assertEqual(main.first, 1)
+        self.assertEqual(main.union.a, 3)
+        self.assertEqual(main.union.b.x, 3)
+        self.assertEqual(main.union.b.y, 4)
+
+
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2024-01-28-02-46-12.gh-issue-112433.FUX-nT.rst

@@ -1 +1 @@
-Add ability to force alignment of :mod:`ctypes.Structure` by way of the new `_align_` attribute on the class.
+Add ability to force alignment of :mod:`ctypes.Structure` by way of the new ``_align_`` attribute on the class.

Modules/_ctypes/stgdict.c

@@ -486,7 +486,7 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
         size = offset = basedict->size;
         align = basedict->align;
         union_size = 0;
-        total_align = align ? align : forced_alignment;
+        total_align = max(align ? align : 1, forced_alignment);
         stgdict->ffi_type_pointer.type = FFI_TYPE_STRUCT;
         stgdict->ffi_type_pointer.elements = PyMem_New(ffi_type *, basedict->length + len + 1);
         if (stgdict->ffi_type_pointer.elements == NULL) {