gh-73939: Fix bitfiled not matching C on mac/linux.

Lib/test/test_ctypes/test_structures.py

@@ -738,6 +738,30 @@ class Test8(Union):
         self.assertEqual(ctx.exception.args[0], 'item 1 in _argtypes_ passes '
                          'a union by value, which is unsupported.')
 
+    def test_bitfield_matches_c(self):
+        class Test9(Structure):
+            _pack_ = 1
+            _fields_ = (('A', c_uint16),     # 2 bytes
+                        ('B', c_uint16, 9),
+                        ('C', c_uint16, 1),
+                        ('D', c_uint16, 1),
+                        ('E', c_uint16, 1),
+                        ('F', c_uint16, 1),
+                        ('G', c_uint16, 3),  # 4 bytes
+                        ('H', c_uint32, 10),
+                        ('I', c_uint32, 20),
+                        ('J', c_uint32, 2))  # 8 bytes
+        dll = CDLL(_ctypes_test.__file__)
+        func = dll._testfunc_bitfield_by_reference3
+        func.restype = c_long
+        func.argtypes = (POINTER(Test9),c_long,)
+        ind = 0
+        for field in Test9._fields_:
+            test9 = Test9()
+            setattr(test9,field[0], 1)
+            self.assertEqual(func(test9, ind), 1)
+            ind += 1
+
 class PointerMemberTestCase(unittest.TestCase):
 
     def test(self):

Misc/NEWS.d/next/C API/2023-03-27-03-52-12.gh-issue-73939.aOzhdU.rst

@@ -0,0 +1,2 @@
+Fixes bitfield ABI for Linux/Mac for certain bitfields.
+Adds unit test to verify C bitfield ABI matches python ABI.

Modules/_ctypes/_ctypes_test.c

@@ -231,6 +231,31 @@ _testfunc_bitfield_by_reference2(Test7 *in) {
     return result;
 }
 
+typedef struct{
+    uint16_t A ;
+    uint16_t B : 9;
+    uint16_t C : 1;
+    uint16_t D : 1;
+    uint16_t E : 1;
+    uint16_t F : 1;
+    uint16_t G : 3;
+    uint32_t H : 10;
+    uint32_t I : 20;
+    uint32_t J : 2;
+} Test9;
+
+EXPORT(long)
+_testfunc_bitfield_by_reference3(Test9 *in, long pos) {
+    long data[] = {in->A , in->B , in->C , in->D , in->E , in->F , in->G , in->H , in->I , in->J};
+    long data_length = (long) (sizeof(data)/sizeof(data[0]));
+    if(pos < 0)
+        return -1;
+    if(pos >= data_length)
+        return -1;
+
+    return data[pos];
+}
+
 typedef union {
     signed int A: 1, B:2, C:3, D:2;
 } Test8;

Modules/_ctypes/cfield.c

@@ -86,6 +86,7 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
 #ifndef MS_WIN32
     } else if (bitsize /* this is a bitfield request */
         && *pfield_size /* we have a bitfield open */
+        && *pfield_size != *pbitofs /* Current field has been filled, start new one */
         && dict->size * 8 >= *pfield_size
         && (*pbitofs + bitsize) <= dict->size * 8) {
         /* expand bit field */