gh-132470: Prevent crash in ctypes.CField when byte_size is incorrect

Lib/test/test_ctypes/test_struct_fields.py

@@ -1,6 +1,6 @@
 import unittest
 import sys
-from ctypes import Structure, Union, sizeof, c_char, c_int, CField
+from ctypes import Structure, Union, sizeof, c_byte, c_char, c_int, CField
 from ._support import Py_TPFLAGS_IMMUTABLETYPE, StructCheckMixin
 
 
@@ -75,6 +75,17 @@ def __init_subclass__(cls, **kwargs):
                                     'ctypes state is not initialized'):
             class Subclass(BrokenStructure): ...
 
+    def test_invalid_byte_size_raises_gh132470(self):
+        with self.assertRaisesRegex(ValueError, r"does not match type size"):
+            CField(
+                name="a",
+                type=c_byte,
+                byte_size=2,  # Wrong size: c_byte is only 1 byte
+                byte_offset=2,
+                index=1,
+                _internal_use=True
+            )
+
     def test_max_field_size_gh126937(self):
         # Classes for big structs should be created successfully.
         # (But they most likely can't be instantiated.)

Misc/NEWS.d/next/C_API/2025-04-13-20-52-39.gh-issue-132470.UqBQjN.rst

@@ -0,0 +1,2 @@
+Creating a :class:`ctypes.CField` with a *byte_size* that does not match the actual
+type size now raises a :exc:`ValueError` instead of crashing the interpreter.

Modules/_ctypes/cfield.c

@@ -99,7 +99,12 @@ PyCField_new_impl(PyTypeObject *type, PyObject *name, PyObject *proto,
                      "type of field %R must be a C type", name);
         goto error;
     }
-    assert(byte_size == info->size);
+    if (byte_size != info->size) {
+        PyErr_Format(PyExc_ValueError,
+                     "byte size of field %R (%zd) does not match type size (%zd)",
+                     name, byte_size, info->size);
+        goto error;
+    }
 
     Py_ssize_t bitfield_size = 0;
     Py_ssize_t bit_offset = 0;