gh-126937: ctypes: fix TypeError when a field's size is >65535 bytes

Lib/test/test_ctypes/test_struct_fields.py

@@ -1,4 +1,5 @@
 import unittest
+import sys
 from ctypes import Structure, Union, sizeof, c_char, c_int
 from ._support import (CField, Py_TPFLAGS_DISALLOW_INSTANTIATION,
  Py_TPFLAGS_IMMUTABLETYPE)
@@ -75,6 +76,28 @@ def __init_subclass__(cls, **kwargs):
  'ctypes state is not initialized'):
  class Subclass(BrokenStructure): ...
 
+ def test_max_field_size_gh126937(self):
+ # Classes for big structs should be created successfully.
+ # (But they most likely can't be instantiated.)
+ # Here we test the exact limit: the number of *bits* must fit
+ # in Py_ssize_t.
+
+ class X(self.cls):
+ _fields_ = [('char', c_char),]
+ max_field_size = sys.maxsize // 8
+
+ class Y(self.cls):
+ _fields_ = [('largeField', X * max_field_size)]
+ class Z(self.cls):
+ _fields_ = [('largeField', c_char * max_field_size)]
+
+ with self.assertRaises(ValueError):
+ class TooBig(self.cls):
+ _fields_ = [('largeField', X * (max_field_size + 1))]
+ with self.assertRaises(ValueError):
+ class TooBig(self.cls):
+ _fields_ = [('largeField', c_char * (max_field_size + 1))]
+
  # __set__ and __get__ should raise a TypeError in case their self
  # argument is not a ctype instance.
  def test___set__(self):

Misc/NEWS.d/next/Core_and_Builtins/2024-11-17-21-35-55.gh-issue-126937.qluVM0.rst

@@ -0,0 +1,3 @@
+Fix :exc:`TypeError` when a :class:`ctypes.Structure` has a field size
+that doesn't fit into an unsigned 16-bit integer.
+Instead, the maximum number of *bits* is :data:`sys.maxsize`.

Modules/_ctypes/cfield.c

@@ -110,10 +110,16 @@ PyCField_new_impl(PyTypeObject *type, PyObject *name, PyObject *proto,
  goto error;
  }
 
- Py_ssize_t bit_size = NUM_BITS(size);
- if (bit_size){
+ if (bit_size_obj != Py_None){
+#ifdef Py_DEBUG
+ Py_ssize_t bit_size = NUM_BITS(size);
  assert(bit_size > 0);
  assert(bit_size <= info->size * 8);
+ // Currently, the bit size is specified redundantly
+ // in NUM_BITS(size) and bit_size_obj.
+ // Verify that they match.
+ assert(PyLong_AsSsize_t(bit_size_obj) == bit_size);
+#endif
  switch(info->ffi_type_pointer.type){
  case FFI_TYPE_UINT8:
  case FFI_TYPE_UINT16:

Modules/_ctypes/stgdict.c

@@ -292,7 +292,7 @@ PyCStructUnionType_update_stginfo(PyObject *type, PyObject *fields, int isStruct
  if (!tmp){
  goto error;
  }
- Py_ssize_t total_align = PyLong_AsInt(tmp);
+ Py_ssize_t total_align = PyLong_AsSsize_t(tmp);
  Py_DECREF(tmp);
  if (total_align < 0){
  if (!PyErr_Occurred()){
@@ -306,7 +306,7 @@ PyCStructUnionType_update_stginfo(PyObject *type, PyObject *fields, int isStruct
  if (!tmp){
  goto error;
  }
- Py_ssize_t total_size = PyLong_AsInt(tmp);
+ Py_ssize_t total_size = PyLong_AsSsize_t(tmp);
  Py_DECREF(tmp);
  if (total_size < 0){
  if (!PyErr_Occurred()){