gh-144100: Fix crash for POINTER(str) used in ctypes argtypes

Lib/test/test_ctypes/test_pointers.py

@@ -3,6 +3,7 @@
 import gc
 import sys
 import unittest
+import os
 from ctypes import (CDLL, CFUNCTYPE, Structure,
                     POINTER, pointer, _Pointer,
                     byref, sizeof,
@@ -472,6 +473,23 @@ class C(Structure):
         ptr.set_type(c_int)
         self.assertIs(ptr._type_, c_int)
 
+class TestPointerStringProto(unittest.TestCase):
+    def test_pointer_string_proto_argtypes_error(self):
+        with self.assertWarns(DeprecationWarning):
+            BadType = ctypes.POINTER("BugTrigger")
+
+        if os.name == "nt":
+            libc = ctypes.WinDLL("kernel32.dll")
+            func = libc.GetCurrentProcessId
+        else:
+            libc = ctypes.CDLL(None)
+            func = libc.getpid
+
+        func.argtypes = (BadType,)
+
+        with self.assertRaises(ctypes.ArgumentError):
+            func(ctypes.byref(ctypes.c_int(0)))
+
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Library/2026-01-21-19-39-07.gh-issue-144100.hLMZ8Y.rst

@@ -0,0 +1,3 @@
+Fixed a crash in ctypes when using a deprecated ``POINTER(str)`` type in
+``argtypes``. Instead of aborting, ctypes now raises a proper Python
+exception when the pointer target type is unresolved.

Modules/_ctypes/_ctypes.c

@@ -1419,7 +1419,13 @@ PyCPointerType_from_param_impl(PyObject *type, PyTypeObject *cls,
     /* If we expect POINTER(<type>), but receive a <type> instance, accept
        it by calling byref(<type>).
     */
-    assert(typeinfo->proto);
+    if(typeinfo->proto == NULL){
+        PyErr_SetString(
+            PyExc_TypeError,
+            "cannot convert argument: POINTER target type is unresolved"
+        );
+        return NULL;
+    }
     switch (PyObject_IsInstance(value, typeinfo->proto)) {
     case 1:
         Py_INCREF(value); /* _byref steals a refcount */