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gh-156933: Widen narrow integer results in ctypes callbacks #157045

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Merged
ZeroIntensity merged 6 commits into from
Sep 27, 2026
Merged

gh-156933: Widen narrow integer results in ctypes callbacks #157045

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72a218b
gh-156933: Widen narrow integer results in ctypes callbacks
lazerg Sep 6, 2026
ebf7568
Add test for widened narrow int callback results
lazerg Sep 25, 2026
9338911
Only run the widening test on s390x
lazerg Sep 25, 2026
d54ca1f
Address review comments
lazerg Sep 27, 2026
6490b5f
Use wrap_dll_function in the widening test
lazerg Sep 27, 2026
5693c85
Merge branch 'main' into fix/issue-156933-ctypes-callback-widen
ZeroIntensity Sep 27, 2026
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10 changes: 10 additions & 0 deletions Lib/test/test_ctypes/test_callbacks.py
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Original file line number Diff line number Diff line change
Expand Up @@ -328,6 +328,16 @@ def func():
f"of ctypes callback function {func!r}")
self.assertIsNone(cm.unraisable.object)

@unittest.skipUnless(support.is_s390x, 's390x only test')
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def test_narrow_int_return_widened(self):
@CFUNCTYPE(c_int)
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def cb():
return -1

addr = ctypes.cast(cb, ctypes.c_void_p).value
wide = CFUNCTYPE(c_longlong)(addr)
self.assertEqual(wide(), -1)
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if __name__ == '__main__':
unittest.main()
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-09-07-00-21-31.gh-issue-156933.OalCjC.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix :mod:`ctypes` callbacks returning an integer narrower than a machine
register: the result is now widened to fill the register, as libffi's
closure contract requires.
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59 changes: 51 additions & 8 deletions Modules/_ctypes/callbacks.c
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Original file line number Diff line number Diff line change
Expand Up @@ -101,6 +101,22 @@ TryAddRef(PyObject *cnv, CDataObject *obj)
}
#endif

static int
is_narrow_int_ffi_type(int type)
{
switch (type) {
case FFI_TYPE_SINT8:
case FFI_TYPE_UINT8:
case FFI_TYPE_SINT16:
case FFI_TYPE_UINT16:
case FFI_TYPE_SINT32:
case FFI_TYPE_UINT32:
return 1;
default:
return 0;
}
}

/******************************************************************************
*
* Call the python object with all arguments
Expand Down Expand Up @@ -222,13 +238,14 @@ static void _CallPythonObject(ctypes_state *st,
if (restype != &ffi_type_void && result) {
assert(setfunc);

#ifdef WORDS_BIGENDIAN
/* See the corresponding code in _ctypes_callproc():
in callproc.c, around line 1219. */
if (restype->type != FFI_TYPE_FLOAT && restype->size < sizeof(ffi_arg)) {
mem = (char *)mem + sizeof(ffi_arg) - restype->size;
}
#endif
/* libffi's closure contract requires integral results narrower
than ffi_arg to fill a whole register, sign-extended if signed;
setfunc() only writes restype->size bytes. Cf. _ctypes_callproc()
in callproc.c. */
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ffi_arg widened = 0;
int narrow = restype->size < sizeof(ffi_arg) &&
is_narrow_int_ffi_type(restype->type);
void *resmem = narrow ? &widened : mem;

/* keep is an object we have to keep alive so that the result
stays valid. If there is no such object, the setfunc will
Expand All @@ -239,7 +256,33 @@ static void _CallPythonObject(ctypes_state *st,
be the result. EXCEPT when restype is py_object - Python
itself knows how to manage the refcount of these objects.
*/
PyObject *keep = setfunc(mem, result, restype->size);
PyObject *keep = setfunc(resmem, result, restype->size);

if (narrow && keep != NULL) {
switch (restype->type) {
case FFI_TYPE_SINT8:
widened = (ffi_arg)(ffi_sarg)*(int8_t *)&widened;
break;
case FFI_TYPE_SINT16:
widened = (ffi_arg)(ffi_sarg)*(int16_t *)&widened;
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break;
case FFI_TYPE_SINT32:
widened = (ffi_arg)(ffi_sarg)*(int32_t *)&widened;
break;
case FFI_TYPE_UINT8:
widened = *(uint8_t *)&widened;
break;
case FFI_TYPE_UINT16:
widened = *(uint16_t *)&widened;
break;
case FFI_TYPE_UINT32:
widened = *(uint32_t *)&widened;
break;
default:
break;
}
memcpy(mem, &widened, sizeof(ffi_arg));
}

if (keep == NULL) {
/* Could not convert callback result. */
Expand Down
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