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Commit0313970

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gh-92135: Fix _Py_reinterpret_cast() for const (#92138)
Fix C++ compiler warnings on cast macros, like _PyObject_CAST(), whencasting a constant expression to a non constant type: useconst_cast<> in C++.* In C++, Py_SAFE_DOWNCAST() now uses static_cast<> rather than reinterpret_cast<>.* Add tests to the _testcppext C++ extension.* test_cppext no longer captures stdout in verbose mode.
1 parentb11243e commit0313970

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5 files changed

+62
-14
lines changed

5 files changed

+62
-14
lines changed

‎Include/methodobject.h‎

Lines changed: 1 addition & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -43,8 +43,7 @@ typedef PyObject *(*PyCMethod)(PyObject *, PyTypeObject *, PyObject *const *,
4343
// it triggers an undefined behavior when Python calls it with 2 parameters
4444
// (bpo-33012).
4545
#define_PyCFunction_CAST(func) \
46-
_Py_reinterpret_cast(PyCFunction, \
47-
_Py_reinterpret_cast(void(*)(void), (func)))
46+
_Py_reinterpret_cast(PyCFunction, _Py_reinterpret_cast(void(*)(void), (func)))
4847

4948
PyAPI_FUNC(PyCFunction)PyCFunction_GetFunction(PyObject*);
5049
PyAPI_FUNC(PyObject*)PyCFunction_GetSelf(PyObject*);

‎Include/objimpl.h‎

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -182,9 +182,9 @@ PyAPI_FUNC(void) PyObject_GC_UnTrack(void *);
182182
PyAPI_FUNC(void)PyObject_GC_Del(void*);
183183

184184
#definePyObject_GC_New(type,typeobj) \
185-
_Py_reinterpret_cast(type*, _PyObject_GC_New(typeobj))
185+
_Py_reinterpret_cast(type*, _PyObject_GC_New(typeobj))
186186
#definePyObject_GC_NewVar(type,typeobj,n) \
187-
_Py_reinterpret_cast(type*, _PyObject_GC_NewVar((typeobj), (n)))
187+
_Py_reinterpret_cast(type*, _PyObject_GC_NewVar((typeobj), (n)))
188188

189189
PyAPI_FUNC(int)PyObject_GC_IsTracked(PyObject*);
190190
PyAPI_FUNC(int)PyObject_GC_IsFinalized(PyObject*);

‎Include/pyport.h‎

Lines changed: 15 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -14,10 +14,20 @@
1414
#endif
1515

1616

17-
// Macro to use C++ static_cast<> and reinterpret_cast<> in the Python C API
17+
// Macro to use C++ static_cast<>, reinterpret_cast<> and const_cast<>
18+
// in the Python C API.
19+
//
20+
// In C++, _Py_reinterpret_cast(type, expr) converts a constant expression to a
21+
// non constant type using const_cast<type>. For example,
22+
// _Py_reinterpret_cast(PyObject*, op) can convert a "const PyObject*" to
23+
// "PyObject*".
24+
//
25+
// The type argument must not be constant. For example, in C++,
26+
// _Py_reinterpret_cast(const PyObject*, expr) fails with a compiler error.
1827
#ifdef__cplusplus
1928
# define_Py_static_cast(type,expr) static_cast<type>(expr)
20-
# define_Py_reinterpret_cast(type,expr) reinterpret_cast<type>(expr)
29+
# define_Py_reinterpret_cast(type,expr) \
30+
const_cast<type>(reinterpret_cast<const type>(expr))
2131
#else
2232
# define_Py_static_cast(type,expr) ((type)(expr))
2333
# define_Py_reinterpret_cast(type,expr) ((type)(expr))
@@ -307,10 +317,10 @@ extern "C" {
307317
*/
308318
#ifdefPy_DEBUG
309319
# definePy_SAFE_DOWNCAST(VALUE,WIDE,NARROW) \
310-
(assert((WIDE)(NARROW)(VALUE) == (VALUE)), (NARROW)(VALUE))
320+
(assert(_Py_static_cast(WIDE, _Py_static_cast(NARROW, (VALUE))) == (VALUE)), \
321+
_Py_static_cast(NARROW, (VALUE)))
311322
#else
312-
# definePy_SAFE_DOWNCAST(VALUE,WIDE,NARROW) \
313-
_Py_reinterpret_cast(NARROW, (VALUE))
323+
# definePy_SAFE_DOWNCAST(VALUE,WIDE,NARROW) _Py_static_cast(NARROW, (VALUE))
314324
#endif
315325

316326

‎Lib/test/_testcppext.cpp‎

Lines changed: 31 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1,6 +1,9 @@
11
// gh-91321: Very basic C++ test extension to check that the Python C API is
22
// compatible with C++ and does not emit C++ compiler warnings.
33

4+
// Always enable assertions
5+
#undef NDEBUG
6+
47
#include"Python.h"
58

69
PyDoc_STRVAR(_testcppext_add_doc,
@@ -20,8 +23,36 @@ _testcppext_add(PyObject *Py_UNUSED(module), PyObject *args)
2023
}
2124

2225

26+
static PyObject *
27+
test_api_casts(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
28+
{
29+
PyObject *obj =Py_BuildValue("(ii)",1,2);
30+
if (obj ==nullptr) {
31+
returnnullptr;
32+
}
33+
34+
// gh-92138: For backward compatibility, functions of Python C API accepts
35+
// "const PyObject*". Check that using it does not emit C++ compiler
36+
// warnings.
37+
const PyObject *const_obj = obj;
38+
Py_INCREF(const_obj);
39+
Py_DECREF(const_obj);
40+
PyTypeObject *type =Py_TYPE(const_obj);
41+
assert(Py_REFCNT(const_obj) >=1);
42+
43+
assert(type == &PyTuple_Type);
44+
assert(PyTuple_GET_SIZE(const_obj) ==2);
45+
PyObject *one =PyTuple_GET_ITEM(const_obj,0);
46+
assert(PyLong_AsLong(one) ==1);
47+
48+
Py_DECREF(obj);
49+
Py_RETURN_NONE;
50+
}
51+
52+
2353
static PyMethodDef _testcppext_methods[] = {
2454
{"add", _testcppext_add, METH_VARARGS, _testcppext_add_doc},
55+
{"test_api_casts", test_api_casts, METH_NOARGS,NULL},
2556
{nullptr,nullptr,0,nullptr}/* sentinel*/
2657
};
2758

‎Lib/test/test_cppext.py‎

Lines changed: 13 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,6 @@
11
# gh-91321: Build a basic C++ test extension to check that the Python C API is
22
# compatible with C++ and does not emit C++ compiler warnings.
3+
importcontextlib
34
importos
45
importsys
56
importunittest
@@ -39,17 +40,24 @@ def build(self):
3940
sources=[SOURCE],
4041
language='c++',
4142
extra_compile_args=CPPFLAGS)
43+
capture_stdout= (notsupport.verbose)
4244

4345
try:
4446
try:
45-
with (support.captured_stdout()asstdout,
46-
support.swap_attr(sys,'argv', ['setup.py','build_ext'])):
47+
ifcapture_stdout:
48+
stdout=support.captured_stdout()
49+
else:
50+
print()
51+
stdout=contextlib.nullcontext()
52+
with (stdout,
53+
support.swap_attr(sys,'argv', ['setup.py','build_ext','--verbose'])):
4754
setup(name="_testcppext",ext_modules=[cpp_ext])
4855
return
4956
except:
50-
# Show output on error
51-
print()
52-
print(stdout.getvalue())
57+
ifcapture_stdout:
58+
# Show output on error
59+
print()
60+
print(stdout.getvalue())
5361
raise
5462
exceptSystemExit:
5563
self.fail("Build failed")

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