GeezerVison is showing github.com — pyatomic_gcc.h with GCC discards const qualifier · Issue #120593 · python/cpython
pyatomic_gcc.h with GCC discards const qualifier #120593DescriptionBug report
Bug description:
Hi,
including Python.h with gcc and -Wcast-qual raises the following issue (Regression from 3.12):
/usr/include/python3.13/cpython/pyatomic_gcc.h: In function '_Py_atomic_load_ptr':
/usr/include/python3.13/cpython/pyatomic_gcc.h:300:34: error: cast discards 'const' qualifier from pointer target type [-Werror=cast-qual]
300 | { return (void *)__atomic_load_n((void **)obj, __ATOMIC_SEQ_CST); }
| ^
/usr/include/python3.13/cpython/pyatomic_gcc.h: In function '_Py_atomic_load_ptr_relaxed':
/usr/include/python3.13/cpython/pyatomic_gcc.h:359:34: error: cast discards 'const' qualifier from pointer target xt (line 8))
$ python3.13 -V
Python 3.13.0b1 This is a follow-up to #120293
CPython versions tested on:
3.13
Operating systems tested on:
Linux
Linked PRs
Activitypicnixz commented on Jun 17, 2024 For the record, this is not the only place where const qualifiers are lost. If you compile cpython with make CFLAGS=-Wcast-qual -j12 you end up with a lot of warnings. For the line 359, the fix would be:
// pyatomic_gcc.h
static inline void *
_Py_atomic_load_ptr_relaxed(const void *obj)
-{ return (void *)__atomic_load_n((void **)obj, __ATOMIC_RELAXED); }
+{ return (void *)__atomic_load_n((void * const *)obj, __ATOMIC_RELAXED); }As you can see you lose constness twice, namely when calling __atomic_load_n and when returning the type. Note that we have
// pyatomic_std.h
static inline void *
_Py_atomic_load_ptr_relaxed(const void *obj)
{
_Py_USING_STD;
return atomic_load_explicit((const _Atomic(void*)*)obj,
memory_order_relaxed);
}As you can see, the cast here is essentially using a const void **, but still the return type should be a const as well (in the case of __atomic_load_n, the signature is type __atomic_load_n(type *ptr, int memorder);).
EDIT: I edited my comment in order to reflect the real signature of _Py_atomic_load_ptr_relaxed in cpython. The return type is a void * and not a const void * but the inner cast fix remains correct. ndparker commented on Jun 17, 2024 ContributorAuthor More actions Yes, I'd like to clarify that this is specifically not about compiling CPython, it's about including Python.h only. picnixz commented on Jun 18, 2024 Yes, I understood that from the other issue but since you only mentioned 2 lines, I wanted to 1) give a way to fix it 2) say that it's not just 2 lines that are affected but a lot of calls. changed the title [-]Python.h with GCC discards const qualifier [/-] [+]pyatomic_gcc.h with GCC discards const qualifier [/+] on Jun 26, 2024vstinner commented on Jun 27, 2024 The warnings were fixed by 9cd2dcb and e51e880.
I also added a check in test_cext for check for non-regression: b7a95df.
Thanks @ndparker for your bug report. It's now fixed, I close the issue. markshannon commented on Jul 15, 2024 I don't think weakening the type of calls that should be const is the correct thing to do here.
Where there is a conflict in types we should be strengthening the type, not weakening it.
Why not strengthen the types in the callees, rather than weakening them in the callers? vstinner commented on Jul 27, 2024 @markshannon:
I don't think weakening the type of calls that should be const is the correct thing to do here.
If you're talking about _PyLong_CompactValue(): I wrote PR gh-122367 to restore the const modifier.
If you're talking about something else, please elaborate which function/code you are referring to.
MetadataMetadataDevelopmentNo branches or pull requestsIssue actions
|