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Re: [libvirt] [PATCH v2] python: add virDomainGetCPUStats python binding API



On 03/18/2012 04:41 AM, Guannan Ren wrote:

>>> +    if (totalbool == Py_False) {
>> Per other code in libvirt-override.c, you can't compare totalbool (type
>> PyObject) with Py_False, at least not on all compilers.  You need
>> something like this instead:
>>
>>              /* Hack - Python's definition of Py_True breaks strict
>>               * aliasing rules, so can't directly compare
>>               */
>>              if (PyBool_Check(value)) {
>>                  PyObject *hacktrue = PyBool_FromLong(1);
>>                  temp->value.b = hacktrue == value ? 1 : 0;
>>                  Py_DECREF(hacktrue);
> 
>      Yes, it did report warning in compiling as follows due to the case
> from PyIntObject* to PyObject*
>      warning :dereferencing type-punned pointer might break
> strict-aliasing rules [-Wstrict-aliasing]

And that would trip up a -Werror compilation, so I'm glad to see you
changed it in v3.

> 
>      GCC command line to reproduce the error:
>      gcc -Wstrict-aliasing=1 -O2 cpythonexample.c
> 
>      Actually PyObject_IsTrue() is a more light-weight approach to do
> the checking instead of
>      creating a intermediate PyObject * for the compare.

Is PyObject_IsTrue() available in the version of python present on RHEL
5?  If so, I'd be in favor of a followup cleanup patch that removes all
our hacks in favor of the python glue code that does the same thing.
And even if not, we should write a decent wrapper in our own
typewrappers.c, so that the rest of our code doesn't have to look so
ugly with so much copy-and-paste.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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