Re: [Qemu-devel] [RFC PATCH V4 2/5] qapi: add event helper functions

[Wenchao Xia <wenchaoqemu@gmail.com>]

于 2014/3/28 16:21, Markus Armbruster 写道:
> Eric Blake <eblake@redhat.com> writes:
> 
>> On 03/27/2014 02:03 AM, Wenchao Xia wrote:
>>> This file holds some functions that do not need to be generated.
>>>
>>> Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
>>> ---
>>>   include/qapi/qmp-event.h |   27 +++++++++++++++++
>>>   qapi/Makefile.objs       |    1 +
>>>   qapi/qmp-event.c         |   70 ++++++++++++++++++++++++++++++++++++++++++++++
>>>   3 files changed, 98 insertions(+), 0 deletions(-)
>>>   create mode 100644 include/qapi/qmp-event.h
>>>   create mode 100644 qapi/qmp-event.c
>>>
>>
>>> +    err = qemu_gettimeofday(&tv);
>>> +    if (err < 0) {
>>> +        /* Put -1 to indicate failure of getting host time */
>>> +        tv.tv_sec = -1;
>>> +        tv.tv_usec = -1;
>>
>> You fixed the problem with C promotion here, but...
>>
>>> +    }
>>> +
>>> +    obj = qobject_from_jsonf("{ 'seconds': %" PRId64 ", "
>>> +                             "'microseconds': %" PRId64 " }",
>>> +                             (int64_t) tv.tv_sec, (int64_t) tv.tv_usec);
>>
>> ...here, C promotion rules bite once again :(  If tv_usec is uint32_t,
>> then it zero-extends rather than sign-extends into int64_t, and you may
>> end up with 0xffffffff instead of the intended -1.  When doing
>> potentially widening casts, it is only safe if you know the signedness
>> of the pre-cast value; but with struct timeval, POSIX doesn't make that
>> easy.
>>
>> Maybe it's easier to just rewrite things with known types:
>>
>> int64_t sec;
>> int usec;
>> qemu_timval tv;
>> err = qemu_gettimeofday(&tv);
>> if (err < 0) {
>>      sec = -1;
>>      usec = -1;
>> } else {
>>      sec = tv.tv_sec;
>>      usec = tv.tv_usec;
>> }
>> qobject_from_jsonf("... %"PRId64 ", ...%d", sec, usec)
>>
>> since 'int' is guaranteed to be large enough for all the usec values we
>> care about on all platforms we compile on (that is, we require 32-bit
>> int, even if C allows for a 16-bit int implementation).
> 
> If you go that route, then why not go all the way nad make both sec and
> usec int64_t?  Look ma, no type casts!
> 
  Yep, in this way code looks better... I will respin it later with a
full conversion.