On Tue, Apr 14, 2015 at 4:48 PM, Russell Bryant <rbry...@redhat.com> wrote:
> On 04/14/2015 06:56 PM, Ben Pfaff wrote:
>> On Tue, Apr 14, 2015 at 03:16:22PM -0700, Andy Zhou wrote:
>>> On Tue, Apr 14, 2015 at 2:55 PM, Russell Bryant <rbry...@redhat.com> wrote:
>>>> On 04/14/2015 05:51 PM, Andy Zhou wrote:
>>>>> Gcc complains about:
>>>>> lib/perf-counter.c:43:13: error: ignoring return value of 'read',
>>>>> declared with attribute warn_unused_result [-Werror=unused-result]
>>>>> read(fd__, counter, sizeof(*counter));
>>>>>
>>>>> Signed-off-by: Andy Zhou <az...@nicira.com>
>>>>>
>>>>> ---
>>>>> v1->v2: Combine two 'if's into using only one.
>>>>> ---
>>>>> lib/perf-counter.c | 6 +++---
>>>>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/lib/perf-counter.c b/lib/perf-counter.c
>>>>> index 7bd7834..a6e63ac 100644
>>>>> --- a/lib/perf-counter.c
>>>>> +++ b/lib/perf-counter.c
>>>>> @@ -39,9 +39,9 @@ static int fd__ = 0;
>>>>> uint64_t
>>>>> perf_counter_read(uint64_t *counter)
>>>>> {
>>>>> - if (fd__ > 0) {
>>>>> - read(fd__, counter, sizeof(*counter));
>>>>> - } else {
>>>>> + int size = sizeof *counter;
>>>>
>>>> Minor nit ... this should be size_t instead of int.
>>>>
>>> Thanks, I wlll make the suggested changes.
>>
>> I'm pretty sure that using size_t instead of int will actually bust
>> this. read returns ssize_t, a signed type, and C says that in an
>> expression like "ssize_t < size_t" the left-hand operand gets
>> converted to size_t. Thus, if "read" returns -1, it gets converted to
>> SIZE_MAX, which is not less than sizeof *counter.
>>
>> If I'm correct, then size should be declared as int or ssize_t.
>>
>
> Oops, thanks for catching that, Ben.
There is very little in C more subtle than the integer promotion rules.
I worry that we get these wrong in important ways.
--
"I don't normally do acked-by's. I think it's my way of avoiding
getting blamed when it all blows up." Andrew Morton
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