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Re: [Qemu-devel] [PATCH v3 4/4] ARRAY_SIZE: check that argument is an ar
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH v3 4/4] ARRAY_SIZE: check that argument is an array |
Date: |
Fri, 20 Jan 2017 13:20:47 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1 |
On 20/01/2017 08:34, Markus Armbruster wrote:
> Eric Blake <address@hidden> writes:
>
>> On 01/19/2017 04:11 PM, Michael S. Tsirkin wrote:
>>
>>>>> +#define QEMU_IS_ARRAY(x) (!__builtin_types_compatible_p(typeof(x), \
>>>>> + typeof(&(x)[0])))
>>>>> #ifndef ARRAY_SIZE
>>>>> -#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
>>>>> +#define ARRAY_SIZE(x) ((sizeof(x) / sizeof((x)[0])) + \
>>>>> + QEMU_BUILD_BUG_ON_ZERO(!QEMU_IS_ARRAY(x)))
>>>>
>>>> We've got some double-negation going on here ("cause a build bug if the
>>>> negation of QEMU_IS_ARRAY() is not 0") which takes some mental
>>>> gymnastics, but it is the correct result. [I kind of like that gnulib
>>>> uses positive logic in its 'verify(x)' meaning "verify that x is true,
>>>> or cause a build error"; compared to the negative logic in the kernal
>>>> 'BUILD_BUG_ON[_ZERO](x)' meaning "cause a build bug if x is non-zero" -
>>>> but that's personal preference and not something for qemu to change]
>>>
>>> I can rename QEMU_IS_ARRAY to QEMU_IS_PTR and reverse the logic - would
>>> this be preferable?
>>
>> No, that's worse. As written, "cause a build bug if x is not an array"
>> is easier than "cause a build bug if x is a pointer", because now you
>> are missing an implicit "(instead of the intended array)". Keep it the
>> way you have it. I guess it's the _ZERO as a suffix that's throwing me;
>> a better name might have been QEMU_ZERO_OR_BUILD_BUG_ON(x) ("give me a
>> zero expression, or a build bug if x is non-zero") rather than
>> QEMU_BUILD_BUG_ON_ZERO (my first read was "give me a build bug if x is
>> zero", but a better read is "give me a build bug if x is not zero, else
>> give me x because it is zero") - but our choice of naming in patch 3/4
>> mirrors the kernel naming, so it's not worth changing.
>
> Two ways to skin the assertion cat:
>
> assert must_be_true
> bug_on must_be_false
>
> The C language picks the first one, both with assert() and with C11's
> _Static_assert(). I'd prefer we stick to that, but I'm not asking you
> to change your series.
We should probably change it to QEMU_STATIC_ASSERT and
QEMU_STATIC_ASSERT_VALUE, but that shouldn't be in this series.
Paolo
- Re: [Qemu-devel] [PATCH v3 2/4] compiler: rework BUG_ON using a struct, (continued)
[Qemu-devel] [PATCH v3 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Michael S. Tsirkin, 2017/01/19
[Qemu-devel] [PATCH v3 4/4] ARRAY_SIZE: check that argument is an array, Michael S. Tsirkin, 2017/01/19
Re: [Qemu-devel] [PATCH v3 0/4] ARRAY_SIZE fixups, Markus Armbruster, 2017/01/20
Re: [Qemu-devel] [PATCH v3 0/4] ARRAY_SIZE fixups, no-reply, 2017/01/20
Re: [Qemu-devel] [PATCH v3 0/4] ARRAY_SIZE fixups, no-reply, 2017/01/20