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Re: [Qemu-devel] [PATCH v3 4/4] ARRAY_SIZE: check that argument is an ar
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH v3 4/4] ARRAY_SIZE: check that argument is an array |
Date: |
Fri, 20 Jan 2017 08:34:57 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) |
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.
- 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