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Re: [Qemu-devel] [PATCH v3 4/4] ARRAY_SIZE: check that argument is an ar

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v3 4/4] ARRAY_SIZE: check that argument is an array
Date: Thu, 19 Jan 2017 17:00:52 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0

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.

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

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