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

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> 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.