Re: strengthening ARRAY_CARDINALITY

[Jim Meyering]

On Thu, Jan 19, 2017 at 5:06 PM, Mike Frysinger <address@hidden> wrote:
> On 19 Jan 2017 22:20, Pádraig Brady wrote:
>> On 19/01/17 21:52, Eric Blake wrote:
>> > On the qemu list, it was pointed out that code that uses
>> > ARRAY_CARDINALITY() might still compile even after it has been
>> > refactored to use a pointer (probably conversion of an array into
>> > dynamic allocation), but that you can add a compile-time check with
>> > new-enough gcc/clang to catch this.
>> >
>> > I'm also wondering if we should promote ARRAY_CARDINALITY into a
>> > full-fledged gnulib module (several gnulib files define it in .c files,
>> > but leave projects to re-define their own; coreutils' is in system.h).
>> >
>> > The qemu list spells their macro ARRAY_SIZE, and
>> > QEMU_BUILD_BUG_ON_ZERO(x) is equivalent to our verify_expr(!x, 0), but
>> > I'm wondering if we should similarly strengthen coreutils' macro (with
>> > appropriate guards for new-enough gcc, since we target more compilers
>> > than qemu):
>> >
>> > https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg04118.html
>> >
>> > +/*
>> > + * &(x)[0] is always a pointer - if it's same type as x then the
>> > argument is a
>> > + * pointer, not an array.
>> > + */
>> > +#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)))
>> >  #endif
>>
>> gnulib module + extra checks + name change to ARRAY_SIZE sound good!
>> For a convenience feature, ARRAY_CARDINALITY is not a convenient name.
>
> yeah, "cardinality" sounds more like someone trying to show off their
> familiarity with the english language ;).  then again, my spell checker
> says it's not a word.
>
> linux, glibc, bootloaders, use ARRAY_SIZE.  types use "size" -- it's
> size_t, not cardinality_t.  this is the first time i've seen this name
> used myself.
> -mike

FYI, I chose "cardinality" over "size" quite deliberately: the former
refers to the number of things (typically in a set), and while "size"
may also refer to that number, it is often used to refer to the number
of bytes, which is usually a larger number.