Re: strengthening ARRAY_CARDINALITY

[Mike Frysinger]

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

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