Array-size macro that rejects pointers

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Linux kernel uses a nice implementation of ARRAY_SIZE to deal with this issue:

#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))

with

#define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))

and 

#define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))

Of course this is portable only in GNU C as it makes use of two instrinsics:
typeof operator and __builtin_types_compatible_p function. Also it uses their “famous” BUILD_BUG_ON_ZERO macro which is only valid in GNU C.

Assuming a compile time evaluation requirement (which is what we want), I don’t know any portable implementation of this macro.

A “semi-portable” implementation (and which would not cover all cases) is:

#define ARRAY_SIZE(arr)  \
    (sizeof(arr) / sizeof((arr)[0]) + STATIC_EXP(IS_ARRAY(arr)))

with

#define IS_ARRAY(arr)  ((void*)&(arr) == &(arr)[0])
#define STATIC_EXP(e)  \
    (0 * sizeof (struct { int ARRAY_SIZE_FAILED:(2 * (e) - 1);}))

With gcc this gives no warning if argument is an array in -std=c99 -Wall but -pedantic would gives a warning. The reason is IS_ARRAY expression is not an integer constant expression (cast to pointer types and subscript operator are not allowed in integer constant expressions) and the bit-field width in STATIC_EXP requires an integer constant expression.

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