C find static array size (preventing mistakes) [duplicate]

Question

Finding the size of a static array is a common operation. see: C find static array size - sizeof(a) / sizeof((a)[0])

This can be wrapped into a macro, eg:

#define ARRAY_SIZE(a) (sizeof(a) / sizeof((a)[0]))

however its possible to accidentally pass in a regular pointer.

eg: void func(SomeArray **foo) { int i = ARRAY_SIZE(foo); }

While its valid C, but often ends up being a logical error.

Its possible to prevent this mistake (taking advantage of per-processor to fail on a zero length bit-field).

---

#define ARRAY_SIZE(a) \
    ((sizeof(struct { int isnt_array : \
     ((const void *)&(a) == &(a)[0]); }) * 0) + \
     (sizeof(a) / sizeof(*(a))))

I've found this macro works with GCC, but fails with Clang, for indirectly referenced members. with error: expression is not an integer constant expression

eg:

• char word[8]; int i = ARRAY_SIZE(word); ok.
• struct Bar { word[8]; }
void func(struct Bar *foo) { int i = ARRAY_SIZE(foo->word); } fails.

Is there a more portable way to to implement this? (working with Clang is good of course though Im interested in general portability... other compilers too).

This seems such a common task that it would be good to have a re-usable portable macro.

Answer 1

Try this:

#define ASSERT_ARRAY(a) \
    sizeof(char[1-2*__builtin_types_compatible_p(__typeof__(a), __typeof__(&(a)[0]))])

#define ARRAY_SIZE(a) \
    (ASSERT_ARRAY(a)*0 + sizeof(a)/sizeof((a)[0]))

It is not portable, but works with both gcc and clang and has fewer side effects than n.m.'s proposal.