Array-size macro that rejects pointers

The standard array-size macro that is often taught is

#define ARRAYSIZE(arr) (sizeof(arr) / sizeof(arr[0]))

or some equivalent formation. However, this kind of thing silently succeeds when a pointer is passed in, and gives results that can seem plausible at runtime until things mysteriously fall apart.

It's all-too-easy to make this mistake: a function that has a local array variable is refactored, moving a bit of array manipulation into a new function called with the array as a parameter.

So, the question is: is there a "sanitary" macro to detect misuse of the ARRAYSIZE macro in C, preferably at compile-time? In C++ we'd just use a template specialized for array arguments only; in C, it seems we'll need some way to distinguish arrays and pointers. (If I wanted to reject arrays, for instance, I'd just do e.g. (arr=arr, ...) because array assignment is illegal).

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.

This version of ARRAYSIZE() returns 0 when arr is a pointer and the size when its a pure array

#include <stdio.h>

#define IS_INDEXABLE(arg) (sizeof(arg[0]))
#define IS_ARRAY(arg) (IS_INDEXABLE(arg) && (((void *) &arg) == ((void *) arg)))
#define ARRAYSIZE(arr) (IS_ARRAY(arr) ? (sizeof(arr) / sizeof(arr[0])) : 0)

int main(void)
{
	int a[5];
	int *b = a;
	int n = 10;
	int c[n]; /* a VLA */

	printf("%zu\n", ARRAYSIZE(a));
	printf("%zu\n", ARRAYSIZE(b));
	printf("%zu\n", ARRAYSIZE(c));
	return 0;
}

Output:

5
0
10

As pointed out by Ben Jackson, you can force a run-time exception (dividing by 0)

#define IS_INDEXABLE(arg) (sizeof(arg[0]))
#define IS_ARRAY(arg) (IS_INDEXABLE(arg) && (((void *) &arg) == ((void *) arg)))
#define ARRAYSIZE(arr) (sizeof(arr) / (IS_ARRAY(arr) ? sizeof(arr[0]) : 0))

Sadly, you can't force a compile-time error (the address of arg must be compared at run-time)

With C11, we can differentiate arrays and pointers using _Generic, but I have only found a way to do it if you supply the element type:

#define ARRAY_SIZE(A, T) \
    _Generic(&(A), \
            T **: (void)0, \
            default: _Generic(&(A)[0], T *: sizeof(A) / sizeof((A)[0])))


int a[2];
printf("%zu\n", ARRAY_SIZE(a, int));

The macro checks: 1) pointer-to-A is not pointer-to-pointer. 2) pointer-to-elem is pointer-to-T. It evaluates to (void)0 and fails statically with pointers.

It's an imperfect answer, but maybe a reader can improve upon it and get rid of that type parameter!

Modification of bluss's answer using typeof instead of a type parameter:

#define ARRAY_SIZE(A) \
    _Generic(&(A), \
    typeof((A)[0]) **: (void)0, \
    default: sizeof(A) / sizeof((A)[0]))

Here's one possible solution using a GNU extension called statement expressions:

#define ARRAYSIZE(arr) \
    ({typedef char ARRAYSIZE_CANT_BE_USED_ON_POINTERS[sizeof(arr) == sizeof(void*) ? -1 : 1]; \
     sizeof(arr) / sizeof((arr)[0]);})

This uses a static assertion to assert that sizeof(arr) != sizeof(void*). This has an obvious limitation -- you can't use this macro on arrays whose size happens to be exactly one pointer (e.g. a 1-length array of pointers/integers, or maybe a 4-length array of bytes on a 32-bit platform). But those particular instances can be worked around easily enough.

This solution is not portable to platforms which don't support this GNU extension. In those cases, I'd recommend just using the standard macro and not worry about accidentally passing in pointers to the macro.

Here's another one which relies on the [tag:gcc] [typeof extension][1]:

#define ARRAYSIZE(arr) ({typeof (arr) arr ## _is_a_pointer __attribute__((unused)) = {}; \
                         sizeof(arr) / sizeof(arr[0]);})

This works by attempting to set up an identical object and initializing it with an array designated initializer. If an array is passed, then the compiler is happy. If pointer is passed the compiler complains with:

arraysize.c: In function 'main':
arraysize.c:11: error: array index in non-array initializer
arraysize.c:11: error: (near initialization for 'p_is_a_pointer')

[1]: http://gcc.gnu.org/onlinedocs/gcc/Typeof.html "typeof extension"

One more example to the collection.

#define LENGTHOF(X) ({ \
    const size_t length = (sizeof X / (sizeof X[0] ?: 1)); \
    typeof(X[0]) (*should_be_an_array)[length] = &X; \
    length; })

Pros:

1. It works with normal arrays, variable-length arrays, multidimensional arrays, arrays of zero sized structs
2. It generates a compilation error (not warning) if you pass any pointer, struct or union
3. It does not depend on any of C11's features
4. It gives you very readable error

Cons:

1. It depends on some of the gcc extensions: Typeof, Statement Exprs, and (if you like it) Conditionals
2. It depends on C99 VLA feature

Awful, yes, but that works and it is portable.

#define ARRAYSIZE(arr) ((sizeof(arr) != sizeof(&arr[0])) ? \
                       (sizeof(arr)/sizeof(*arr)) : \
                       -1+0*fprintf(stderr, "\n\n** pointer in ARRAYSIZE at line %d !! **\n\n", __LINE__))

This will not detect anything at compile time but will print out an error message in stderr and return -1 if it is a pointer or if the array length is 1.

==> DEMO <==

my personal favorite, tried gcc 4.6.3 and 4.9.2:

#define STR_(tokens) # tokens

#define ARRAY_SIZE(array) \
	({ \
		_Static_assert \
		( \
			! __builtin_types_compatible_p(typeof(array), typeof(& array[0])), \
			"ARRAY_SIZE: " STR_(array) " [expanded from: " # array "] is not an array" \
		); \
		sizeof(array) / sizeof((array)[0]); \
	})

/*
 * example
 */

#define not_an_array ((char const *) "not an array")

int main () {
    return ARRAY_SIZE(not_an_array);
}

compiler prints

x.c:16:12: error: static assertion failed: "ARRAY_SIZE: ((char const *) \"not an array\") [expanded from: not_an_array] is not an array"