Abusing macros for typechecking

One commonly used macro in C programming is ASIZE(), generally defined as something like this

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

and used to calculate the number of elements in an array.

The main problem with this macro, as written, is that it doesn’t distinguish between arrays and pointers. If passed a pointer, it will silently produce wrong results:

Code

#include <stdio.h>

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

int main(void)
{
	short a[3];
	short *b;
	int c[2];
	int *d;
	long long e[5][4];
	char *f[4];
	char (*g)[4];
	(void)a; (void)b; (void)c; (void)d; (void)e; (void)f; (void)g;
	printf("ASIZE() accepts pointers, producing invalid results.\n");
	printf("%zu\n", ASIZE( a ));
	printf("%zu\n", ASIZE( b ));
	printf("%zu\n", ASIZE( c ));
	printf("%zu\n", ASIZE( d ));
	printf("%zu\n", ASIZE( e ));
	printf("%zu\n", ASIZE( f ));
	printf("%zu\n", ASIZE( g ));
	return 0;
}

Output

3
2
2
1
5
4
1

By adding a new macro checking if the parameter is an array, we can define a safer ASIZE():

#define CHECK_ARRAY(a) ((void)(0&&((int (*)(__typeof__(a[0])(*)[ASIZE(a)]))NULL)(&(a))))
#define ASIZE_SAFE(a) (CHECK_ARRAY(a), ASIZE(a))

Checking this new version, we see it gets the correct results when passed arrays, but now the compilation fails when applied to pointers:

Code

#include <stdio.h>

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

#define CHECK_ARRAY(a) ((void)(0&&((int (*)(__typeof__(a[0])(*)[ASIZE(a)]))NULL)(&(a))))

#define ASIZE_SAFE(a) (CHECK_ARRAY(a), ASIZE(a))

int main(void)
{
	short a[3];
	short *b;
	int c[2];
	int *d;
	long long e[5][4];
	char *f[4];
	char (*g)[4];
	(void)a; (void)b; (void)c; (void)d; (void)e; (void)f; (void)g;
	printf("ASIZE() accepts pointers, producing invalid results.\n");
	printf("%zu\n", ASIZE( a ));
	printf("%zu\n", ASIZE( b ));
	printf("%zu\n", ASIZE( c ));
	printf("%zu\n", ASIZE( d ));
	printf("%zu\n", ASIZE( e ));
	printf("%zu\n", ASIZE( f ));
	printf("%zu\n", ASIZE( g ));
	printf("ASIZE_SAFE() only accepts arrays (try uncommenting).\n");
	printf("%zu\n", ASIZE_SAFE( a ));
	//printf("%zu\n", ASIZE_SAFE( b ));
	printf("%zu\n", ASIZE_SAFE( c ));
	//printf("%zu\n", ASIZE_SAFE( d ));
	printf("%zu\n", ASIZE_SAFE( e ));
	//printf("%zu\n", ASIZE_SAFE( f ));
	//printf("%zu\n", ASIZE_SAFE( g ));
	return 0;
}

Output

ASIZE() accepts pointers, producing invalid results.
3
2
2
1
5
4
1
ASIZE_SAFE() only accepts arrays (try uncommenting).
3
2
5

It works in a relatively straightforward way, though I have put the details in a gist to avoid spoiling them.

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