#include <stdio.h>

#define SENTINEL -1

void foo(int *a) {
	size_t n = 0; 
	while (*a != SENTINEL) {
		a++;
		n++;
	}
	printf("%zu elements.\n", n);
}

int main(void) {
	foo((int[]){4, 5, 6, 7, 8, SENTINEL});
	foo((int[]){-1, -2, SENTINEL});
	foo((int[]){1, 2, 3}); // no sentinel -- UB!!
}