Question

I have a macro for calculating array sizes in my C code:

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

When I test it, it works fine for statically defined arrays, but not so for dynamically defined arrays (see below). Not sure I understand why this is the case. Is there any way of calculating the size of an array allocated on the heap?

/* test sizearray macro */
void testSIZEARRAY(void)
{
    /* test case for statically defined array */
    int a[5] = {0,0,0,0,0};

    assert(sizearray(a) == 5);

    /* test case for dynamically defined array */
    int *b;

    b = calloc(5, sizeof(int));

    assert(sizearray(b) == 5);

    free(b);
}

Answer 1

The answer is no, there is no standard way to get the size of a dynamically allocated array.
For all practical purposes, you have to keep track it yourself.

However, there are some compiler-specific methods that do that:

Windows (Visual Studio): _msize()

GCC: msize() Can’t find it in the GCC docs…