I have the following code (C11):

#define vector_grow(vec, count)
do {
if(!(vec)) {
size_t __p = (size_t) malloc ((size_t)(count) * sizeof(*(vec)) + (sizeof(size_t) * 2));
(vec) = (void *)(&__p[2]);
vector_set_capacity((vec), (count));
vector_set_size((vec), 0);
} else {
size_t *__p1 = &((size_t *)(vec))[-2]; \ <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
size_t __p2 = realloc(__p1, ((size_t)(count) * sizeof((vec))+ (sizeof(size_t) * 2)));
(vec) = (void *)(&__p2[2]);
vector_set_capacity((vec), (count));
} while(0)

With -Wall it generates the following warning: cast from ‘float *’ to ‘size_t *’ (aka ‘unsigned long long *’) increases required alignment from 4 to 8 [-Wcast-align], triggered by the indicated line, in case the type of pointer is smaller than size_t.

I get the gist of it, but reading the std (taking cppreference.com’s word for it), this seems un-necessary.

malloc: “If allocation succeeds, returns a pointer to the lowest (first) byte in the allocated memory block that is suitably aligned for any object type.”.
and aligned_alloc: “Regular malloc aligns memory suitable for any object type (which, in practice, means that it is aligned to alignof(max_align_t)).”, which means long double.
It appears that even on Windows (without a real long double), alignment is at least 16.

No warning on this issue from VS 15.6.6.