I think I would make grow() like this to prevent the mistake of accidentally laying out the array fields in the wrong order. void grow(void **ptr, ptrdiff_t

I think I would make grow() like this to prevent the mistake of accidentally laying out the array fields in the wrong order. void grow(void **ptr, ptrdiff_t *cap, ptrdiff_t *len, arena *a) ... #define push(s, a) ... grow((void **)&s->data, &s->cap, &s->len)