This is where hand-rolled intrusive data structures, as are traditional in C, really shine.
Even in C, if you want differently sized data to be indexable in O(1), you're stuck leaving them as pointers. You definitely could just have a variable-sized area for this, but that level of optimization is pretty seldomly done in C.
Even in C, if you want differently sized data to be indexable in O(1), you're stuck leaving them as pointers. You definitely could just have a variable-sized area for this, but that level of optimization is pretty seldomly done in C.