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Is this a suitable use of const qualifiers in C?

I have a simple vector implementation in C that contains a void * array. It is up to the user to take care of the actual type.

I want to “promise” that the vector will not change its contents, so I store the data as:

Struct _Vector{
    UInt32 used;
    UInt32 size;
    const void** arr;
};

I don’t know if he considered "overdoing" the rule of his constant, but I try to maintain constant correctness with my accessories / modifiers:

void vector_add(Vector *v, const void* const elem);   
void vector_set(Vector *v, const UInt32 idx, const void* const elem);

When I return an element from a vector, it is a pointer to user data, so I let the user change the displayed data by discarding the constant in the internal array.

void* vector_get(const Vector *v, const UInt32 idx){
    if ( idx >= v->used ) 
        exitAtError("Vector","Array out of bounds");
    return (void*)v->arr[idx];
}

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const int x = 5;
vector_add(&v, &x);

int *p = vector_get(&v, 0);
*p = 6;   // oops

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