Is it true, that modern OS may skip copy when realloc is called

Question

While reading the https://stackoverflow.com/a/3190489/196561 I have a question. What the Qt authors says in the Inside the Qt 4 Containers:

... QVecto

Answer 1

For Linux, see man 2 mremap:

   void *mremap(void *old_address, size_t old_size,
                size_t new_size, int flags, ... /* void *new_address */);

mremap() expands (or shrinks) an existing memory mapping, potentially moving it at the same time (controlled by the flags argument and the available virtual address space).