can a shared buffer be thread safe using std::atomic flags? is there a more efficient way to do it?

Hi SO! Is there, by any change, a possibility that the "waiting thread" does not get the "buff" updated? should i put a lock_guard in the thread that changes the "buff"? I've been reading about memory fences on some articles, like this one: http://ift.tt/2a6UBlN and i am just worried about it. Thanks in advance...

#include <iostream>
#include <thread>
#include <chrono>
#include <atomic>

int main(){
    std::atomic<bool> ready(false);
    char buff[20]= "hello";

    std::cout << "creating thread to change \"buff\"..." << std::endl;
    std::thread tr(
        [ & ready, & buff ](){
            std::this_thread::sleep_for(std::chrono::milliseconds(100));
            //should i put a lock_guard here?
            buff[0]= 'w'; buff[1]= 'o';
            buff[2]= 'r'; buff[3]= 'l';
            buff[4]= 'd'; buff[5]= '\0';
            std::cout << "buffer changed! sending signal to waiting thread..." << std::endl;
            ready= true;
    });

    std::cout << "...waiting for the change..." << std::endl;
    while(!ready);

    std::cout << "...got it! buff=" << buff << std::endl;
}