title: I can’t use mmap to share a Hash between processes
body: <p>I am implementing a multi-process library that provides a data structure for shared memory.
  But I am having trouble now, I modified the shared Hash object in the child process, but the parent process still has not read the changed value.</p>
  
  <p>Sample code: <a href="https://play.crystal-lang.org/#/r/6n34" rel="nofollow noreferrer">https://play.crystal-lang.org/#/r/6n34</a></p>
  
  <p>Modified with the same pointer, why is it not effective?</p>