Method Stdio.Fd()->is_open()
- Method is_open
 intis_open()- Description
 Returns true if the file is open.
- Note
 If the file is a socket that has been closed from the remote side, this function might still return true.
- Note
 Most methods can't be called for a file descriptor that isn't open. Notable exceptions errno, mode, and the set and query functions for callbacks and backend.