close t prevents further use of t, and makes shutdown() and close() system calls on t's underlying file descriptor according to the file_descriptor_handling argument and whether or not t is a socket, i.e., kind t = Socket `Active:

  | file_descriptor_handling                     | shutdown() | close() |
  |----------------------------------------------+------------+---------|
  | Do_not_close_file_descriptor                 | no         | no      |
  | Close_file_descriptor Shutdown_socket        | if socket  | yes     |
  | Close_file_descriptor Do_not_shutdown_socket | no         | yes     |

The result of close becomes determined once the system calls complete. It is OK to call close multiple times on the same t; calls subsequent to the initial call will have no effect, but will return the same deferred as the original call.