The receiver's write file handle.
- iOS 2.0+
- macOS 10.0+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
This object is automatically deallocated when the receiver is deallocated.
You use the returned file handle to write to the pipe using
write(_:) method. When you are finished writing data to this object, send it a
close message to delete the descriptor. Deleting the descriptor causes the reading process to receive an end-of-data signal (an empty