Two parameterized locking and unlocking manipulators, lock and unlock,
   are available for use in synchronizing access to user-defined stream
   objects.  To use these manipulators, you must first define a Mutex
   object which you then pass to the manipulator.  The association
   of a Mutex object with a stream object is not enforced by the iostream
   package. This association is enforced only by a program developer.
   For more information, see help on the Mutex_package.