VDM-10 Language Manual

Chapter 16
Threads (VDM++ and VDM-RT)
Objects instantiated from a class with a thread part are called active objects. The scope of the
instance variables and operations of the current class is considered to extend to the thread specification.
Syntax: thread definitions = ‘thread’, [ thread definition ] ;
thread definition = periodic thread definition
| procedural thread definition ;
Subclasses inherit threads from superclasses. If a class inherits from several classes only one of
these may declare its own thread (possibly through inheritance). Furthermore, explicitly declaring
a thread in a subclass will override any inherited thread.
16.1 Periodic Thread Definitions
The periodic thread definition can be regarded as the implicit way of describing the activities in a
thread.
Syntax: periodic thread definition = periodic obligation ;
periodic obligation = ‘periodic’,‘(’, numeral, numeral, numeral,
numeral, ‘)’, ‘(’, name, ‘)’ ;
Semantics: For each periodic thread four different numbers are used. They are in order of appearance
(also illustrated in Figure 16.1):
1. period: This is a non-negative, non-zero value that describes the length of the time
interval between two adjacent events in a strictly periodic event stream (where jitter =
0)
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