VDM-10 Language Manual

VDM-10 Language Manual
start statement = ‘start’, ‘(’, expression, ‘)’ ;
start list statement = ‘startlist’, ‘(’, expression, ‘)’ ;
Semantics: The start and start list statements have the form:
✞
✡✝
start(aRef)
startlist(aRef_s)
✆
If a class description includes a thread (see section 16), each object created from this class
will have the ability to operate as a stand-alone virtual machine, or in other terms: the object
has its own processing capability. In this situation, a new expression creates the ‘process’
leaving it in a waiting state. For such objects VDM++ and VDM-RT has a mechanism to
change the waiting state into an active state 5 in terms of a predefined operation, which can
be invoked through a start statement.
The explicit separation of object creation and start provides the possibility to complete the
initialisation of a (concurrent) system before the objects start exhibiting their described behaviour,
in this way avoiding problems that may arise when objects are referred to that are
not yet created and/or connected.
A syntactic variant of the start statement is available to start up a number of active objects in
arbitrary order: the start list statement. The parameter aRef s to startlist must be a
set of object references to objects instantiated from classes containing a thread.
Examples: Consider the specification of an operating system. A component of this would be the
daemons and other processes started up during the boot sequence. From this perspective, the
following definitions are relevant:
✞
types
runLevel = nat;
Process = Kerneld | Ftpd | Syslogd | Lpd | Httpd
✡✝
instance variables
pInit : map runLevel to set of Process
✆
where Kerneld is an object reference type specified elsewhere, and similarly for the other
processes listed.
We can then model the boot sequence as an operation:
5 When an object is in an active state, its behaviour can be described using a thread (see section 16).
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