A State-Based Programming Model for Wireless Sensor Networks

104 Chapter 6. Implementation
Program 6.2: A hierarchical state machine and its representation in the OSM textual
language.
PROG_2
A
int a
outB()
B
int b
prB()
f
C
int c
inB(a,b)
f
outC(a,c)
e
e
D
1 state PROG_2 {
2 initial state A {
3 var int a;
4
5 initial state B {
6 var int b;
7 onEntry: f / inB(a, b);
8 f / outB() -> C;
9 onPreemption: prB();
10 } // end state B
11 state C {
12 extern var int a;
13 var int c;
14 f / outC(a, c) -> B;
15 } // end state C
16
17 e / -> D;
18 } // end state A
19
20 state D { e / -> A; }
21 }
state. If desired, the external variable interface may be declared explicitly, but if
it is declared, external variables must be marked with the extern keyword. In
the example, the integer variable a, defined in state A (line 3), is visible in A’s
substates B and C. Therefore a can be used in their actions, such as inB() and
outC() in lines 7 and 14 of the example Prog. 6.2, respectively. In state C the
external variable a is declared explicitly.
Preemption Actions
Substates may declare preemption actions that are executed at runtime when
the substate is left because its superstate is left through a transition. Preemption
actions are declared in the context of states—they are not explicitly associated
with particular transitions. Definitions of preemption actions have the following
form (where apreemption() is the preemption action):
onPreemption: / apreemption();
In the example Prog. 6.2, state B declares a preemption action prB() in line 9. It
could be used to release resources previously allocated in the entry action inB()
when B is left implicitly by preemption, that is, when leaving B’s superstate A
through a transition to D.