A State-Based Programming Model for Wireless Sensor Networks

110 Chapter 6. Implementation
Program 6.5: Example of an OSM program containing several state variables
side-by-side with the its variable mapping in the host language C.
Every state mapps into a corresponding union, containing both
the variables of that state plus a representation of the state’s substates.
Sequential and parallel compositions of substates are represented
as a union and a struct of substate representations, respectively.
1 state PROG_5 {
2
3 state C {
4 var int c;
5
6 state A {
7 var int a1, a2;
8 e / outA(c) -> B;
9 } // end state A
10 state B {
11 var int b1, b2;
12 f / -> A;
13 } // end state B
14
15 } // end state C
16 ||
17 state D {
18 var int d;
19 }
20
21 } // end state PROG_5
1 struct statePROG_5 {
2 struct par_C_D {
3 struct stateC {
4 int c;
5 union seq_A_B {
6 struct stateA {
7 int a1, a2;
8
9 } _stateA;
10 struct stateB {
11 int b1, b2;
12
13 } _stateB;
14 } _seq_A_B;
15 } _stateC;
16
17 struct stateD {
18 int d;
19 } _stateD;
20 } _par_C_D;
21 } _statePROG_5;
is only true for the variable mapping. Control structures are first mapped to an
intermediate representation in the imperative language Esterel [19, 18, 26]. From
this representation, C code is generated by an Esterel compiler.
The Esterel Synchronous Language
Esterel is a synchronous language (see, for example, [17, 50, 95]) for the specification
of reactive systems. Synchronous languages are reactive in that they react
to input stimuli (i.e., events) in order to compute output events, typically also
changing the internal program state. Synchronous languages are typically used
to implement control systems, such as, industry process control, airplane and
automobile control, embedded systems, bus interfaces, etc. They are built on
the hypothesis that operations take no (real) time, that is, operations are atomic
and the output of an operation is synchronous with its input (hence the name
synchronous language). Synchronous languages have a discrete model of time
where time progresses only on the occurrence of events.
The Esterel language allows to specify concurrent processes, where individ-