Object-oriented Software in Ada 95

Tasks 291
The execution of the above program can be visualized as Figure 20.1
Thread_1 {Factorial(5)}
Thread_2 {Factorial(7)}
Main program thread
Thread_3 {Prime(97)}
Figure 20.1 Illustration of active threads in the above program.
Once started, each of the threads will execute concurrently until the Finish rendezvous is encountered,
which is used to deliver the result to the initiator of the tasks.
Note:
20.1.2 Task rendezvous
The actual implementation of the concurrency will depend on the underlying architecture, both
software and hardware, of the platform on which the program is executed.
The rendezvous mechanism is used for:
• synchronizing two separate threads so that information may be exchanged.
• synchronizing the execution of two threads.
A rendezvous is achieved by one task having an entry statement and the other task performing a call on this
entry. For example, the code for a rendezvous to pass a Positive number to the task object thread_1 the
code would be:
Main program (client) which
elaborates thread1
Thread_1.Start(5);
Body of task Thread_1 (server)
accept Start(F:in Positive) do
Factorial := F;
end Start;
To achieve this effect, one of the threads of control will be suspended until the other thread catches up. Then at
the rendezvous, data, in this case the number 5, is transferred between the tasks. The code between do and end is
executed with the client task suspended. After the code between do and end has been executed both tasks resume
their independent execution.
This rendezvous between the two tasks is illustrated in Figure 20.2 in which the main program task
rendezvous with an instance of the task Factorial.
© M A Smith - May not be reproduced without permission