Object-oriented Software in Ada 95

294 Tasks
20.2.1 Putting it all together
Using the new definition of the task type in the package Pack_factorial the following code can now be
written:
with Ada.Text_Io, Ada.Integer_Text_Io, Pack_Factorial;
use Ada.Text_Io, Ada.Integer_Text_Io, Pack_Factorial;
procedure Main is
Num : Positive;
begin
Num := 7;
declare
Factorial: Positive;
Thread_1 : Task_Factorial(Num);
begin
--Do some other work as well
Put("Factorial "); Put(Num); Put(" is ");
Thread_1.Finish( Factorial );
Put( Factorial ); New_Line;
end;
--Answer
--Task is
--Obtain result
end Main;
20.3 Mutual exclusion and critical sections
In many cases of real time working, sections of code must not be executed concurrently. The classic example is
the adding or removing of data in a shared buffer. For example, to perform a copy operation between two separate
devices a shared buffer can be used to even out the differences in response-time. This can be illustrated
diagrammatically as shown in Figure 20.3.
Disk
Reader
task
Writer
task
Disk
Buffer
Figure 20.3 Illustration of copy with a buffer to even out the differences in read and write rates.
The problem is how to prevent both the read and write tasks accessing the buffer simultaneously, causing the
consequential corruption of indices and data. The solution is to have the buffer as an instance of a protected type.
© M A Smith - May not be reproduced without permission