Object-oriented Software in Ada 95

Tasks 295
20.4 Protected type
In essence, an instance of a protected type is an object whose methods have strict concurrency access rules. A
protected object, an instance of a protected type, is composed of data and the procedures and functions that access
the data. The table below summarizes the concurrent access rules for procedures and functions in a protected
object.
Unit Commentary Access
procedure A procedure will only execute when no other Read and write.
units are being executed. If necessary the
procedure will wait until the currently executing
unit(s) have finished.
function A function may execute simultaneously with Read only.
other executing functions. However, a function
cannot execute if a procedure is currently
executing.
entry Like a procedure but may also have a barrier
condition associated with the entry. If the barrier
condition is false the entry is queued until the
barrier becomes true.
Read and write
20.5 Implementation
The implementation of a program to perform an efficient copy using an in store buffer to even out differences in
response rates can be implemented as two tasks and a protected object, as illustrated in Figure 20.4.
Disk
Reader
task
Writer
task
Disk
Put
Buffer
Ge t
Figure 20.4 Copy implemented using two tasks and a protected object buffer.
The responsibilities of the components are as follows:
Name Object is Responsibilities
Task_Reader Task Read data from the file and then pass the data to the buffer.
Note: The task will block if the buffer is full.
Task_Writer Task Take data from the buffer task and write the data to the file.
Note: The task will block if there is no data in the buffer.
PT_Buffer Protected Serialize the storing and retrieving of data to and from a
type buffer.
Note:
The blocking is achieved with a guard to the accept statement. This is described in the section on
guarded accepts.
© M A Smith - May not be reproduced without permission