Object-oriented Software in Ada 95

Tasks 299
to the entry prevents the entry being processed until there is room in the buffer. If the buffer is full then the
reader task is suspended (blocked) until a successful get entry is made. The guards for an entry statement are
re-evaluated after a successful call on the protected object. The full implementation of the protected type
PT_Buffer is as follows:
protected body PT_Buffer is
The queue is implemented in sequential store with the two indices head and tail keeping track of the
current extraction and insertion points respectively. A count of the active cells used in the buffer is held in
no_in_queue. Figure 20.5 illustrates the queue after adding the characters ‘t’, ‘e’, ‘x’, ‘t’.
Head
Tail
T e
x t
Figure 20.5 Queue holding the characters ‘text’.
The procedure Put in the body of the protected object adds new data to the queue. Data can only be added to
the queue when there is room. The index Tail marks the position of the next data item to be added. When no
more data is available to add to the queue the variable Fin is set to true.
entry Put( Ch:in Character; No_More:in Boolean )
when No_In_Queue < Queue_Size is
begin
if No_More then
--Last
Fin := True;
--Set flag
else
Elements( Tail ) := Ch; --Add to queue
Tail := Tail+1; --Next position
No_In_Queue := No_In_Queue + 1; --
end if;
end;
The procedure Get extracts data from the queue. The head indexes the data item at the front of the queue.
The parameter eof is set to true when no more data is available. This is different from a temporary unavailability
of data due to the reader task blocking.
entry Get(Ch:in out Character; No_More:out Boolean)
when No_In_Queue > 0 or else Fin is
begin
if No_In_Queue > 0 then --Item available
Ch := Elements( Head ); --Get item
Head := Head+1; --Next position
No_In_Queue := No_In_Queue - 1; --
No_More := False; --Not end
else
No_More := True;
end if;
end;
end PT_Buffer ;
end Pack_Threads;
--End of items
Note:
When all the data has been exhausted from the buffer, the procedure get will return false in its second
parameter.
© M A Smith - May not be reproduced without permission