Example of Tasking and Synchronization
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

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# 9.11 Example of Tasking and Synchronization

#### Examples

1
The following example defines a buffer protected object to smooth variations between the speed of output of a producing task and the speed of input of some consuming task. For instance, the producing task might have the following structure:
2
task Producer;
3/2
task body Producer is
Person : Person_Name; -- see 3.10.1
begin
loop
... -- simulate arrival of the next customer
Buffer.Append_Wait(Person);
exit when Person = null;
end loop;
end Producer;
4
and the consuming task might have the following structure:
5
task Consumer;
6/2
task body Consumer is
Person : Person_Name;
begin
loop
Buffer.Remove_First_Wait(Person);
exit when Person = null;
... -- simulate serving a customer
end loop;
end Consumer;
7/2
The buffer object contains an internal array of person names managed in a round-robin fashion. The array has two indices, an In_Index denoting the index for the next input person name and an Out_Index denoting the index for the next output person name.
7.1/2
The Buffer is defined as an extension of the Synchronized_Queue interface (see 3.9.4), and as such promises to implement the abstraction defined by that interface. By doing so, the Buffer can be passed to the Transfer class-wide operation defined for objects of a type covered by Queue'Class.
8/2
protected Buffer is new Synchronized_Queue with -- see 3.9.4
entry Append_Wait(Person : in Person_Name);
entry Remove_First_Wait(Person : out Person_Name);
function Cur_Count return Natural;
function Max_Count return Natural;
procedure Append(Person : in Person_Name);
procedure Remove_First(Person : out Person_Name);
private
Pool : Person_Name_Array(1 .. 100);
Count : Natural := 0;
In_Index, Out_Index : Positive := 1;
end Buffer;
9/2
protected body Buffer is
entry Append_Wait(Person : in Person_Name)
when Count < Pool'Length is
begin
Append(Person);
end Append_Wait;
9.1/2
procedure Append(Person : in Person_Name) is
begin
if Count = Pool'Length then
raise Queue_Error with "Buffer Full"; -- see 11.3
end if;
Pool(In_Index) := Person;
In_Index := (In_Index mod Pool'Length) + 1;
Count := Count + 1;
end Append;
10/2
entry Remove_First_Wait(Person : out Person_Name)
when Count > 0 is
begin
Remove_First(Person);
end Remove_First_Wait;
11/2
procedure Remove_First(Person : out Person_Name) is
begin
if Count = 0 then
raise Queue_Error with "Buffer Empty"; -- see 11.3
end if;
Person := Pool(Out_Index);
Out_Index := (Out_Index mod Pool'Length) + 1;
Count := Count - 1;
end Remove_First;
12/2
function Cur_Count return Natural is
begin
return Buffer.Count;
end Cur_Count;
13/2
function Max_Count return Natural is
begin
return Pool'Length;
end Max_Count;
end Buffer;

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