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9.11 Example of Tasking and Synchronization


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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 can have the following structure:

task Producer;
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;

and the consuming task can have the following structure:

task Consumer;
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;

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.


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.

type Person_Name_Array is array (Positive range <>)
   of Person_Name;  -- see 3.10.1
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;