Ada '83 Language Reference Manual

Copyright 1980, 1982, 1983 owned by the United States Government. Direct reproduction and usage requests to the Ada Information Clearinghouse.

9.12. Example of Tasking

The following example defines a buffering task 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 may contain the statements

    loop
       --  produce the next character CHAR
       BUFFER.WRITE(CHAR);
       exit when CHAR = ASCII.EOT;
    end loop;   

and the consuming task may contain the statements

    loop
       BUFFER.READ(CHAR);
       --  consume the character CHAR
       exit when CHAR = ASCII.EOT;
    end loop; 

The buffering task contains an internal pool of characters processed in a round-robin fashion. The pool has two indices, an IN_INDEX denoting the space for the next input character and an OUT_INDEX denoting the space for the next output character.

    task BUFFER is
       entry READ (C : out CHARACTER);
       entry WRITE(C : in  CHARACTER);
    end; 

    task body BUFFER is
       POOL_SIZE : constant INTEGER := 100;
       POOL      : array(1 .. POOL_SIZE) of CHARACTER;
       COUNT     : INTEGER range 0 .. POOL_SIZE := 0;
       IN_INDEX, OUT_INDEX : INTEGER range 1 .. POOL_SIZE := 1;
    begin
       loop
          select
             when COUNT < POOL_SIZE =>
                accept WRITE(C : in CHARACTER) do
                   POOL(IN_INDEX) := C;
                end;
                IN_INDEX := IN_INDEX mod POOL_SIZE + 1;
                COUNT    := COUNT + 1;
          or when COUNT > 0 =>
                accept READ(C : out CHARACTER) do
                   C := POOL(OUT_INDEX);
                end;
                OUT_INDEX := OUT_INDEX mod POOL_SIZE + 1;
                COUNT     := COUNT - 1;
          or
             terminate;
          end select;  
       end loop;
    end BUFFER;                                                       

Address any questions or comments to adainfo@sw-eng.falls-church.va.us.