My code raised TASKING_ERROR?

Question

Suppose the following situation: There is a box that is used to store coins. People take money out of the box. People put money in the box. I took a task entry like Add, Subtract, Read and Write. below is my code. Now I am confused how to name the record, add, subtract and read to achieve the above requirements, please help

   with Ada.Text_IO,ada.integer_text_io;
   use ada.text_io,ada.integer_text_io;
     procedure protected_imp is
     task type box(initial_value:integer;min_value:integer;max_value:integer) is
     entry add(number:integer); 
     entry subtract(number:integer);
     entry read;
      end box;
  task body box is 
  x:integer:=initial_value; --shared variable
   begin
   loop                  --why raised TASKING_ERROR when loop is removed?
select
   when x<max_value=>
   accept add(number:integer)do
   x:=x+number;
   end add;
or  
    when x>min_value=>
    accept subtract(number:integer) do
    x:=x-number;
    end subtract;
or
    accept read do
    put("coins");
    put_line(integer'image(x));
    end read;
or
   delay(5.0);
  put("no request received for 5 seconds");
  end select;
 end loop;
 end box;
  go:box(1,0,10);
  begin ----- how to call? and why  i am getting "no request received for 5 seconds " even i have activate go .add(1) ?
  for i in 1..5 loop
   go.add(1);
   end loop;
    go.read;
  end protected_imp;

Answer 1

Here your program produces output

$ ./protected_imp 
coins 6
no request received for 5 secondsno request received for 5 secondsno request received for 5 seconds^C

(I stopped him at that moment).

"coins 6" - this is exactly the result that he should give; You have added 1 to the starting value (1) 5 times.

, Tasking_Error, , , select ; , go.add, select , go.add , .

task arrives at select; all the alternatives are closed
main calls go.add (1)'
task accepts the entry, increments x, exits the select, and goes round the loop again to wait at the select
main calls go.add (1)
task accepts the entry, increments x, exits the select, and goes round the loop again to wait at the select
main calls go.add (1)
task accepts the entry, increments x, exits the select, and goes round the loop again to wait at the select
main calls go.add (1)
task accepts the entry, increments x, exits the select, and goes round the loop again to wait at the select
main calls go.add (1)
task accepts the entry, increments x, exits the select, and goes round the loop again to wait at the select
main calls go.read
task accepts the entry, prints out "coins 6", and goes round the loop again to wait at the select
main finishes and waits until the task terminates, which it doesn’t, because
after 5 seconds, the select’s delay alternative opens, so the task takes it, prints the message (it should really use Put_Line) and goes round the loop again to wait at the select
after 5 seconds, the select’s delay alternative opens, so the task takes it, prints the message and goes round the loop again to wait at the select
...