Simple Monitor

program example;

monitor resource_allocator;

var busy: boolean;

mon_queue: condition_variable;

(* monitor procedures get_resource and return resource are printed side-by-side to

conserve space *)

procedure get_resource; procedure return_resource;

begin begin

if busy then wait(mon_queue); busy := false;

busy := true; signal(mon_queue);

end; end;

begin (* monitor executable code *)

busy := false;

end;

(* The monitor ends here. The code below belongs to the main program not the

monitor. *)

procedure user;

begin

while(true) do

begin

resource_allocator.get_resource;

critical section

resource_allocator.return_resource;

end;

end;

(* The calls to user inside cobegin and coend invoke four threads of the user code all

competing with each other for repeated access to the critical section. *)

cobegin

user; user; user; user;

coend.