## NAME

ACE_Select_Reactor_Token_T - Used as a synchronization mechanism to coordinate concurrent access to a Select_Reactor object.

## SYNOPSIS

    #include <ace/Select_Reactor_T.h>

    template<class ACE_SELECT_REACTOR_MUTEX>
    class ACE_Select_Reactor_Token_T : public ACE_SELECT_REACTOR_MUTEX
    {
      public:
        ACE_Select_Reactor_Token_T (ACE_Select_Reactor_Impl &r);
        ACE_Select_Reactor_Token_T (void);
        virtual ~ACE_Select_Reactor_Token_T (void);
        virtual void sleep_hook (void);
        ACE_Select_Reactor_Impl &select_reactor (void);
        void select_reactor (ACE_Select_Reactor_Impl &);
        virtual void dump (void) const;
        ACE_ALLOC_HOOK_DECLARE;
      private:
        ACE_Select_Reactor_Impl *select_reactor_;
    };

## DESCRIPTION

This class is used to make the `ACE_Select_Reactor` thread-safe. By default, the thread that runs the `handle_events` loop holds the token, even when it is blocked in the `select` call. Whenever another thread wants to access the `ACE_Reactor` via its `register_handler`, `remove_handler`, etc. methods) it must ask the token owner for temporary release of the token. To accomplish this, the owner of a token must define a `sleep_hook` through which it can be notified to temporarily release the token if the current situation permits this.

The owner of the token is responsible for deciding which request for the token can be granted. By using the `ACE_Token::renew` API, the thread that releases the token temporarily can specify to get the token back right after the other thread has completed using the token. Thus, there is a dedicated thread that owns the token ``by default.'' This thread grants other threads access to the token by ensuring that whenever somebody else has finished using the token the ``default owner'' first holds the token again, i.e., the owner has the chance to schedule other threads.

The thread that most likely needs the token most of the time is the thread running the dispatch loop. Typically the token gets released prior to entering the `select` call and gets ``re-acquired'' as soon as the `select` call returns, which results probably in many calls to `release`/`acquire` that are not really needed since no other thread would need the token in the meantime. That's why the dispatcher thread is chosen to be the owner of the token.

In case the token would have been released while in `select` there would be a good chance that the `fd_set` could have been modified while the `select` returns from blocking and trying to re-acquire the lock. Through the token mechanism it is ensured that while another thread is holding the token, the dispatcher thread is blocked in the `renew` call and not in `select`. Thus, it is not critical to change the `fd_set`. The implementation of the `sleep_hook` mechanism provided by the `ACE_Select_Reactor_Token` enables the default owner to be the thread that executes the dispatch loop.

## PUBLIC MEMBERS

    ACE_Select_Reactor_Token_T (ACE_Select_Reactor_Impl &r);

    ACE_Select_Reactor_Token_T (void);

    virtual ~ACE_Select_Reactor_Token_T (void);

    virtual void sleep_hook (void);

Called just before the ACE_Event_Handler goes to sleep.

    ACE_Select_Reactor_Impl &select_reactor (void);

    void select_reactor (ACE_Select_Reactor_Impl &);

Set/Get methods

    virtual void dump (void) const;

Dump the state of an object.

    ACE_ALLOC_HOOK_DECLARE;

Declare the dynamic allocation hooks.

## PRIVATE MEMBERS

    ACE_Select_Reactor_Impl *select_reactor_;

## AUTHOR

Doug Schmidt

## LIBRARY

ace