condition_variables.qbk

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[/
  (C) Copyright 2007-8 Anthony Williams.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:condvar_ref Condition Variables]

[heading Synopsis]

The classes `condition_variable` and `condition_variable_any` provide a
mechanism for one thread to wait for notification from another thread that a
particular condition has become true. The general usage pattern is that one
thread locks a mutex and then calls `wait` on an instance of
`condition_variable` or `condition_variable_any`. When the thread is woken from
the wait, then it checks to see if the appropriate condition is now true, and
continues if so. If the condition is not true, then the thread then calls `wait`
again to resume waiting. In the simplest case, this condition is just a boolean
variable:

    boost::condition_variable cond;
    boost::mutex mut;
    bool data_ready;

    void process_data();

    void wait_for_data_to_process()
    {
        boost::unique_lock<boost::mutex> lock(mut);
        while(!data_ready)
        {
            cond.wait(lock);
        }
        process_data();
    }

Notice that the `lock` is passed to `wait`: `wait` will atomically add the
thread to the set of threads waiting on the condition variable, and unlock the
mutex. When the thread is woken, the mutex will be locked again before the call
to `wait` returns. This allows other threads to acquire the mutex in order to
update the shared data, and ensures that the data associated with the condition
is correctly synchronized.

In the mean time, another thread sets the condition to `true`, and then calls
either `notify_one` or `notify_all` on the condition variable to wake one
waiting thread or all the waiting threads respectively.

    void retrieve_data();
    void prepare_data();

    void prepare_data_for_processing()
    {
        retrieve_data();
        prepare_data();
        {
            boost::lock_guard<boost::mutex> lock(mut);
            data_ready=true;
        }
        cond.notify_one();
    }

Note that the same mutex is locked before the shared data is updated, but that
the mutex does not have to be locked across the call to `notify_one`.

This example uses an object of type `condition_variable`, but would work just as
well with an object of type `condition_variable_any`: `condition_variable_any`
is more general, and will work with any kind of lock or mutex, whereas
`condition_variable` requires that the lock passed to `wait` is an instance of
`boost::unique_lock<boost::mutex>`. This enables `condition_variable` to make
optimizations in some cases, based on the knowledge of the mutex type;
`condition_variable_any` typically has a more complex implementation than
`condition_variable`.

[section:condition_variable Class `condition_variable`]

    #include <boost/thread/condition_variable.hpp>

    namespace boost
    {
        class condition_variable
        {
        public:
            condition_variable();
            ~condition_variable();

            void notify_one();
            void notify_all();

            void wait(boost::unique_lock<boost::mutex>& lock);

            template<typename predicate_type>
            void wait(boost::unique_lock<boost::mutex>& lock,predicate_type predicate);

            bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::system_time const& abs_time);

            template<typename duration_type>
            bool timed_wait(boost::unique_lock<boost::mutex>& lock,duration_type const& rel_time);

            template<typename predicate_type>
            bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::system_time const& abs_time,predicate_type predicate);

            template<typename duration_type,typename predicate_type>
            bool timed_wait(boost::unique_lock<boost::mutex>& lock,duration_type const& rel_time,predicate_type predicate);

        // backwards compatibility

            bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::xtime const& abs_time);

            template<typename predicate_type>
            bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::xtime const& abs_time,predicate_type predicate);
        };
    }

[section:constructor `condition_variable()`]

[variablelist

[[Effects:] [Constructs an object of class `condition_variable`.]]

[[Throws:] [__thread_resource_error__ if an error occurs.]]

]

[endsect]

[section:destructor `~condition_variable()`]

[variablelist

[[Precondition:] [All threads waiting on `*this` have been notified by a call to
`notify_one` or `notify_all` (though the respective calls to `wait` or
`timed_wait` need not have returned).]]

[[Effects:] [Destroys the object.]]

[[Throws:] [Nothing.]]

]

[endsect]

[section:notify_one `void notify_one()`]

[variablelist

[[Effects:] [If any threads are currently __blocked__ waiting on `*this` in a call
to `wait` or `timed_wait`, unblocks one of those threads.]]

[[Throws:] [Nothing.]]

]

[endsect]

[section:notify_all `void notify_all()`]

[variablelist

[[Effects:] [If any threads are currently __blocked__ waiting on `*this` in a call
to `wait` or `timed_wait`, unblocks all of those threads.]]

[[Throws:] [Nothing.]]

]

[endsect]

[section:wait `void wait(boost::unique_lock<boost::mutex>& lock)`]

[variablelist

[[Precondition:] [`lock` is locked by the current thread, and either no other
thread is currently waiting on `*this`, or the execution of the `mutex()` member
function on the `lock` objects supplied in the calls to `wait` or `timed_wait`
in all the threads currently waiting on `*this` would return the same value as
`lock->mutex()` for this call to `wait`.]]

[[Effects:] [Atomically call `lock.unlock()` and blocks the current thread. The
thread will unblock when notified by a call to `this->notify_one()` or
`this->notify_all()`, or spuriously. When the thread is unblocked (for whatever
reason), the lock is reacquired by invoking `lock.lock()` before the call to
`wait` returns. The lock is also reacquired by invoking `lock.lock()` if the
function exits with an exception.]]

[[Postcondition:] [`lock` is locked by the current thread.]]

[[Throws:] [__thread_resource_error__ if an error
occurs. __thread_interrupted__ if the wait was interrupted by a call to
__interrupt__ on the __thread__ object associated with the current thread of execution.]]

]

[endsect]

[section:wait_predicate `template<typename predicate_type> void wait(boost::unique_lock<boost::mutex>& lock, predicate_type pred)`]

[variablelist

[[Effects:] [As-if ``
while(!pred())
{
    wait(lock);
}
``]]

]

[endsect]

[section:timed_wait `bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::system_time const& abs_time)`]

[variablelist

[[Precondition:] [`lock` is locked by the current thread, and either no other
thread is currently waiting on `*this`, or the execution of the `mutex()` member
function on the `lock` objects supplied in the calls to `wait` or `timed_wait`
in all the threads currently waiting on `*this` would return the same value as
`lock->mutex()` for this call to `wait`.]]

[[Effects:] [Atomically call `lock.unlock()` and blocks the current thread. The
thread will unblock when notified by a call to `this->notify_one()` or
`this->notify_all()`, when the time as reported by `boost::get_system_time()`
would be equal to or later than the specified `abs_time`, or spuriously. When
the thread is unblocked (for whatever reason), the lock is reacquired by
invoking `lock.lock()` before the call to `wait` returns. The lock is also
reacquired by invoking `lock.lock()` if the function exits with an exception.]]

[[Returns:] [`false` if the call is returning because the time specified by
`abs_time` was reached, `true` otherwise.]]

[[Postcondition:] [`lock` is locked by the current thread.]]

[[Throws:] [__thread_resource_error__ if an error
occurs. __thread_interrupted__ if the wait was interrupted by a call to
__interrupt__ on the __thread__ object associated with the current thread of execution.]]

]

[endsect]

[section:timed_wait_rel `template<typename duration_type> bool timed_wait(boost::unique_lock<boost::mutex>& lock,duration_type const& rel_time)`]

[variablelist

[[Precondition:] [`lock` is locked by the current thread, and either no other
thread is currently waiting on `*this`, or the execution of the `mutex()` member
function on the `lock` objects supplied in the calls to `wait` or `timed_wait`
in all the threads currently waiting on `*this` would return the same value as
`lock->mutex()` for this call to `wait`.]]

[[Effects:] [Atomically call `lock.unlock()` and blocks the current thread. The
thread will unblock when notified by a call to `this->notify_one()` or
`this->notify_all()`, after the period of time indicated by the `rel_time`
argument has elapsed, or spuriously. When the thread is unblocked (for whatever
reason), the lock is reacquired by invoking `lock.lock()` before the call to
`wait` returns. The lock is also reacquired by invoking `lock.lock()` if the