Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Synchronization

Mutex Concepts
Lock Types
Mutex Types
Condition Variables
One-time Initialization
Barriers

A mutex object facilitates protection against data races and allows thread-safe synchronization of data between threads. A thread obtains ownership of a mutex object by calling one of the lock functions and relinquishes ownership by calling the corresponding unlock function. Mutexes may be either recursive or non-recursive, and may grant simultaneous ownership to one or many threads. Boost.Thread supplies recursive and non-recursive mutexes with exclusive ownership semantics, along with a shared ownership (multiple-reader / single-writer) mutex.

Boost.Thread supports four basic concepts for lockable objects: Lockable, TimedLockable, SharedLockable and UpgradeLockable. Each mutex type implements one or more of these concepts, as do the various lock types.

The Lockable concept models exclusive ownership. A type that implements the Lockable concept shall provide the following member functions:

Lock ownership acquired through a call to lock() or try_lock() must be released through a call to unlock().

Effects:

The current thread blocks until ownership can be obtained for the current thread.

Postcondition:

The current thread owns *this.

Throws:

boost::thread_resource_error if an error occurs.

Effects:

Attempt to obtain ownership for the current thread without blocking.

Returns:

true if ownership was obtained for the current thread, false otherwise.

Postcondition:

If the call returns true, the current thread owns the *this.

Throws:

boost::thread_resource_error if an error occurs.

Precondition:

The current thread owns *this.

Effects:

Releases ownership by the current thread.

Postcondition:

The current thread no longer owns *this.

Throws:

Nothing

The TimedLockable concept refines the Lockable concept to add support for timeouts when trying to acquire the lock.

A type that implements the TimedLockable concept shall meet the requirements of the Lockable concept. In addition, the following member functions must be provided:

Lock ownership acquired through a call to timed_lock() must be released through a call to unlock().

Effects:

Attempt to obtain ownership for the current thread. Blocks until ownership can be obtained, or the specified time is reached. If the specified time has already passed, behaves as try_lock().

Returns:

true if ownership was obtained for the current thread, false otherwise.

Postcondition:

If the call returns true, the current thread owns *this.

Throws:

boost::thread_resource_error if an error occurs.

The SharedLockable concept is a refinement of the TimedLockable concept that allows for shared ownership as well as exclusive ownership. This is the standard multiple-reader / single-write model: at most one thread can have exclusive ownership, and if any thread does have exclusive ownership, no other threads can have shared or exclusive ownership. Alternatively, many threads may have shared ownership.

For a type to implement the SharedLockable concept, as well as meeting the requirements of the TimedLockable concept, it must also provide the following member functions:

Lock ownership acquired through a call to lock_shared(), try_lock_shared() or timed_lock_shared() must be released through a call to unlock_shared().

Effects:

The current thread blocks until shared ownership can be obtained for the current thread.

Postcondition:

The current thread has shared ownership of *this.

Throws:

boost::thread_resource_error if an error occurs.

Effects:

Attempt to obtain shared ownership for the current thread without blocking.

Returns:

true if shared ownership was obtained for the current thread, false otherwise.

Postcondition:

If the call returns true, the current thread has shared ownership of *this.

Throws:

boost::thread_resource_error if an error occurs.

Effects:

Attempt to obtain shared ownership for the current thread. Blocks until shared ownership can be obtained, or the specified time is reached. If the specified time has already passed, behaves as try_lock_shared().

Returns:

true if shared ownership was acquired for the current thread, false otherwise.

Postcondition:

If the call returns true, the current thread has shared ownership of *this.

Throws:

boost::thread_resource_error if an error occurs.

Precondition:

The current thread has shared ownership of *this.

Effects:

Releases shared ownership of *this by the current thread.

Postcondition:

The current thread no longer has shared ownership of *this.

Throws:

Nothing

The UpgradeLockable concept is a refinement of the SharedLockable concept that allows for upgradable ownership as well as shared ownership and exclusive ownership. This is an extension to the multiple-reader / single-write model provided by the SharedLockable concept: a single thread may have upgradable ownership at the same time as others have shared ownership. The thread with upgradable ownership may at any time attempt to upgrade that ownership to exclusive ownership. If no other threads have shared ownership, the upgrade is completed immediately, and the thread now has exclusive ownership, which must be relinquished by a call to unlock(), just as if it had been acquired by a call to lock().

If a thread with upgradable ownership tries to upgrade whilst other threads have shared ownership, the attempt will fail and the thread will block until exclusive ownership can be acquired.

Ownership can also be downgraded as well as upgraded: exclusive ownership of an implementation of the UpgradeLockable concept can be downgraded to upgradable ownership or shared ownership, and upgradable ownership can be downgraded to plain shared ownership.

For a type to implement the UpgradeLockable concept, as well as meeting the requirements of the SharedLockable concept, it must also provide the following member functions:

Lock ownership acquired through a call to lock_upgrade() must be released through a call to unlock_upgrade(). If the ownership type is changed through a call to one of the unlock_xxx_and_lock_yyy() functions, ownership must be released through a call to the unlock function corresponding to the new level of ownership.

Effects:

The current thread blocks until upgrade ownership can be obtained for the current thread.

Postcondition:

The current thread has upgrade ownership of *this.

Throws:

boost::thread_resource_error if an error occurs.

Precondition:

The current thread has upgrade ownership of *this.

Effects:

Releases upgrade ownership of *this by the current thread.

Postcondition:

The current thread no longer has upgrade ownership of *this.

Throws:

Nothing

Precondition:

The current thread has upgrade ownership of *this.

Effects:

Atomically releases upgrade ownership of *this by the current thread and acquires exclusive ownership of *this. If any other threads have shared ownership, blocks until exclusive ownership can be acquired.

Postcondition:

The current thread has exclusive ownership of *this.

Throws:

Nothing

Precondition:

The current thread has upgrade ownership of *this.

Effects:

Atomically releases upgrade ownership of *this by the current thread and acquires shared ownership of *this without blocking.

Postcondition:

The current thread has shared ownership of *this.

Throws:

Nothing

Precondition:

The current thread has exclusive ownership of *this.

Effects:

Atomically releases exclusive ownership of *this by the current thread and acquires upgrade ownership of *this without blocking.

Postcondition:

The current thread has upgrade ownership of *this.

Throws:

Nothing

template<typename Lockable>
class lock_guard
{
public:
    explicit lock_guard(Lockable& m_);
    lock_guard(Lockable& m_,boost::adopt_lock_t);

    ~lock_guard();
};

boost::lock_guard is very simple: on construction it acquires ownership of the implementation of the Lockable concept supplied as the constructor parameter. On destruction, the ownership is released. This provides simple RAII-style locking of a Lockable object, to facilitate exception-safe locking and unlocking. In addition, the lock_guard(Lockable & m,boost::adopt_lock_t) constructor allows the boost::lock_guard object to take ownership of a lock already held by the current thread.

Effects:

Stores a reference to m. Invokes m.lock().

Throws:

Any exception thrown by the call to m.lock().

Precondition:

The current thread owns a lock on m equivalent to one obtained by a call to m.lock().

Effects:

Stores a reference to m. Takes ownership of the lock state of m.

Throws:

Nothing.

Effects:

Invokes m.unlock() on the Lockable object passed to the constructor.

Throws:

Nothing.

template<typename Lockable>
class unique_lock
{
public:
    explicit unique_lock(Lockable& m_);
    unique_lock(Lockable& m_,adopt_lock_t);
    unique_lock(Lockable& m_,defer_lock_t);
    unique_lock(Lockable& m_,try_to_lock_t);
    unique_lock(Lockable& m_,system_time const& target_time);

    ~unique_lock();

    unique_lock(detail::thread_move_t<unique_lock<Lockable> > other);
    unique_lock(detail::thread_move_t<upgrade_lock<Lockable> > other);

    operator detail::thread_move_t<unique_lock<Lockable> >();
    detail::thread_move_t<unique_lock<Lockable> > move();
    unique_lock& operator=(detail::thread_move_t<unique_lock<Lockable> > other);
    unique_lock& operator=(detail::thread_move_t<upgrade_lock<Lockable> > other);

    void swap(unique_lock& other);
    void swap(detail::thread_move_t<unique_lock<Lockable> > other);

    void lock();
    bool try_lock();

    template<typename TimeDuration>
    bool timed_lock(TimeDuration const& relative_time);
    bool timed_lock(::boost::system_time const& absolute_time);

    void unlock();

    bool owns_lock() const;
    operator unspecified-bool-type() const;
    bool operator!() const;

    Lockable* mutex() const;
    Lockable* release();
};

boost::unique_lock is more complex than boost::lock_guard: not only does it provide for RAII-style locking, it also allows for deferring acquiring the lock until the lock() member function is called explicitly, or trying to acquire the lock in a non-blocking fashion, or with a timeout. Consequently, unlock() is only called in the destructor if the lock object has locked the Lockable object, or otherwise adopted a lock on the Lockable object.

Specializations of boost::unique_lock model the TimedLockable concept if the supplied Lockable type itself models TimedLockable concept (e.g. boost::unique_lock<boost::timed_mutex>), or the Lockable concept otherwise (e.g. boost::unique_lock<boost::mutex>).

An instance of boost::unique_lock is said to own the lock state of a Lockable m if mutex() returns a pointer to m and owns_lock() returns true. If an object that owns the lock state of a Lockable object is destroyed, then the destructor will invoke mutex()->unlock().

The member functions of boost::unique_lock are not thread-safe. In particular, boost::unique_lock is intended to model the ownership of a Lockable object by a particular thread, and the member functions that release ownership of the lock state (including the destructor) must be called by the same thread that acquired ownership of the lock state.

Effects:

Stores a reference to m. Invokes m.lock().

Postcondition:

owns_lock() returns true. mutex() returns &m.

Throws:

Any exception thrown by the call to m.lock().

Precondition:

The current thread owns an exclusive lock on m.

Effects:

Stores a reference to m. Takes ownership of the lock state of m.

Postcondition:

owns_lock() returns true. mutex() returns &m.

Throws:

Nothing.

Effects:

Stores a reference to m.

Postcondition:

owns_lock() returns false. mutex() returns &m.

Throws:

Nothing.

Effects:

Stores a reference to m. Invokes m.try_lock(), and takes ownership of the lock state if the call returns true.

Postcondition:

mutex() returns &m. If the call to try_lock() returned true, then owns_lock() returns true, otherwise owns_lock() returns false.

Throws:

Nothing.

Effects:

Stores a reference to m. Invokes m.timed_lock(abs_time), and takes ownership of the lock state if the call returns true.

Postcondition:

mutex() returns &m. If the call to timed_lock() returned true, then owns_lock() returns true, otherwise owns_lock() returns false.

Throws:

Any exceptions thrown by the call to m.timed_lock(abs_time).

Effects:

Invokes mutex()-> unlock() if owns_lock() returns true.

Throws:

Nothing.

Returns:

true if the *this owns the lock on the Lockable object associated with *this.

Throws:

Nothing.

Returns:

A pointer to the Lockable object associated with *this, or NULL if there is no such object.

Throws:

Nothing.

Returns:

If owns_lock() would return true, a value that evaluates to true in boolean contexts, otherwise a value that evaluates to false in boolean contexts.

Throws:

Nothing.

Returns:

! owns_lock().

Throws:

Nothing.

Effects:

The association between *this and the Lockable object is removed, without affecting the lock state of the Lockable object. If owns_lock() would have returned true, it is the responsibility of the calling code to ensure that the Lockable is correctly unlocked.

Returns:

A pointer to the Lockable object associated with *this at the point of the call, or NULL if there is no such object.

Throws:

Nothing.

Postcondition:

*this is no longer associated with any Lockable object. mutex() returns NULL and owns_lock() returns false.

template<typename Lockable>
class shared_lock
{
public:
    explicit shared_lock(Lockable& m_);
    shared_lock(Lockable& m_,adopt_lock_t);
    shared_lock(Lockable& m_,defer_lock_t);
    shared_lock(Lockable& m_,try_to_lock_t);
    shared_lock(Lockable& m_,system_time const& target_time);
    shared_lock(detail::thread_move_t<shared_lock<Lockable> > other);
    shared_lock(detail::thread_move_t<unique_lock<Lockable> > other);
    shared_lock(detail::thread_move_t<upgrade_lock<Lockable> > other);

    ~shared_lock();

    operator detail::thread_move_t<shared_lock<Lockable> >();
    detail::thread_move_t<shared_lock<Lockable> > move();

    shared_lock& operator=(detail::thread_move_t<shared_lock<Lockable> > other);
    shared_lock& operator=(detail::thread_move_t<unique_lock<Lockable> > other);
    shared_lock& operator=(detail::thread_move_t<upgrade_lock<Lockable> > other);
    void swap(shared_lock& other);

    void lock();
    bool try_lock();
    bool timed_lock(boost::system_time const& target_time);
    void unlock();

    operator unspecified-bool-type() const;
    bool operator!() const;
    bool owns_lock() const;
};

Like boost::unique_lock, boost::shared_lock models the Lockable concept, but rather than acquiring unique ownership of the supplied Lockable object, locking an instance of boost::shared_lock acquires shared ownership.

Like boost::unique_lock, not only does it provide for RAII-style locking, it also allows for deferring acquiring the lock until the lock() member function is called explicitly, or trying to acquire the lock in a non-blocking fashion, or with a timeout. Consequently, unlock() is only called in the destructor if the lock object has locked the Lockable object, or otherwise adopted a lock on the Lockable object.

An instance of boost::shared_lock is said to own the lock state of a Lockable m if mutex() returns a pointer to m and owns_lock() returns true. If an object that owns the lock state of a Lockable object is destroyed, then the destructor will invoke mutex()->unlock_shared().

The member functions of boost::shared_lock are not thread-safe. In particular, boost::shared_lock is intended to model the shared ownership of a Lockable object by a particular thread, and the member functions that release ownership of the lock state (including the destructor) must be called by the same thread that acquired ownership of the lock state.

Effects:

Stores a reference to m. Invokes m.lock_shared().

Postcondition:

owns_lock() returns true. mutex() returns &m.

Throws:

Any exception thrown by the call to m.lock_shared().

Precondition:

The current thread owns an exclusive lock on m.

Effects:

Stores a reference to m. Takes ownership of the lock state of m.

Postcondition:

owns_lock() returns true. mutex() returns &m.

Throws:

Nothing.

Effects:

Stores a reference to m.

Postcondition:

owns_lock() returns false. mutex() returns &m.

Throws:

Nothing.

Effects:

Stores a reference to m. Invokes m.try_lock_shared(), and takes ownership of the lock state if the call returns true.

Postcondition:

mutex() returns &m. If the call to try_lock_shared() returned true, then owns_lock() returns true, otherwise owns_lock() returns false.

Throws:

Nothing.

Effects:

Stores a reference to m. Invokes m.timed_lock(abs_time), and takes ownership of the lock state if the call returns true.

Postcondition:

mutex() returns &m. If the call to timed_lock_shared() returned true, then owns_lock() returns true, otherwise owns_lock() returns false.

Throws:

Any exceptions thrown by the call to m.timed_lock(abs_time).

Effects:

Invokes mutex()-> unlock_shared() if owns_lock() returns true.

Throws:

Nothing.

Returns:

true if the *this owns the lock on the Lockable object associated with *this.

Throws:

Nothing.

Returns:

A pointer to the Lockable object associated with *this, or NULL if there is no such object.

Throws:

Nothing.

Returns:

If owns_lock() would return true, a value that evaluates to true in boolean contexts, otherwise a value that evaluates to false in boolean contexts.

Throws:

Nothing.

Returns:

! owns_lock().

Throws:

Nothing.

Effects:

The association between *this and the Lockable object is removed, without affecting the lock state of the Lockable object. If owns_lock() would have returned true, it is the responsibility of the calling code to ensure that the Lockable is correctly unlocked.

Returns:

A pointer to the Lockable object associated with *this at the point of the call, or NULL if there is no such object.

Throws:

Nothing.

Postcondition:

*this is no longer associated with any Lockable object. mutex() returns NULL and owns_lock() returns false.

template<typename Lockable>
class upgrade_lock
{
public:
    explicit upgrade_lock(Lockable& m_);

    upgrade_lock(detail::thread_move_t<upgrade_lock<Lockable> > other);
    upgrade_lock(detail::thread_move_t<unique_lock<Lockable> > other);

    ~upgrade_lock();

    operator detail::thread_move_t<upgrade_lock<Lockable> >();
    detail::thread_move_t<upgrade_lock<Lockable> > move();

    upgrade_lock& operator=(detail::thread_move_t<upgrade_lock<Lockable> > other);
    upgrade_lock& operator=(detail::thread_move_t<unique_lock<Lockable> > other);

    void swap(upgrade_lock& other);

    void lock();
    void unlock();

    operator unspecified-bool-type() const;
    bool operator!() const;
    bool owns_lock() const;
};

Like boost::unique_lock, boost::upgrade_lock models the Lockable concept, but rather than acquiring unique ownership of the supplied Lockable object, locking an instance of boost::upgrade_lock acquires upgrade ownership.

Like boost::unique_lock, not only does it provide for RAII-style locking, it also allows for deferring acquiring the lock until the lock() member function is called explicitly, or trying to acquire the lock in a non-blocking fashion, or with a timeout. Consequently, unlock() is only called in the destructor if the lock object has locked the Lockable object, or otherwise adopted a lock on the Lockable object.

An instance of boost::upgrade_lock is said to own the lock state of a Lockable m if mutex() returns a pointer to m and owns_lock() returns true. If an object that owns the lock state of a Lockable object is destroyed, then the destructor will invoke mutex()->unlock_upgrade().

The member functions of boost::upgrade_lock are not thread-safe. In particular, boost::upgrade_lock is intended to model the upgrade ownership of a Lockable object by a particular thread, and the member functions that release ownership of the lock state (including the destructor) must be called by the same thread that acquired ownership of the lock state.

template <class Lockable>
class upgrade_to_unique_lock
{
public:
    explicit upgrade_to_unique_lock(upgrade_lock<Lockable>& m_);

    ~upgrade_to_unique_lock();

    upgrade_to_unique_lock(detail::thread_move_t<upgrade_to_unique_lock<Lockable> > other);
    upgrade_to_unique_lock& operator=(detail::thread_move_t<upgrade_to_unique_lock<Lockable> > other);
    void swap(upgrade_to_unique_lock& other);

    operator unspecified-bool-type() const;
    bool operator!() const;
    bool owns_lock() const;
};

boost::upgrade_to_unique_lock allows for a temporary upgrade of an boost::upgrade_lock to exclusive ownership. When constructed with a reference to an instance of boost::upgrade_lock, if that instance has upgrade ownership on some Lockable object, that ownership is upgraded to exclusive ownership. When the boost::upgrade_to_unique_lock instance is destroyed, the ownership of the Lockable is downgraded back to upgrade ownership.

class mutex:
    boost::noncopyable
{
public:
    mutex();
    ~mutex();
    
    void lock();
    bool try_lock();
    void unlock();
    
    typedef unique_lock<mutex> scoped_lock;
    typedef scoped_lock scoped_try_lock;
};

boost::mutex implements the Lockable concept to provide an exclusive-ownership mutex. At most one thread can own the lock on a given instance of boost::mutex at any time. Multiple concurrent calls to lock(), try_lock() and unlock() shall be permitted.

typedef mutex try_mutex;

boost::try_mutex is a typedef to boost::mutex, provided for backwards compatibility with previous releases of boost.

class timed_mutex:
    boost::noncopyable
{
public:
    timed_mutex();
    ~timed_mutex();

    void lock();
    void unlock();
    bool try_lock();
    bool timed_lock(system_time const & abs_time);

    template<typename TimeDuration>
    bool timed_lock(TimeDuration const & relative_time);

    typedef unique_lock<timed_mutex> scoped_timed_lock;
    typedef scoped_timed_lock scoped_try_lock;
    typedef scoped_timed_lock scoped_lock;
};

boost::timed_mutex implements the TimedLockable concept to provide an exclusive-ownership mutex. At most one thread can own the lock on a given instance of boost::timed_mutex at any time. Multiple concurrent calls to lock(), try_lock(), timed_lock(), timed_lock() and unlock() shall be permitted.

class recursive_mutex:
    boost::noncopyable
{
public:
    recursive_mutex();
    ~recursive_mutex();
    
    void lock();
    bool try_lock();
    void unlock();
    
    typedef unique_lock<recursive_mutex> scoped_lock;
    typedef scoped_lock scoped_try_lock;
};

boost::recursive_mutex implements the Lockable concept to provide an exclusive-ownership recursive mutex. At most one thread can own the lock on a given instance of boost::recursive_mutex at any time. Multiple concurrent calls to lock(), try_lock() and unlock() shall be permitted. A thread that already has exclusive ownership of a given boost::recursive_mutex instance can call lock() or try_lock() to acquire an additional level of ownership of the mutex. unlock() must be called once for each level of ownership acquired by a single thread before ownership can be acquired by another thread.

typedef recursive_mutex recursive_try_mutex;

boost::recursive_try_mutex is a typedef to boost::recursive_mutex, provided for backwards compatibility with previous releases of boost.

class recursive_timed_mutex:
    boost::noncopyable
{
public:
    recursive_timed_mutex();
    ~recursive_timed_mutex();
    
    void lock();
    bool try_lock();
    void unlock();

    bool timed_lock(system_time const & abs_time);

    template<typename TimeDuration>
    bool timed_lock(TimeDuration const & relative_time);
    
    typedef unique_lock<recursive_timed_mutex> scoped_lock;
    typedef scoped_lock scoped_try_lock;
    typedef scoped_lock scoped_timed_lock;
};

boost::recursive_timed_mutex implements the TimedLockable concept to provide an exclusive-ownership recursive mutex. At most one thread can own the lock on a given instance of boost::recursive_timed_mutex at any time. Multiple concurrent calls to lock(), try_lock(), timed_lock(), timed_lock() and unlock() shall be permitted. A thread that already has exclusive ownership of a given boost::recursive_timed_mutex instance can call lock(), timed_lock(), timed_lock() or try_lock() to acquire an additional level of ownership of the mutex. unlock() must be called once for each level of ownership acquired by a single thread before ownership can be acquired by another thread.

class shared_mutex
{
public:
    shared_mutex();
    ~shared_mutex();

    void lock_shared();
    bool try_lock_shared();
    bool timed_lock_shared(system_time const& timeout);
    void unlock_shared();

    void lock();
    bool try_lock();
    bool timed_lock(system_time const& timeout);
    void unlock();

    void lock_upgrade();
    void unlock_upgrade();

    void unlock_upgrade_and_lock();
    void unlock_and_lock_upgrade();
    void unlock_and_lock_shared();
    void unlock_upgrade_and_lock_shared();
};

The class boost::shared_mutex provides an implementation of a multiple-reader / single-writer mutex. It implements the UpgradeLockable concept.

Multiple concurrent calls to lock(), try_lock(), timed_lock(), lock_shared(), try_lock_shared() and timed_lock_shared() shall be permitted.

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.

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

        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);
    };
}

Effects:

Constructs an object of class condition_variable.

Throws:

boost::thread_resource_error if an error occurs.

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.

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.

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.

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:

boost::thread_resource_error if an error occurs. boost::thread_interrupted if the wait was interrupted by a call to interrupt() on the boost::thread object associated with the current thread of execution.

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:

boost::thread_resource_error if an error occurs. boost::thread_interrupted if the wait was interrupted by a call to interrupt() on the boost::thread object associated with the current thread of execution.

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 function exits with an exception.

Returns:

false if the call is returning because the time period specified by rel_time has elapsed, true otherwise.

Postcondition:

lock is locked by the current thread.

Throws:

boost::thread_resource_error if an error occurs. boost::thread_interrupted if the wait was interrupted by a call to interrupt() on the boost::thread object associated with the current thread of execution.

[Note] Note

The duration overload of timed_wait is difficult to use correctly. The overload taking a predicate should be preferred in most cases.

Effects:

As-if

while(!pred())
{
    if(!timed_wait(lock,abs_time))
    {
        return pred();
    }
}
return true;

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

        template<typename lock_type>
        void wait(lock_type& lock);

        template<typename lock_type,typename predicate_type>
        void wait(lock_type& lock,predicate_type predicate);

        template<typename lock_type>
        bool timed_wait(lock_type& lock,boost::system_time const& abs_time);

        template<typename lock_type,typename duration_type>
        bool timed_wait(lock_type& lock,duration_type const& rel_time);

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

        template<typename lock_type,typename duration_type,typename predicate_type>
        bool timed_wait(lock_type& lock,duration_type const& rel_time,predicate_type predicate);

    // backwards compatibility

        template<typename lock_type>
        bool timed_wait(lock_type>& lock,boost::xtime const& abs_time);

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

Effects:

Constructs an object of class condition_variable_any.

Throws:

boost::thread_resource_error if an error occurs.

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.

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.

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.

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:

boost::thread_resource_error if an error occurs. boost::thread_interrupted if the wait was interrupted by a call to interrupt() on the boost::thread object associated with the current thread of execution.

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:

boost::thread_resource_error if an error occurs. boost::thread_interrupted if the wait was interrupted by a call to interrupt() on the boost::thread object associated with the current thread of execution.

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 function exits with an exception.

Returns:

false if the call is returning because the time period specified by rel_time has elapsed, true otherwise.

Postcondition:

lock is locked by the current thread.

Throws:

boost::thread_resource_error if an error occurs. boost::thread_interrupted if the wait was interrupted by a call to interrupt() on the boost::thread object associated with the current thread of execution.

[Note] Note

The duration overload of timed_wait is difficult to use correctly. The overload taking a predicate should be preferred in most cases.

Effects:

As-if

while(!pred())
{
    if(!timed_wait(lock,abs_time))
    {
        return pred();
    }
}
return true;

typedef condition_variable_any condition;

The typedef condition is provided for backwards compatibility with previous boost releases.

boost::call_once provides a mechanism for ensuring that an initialization routine is run exactly once without data races or deadlocks.

typedef platform-specific-type once_flag;
#define BOOST_ONCE_INIT platform-specific-initializer

Objects of type boost::once_flag shall be initialized with BOOST_ONCE_INIT:

boost::once_flag f=BOOST_ONCE_INIT;
template<typename Callable>
void call_once(once_flag& flag,Callable func);

Requires:

Callable is CopyConstructible. Copying func shall have no side effects, and the effect of calling the copy shall be equivalent to calling the original.

Effects:

Calls to call_once on the same once_flag object are serialized. If there has been no prior effective call_once on the same once_flag object, the argument func (or a copy thereof) is called as-if by invoking func(args), and the invocation of call_once is effective if and only if func(args) returns without exception. If an exception is thrown, the exception is propagated to the caller. If there has been a prior effective call_once on the same once_flag object, the call_once returns without invoking func.

Synchronization:

The completion of an effective call_once invocation on a once_flag object, synchronizes with all subsequent call_once invocations on the same once_flag object.

Throws:

thread_resource_error when the effects cannot be achieved. or any exception propagated from func.

void call_once(void (*func)(),once_flag& flag);

This second overload is provided for backwards compatibility. The effects of call_once(func,flag) shall be the same as those of call_once(flag,func).

A barrier is a simple concept. Also known as a rendezvous, it is a synchronization point between multiple threads. The barrier is configured for a particular number of threads (n), and as threads reach the barrier they must wait until all n threads have arrived. Once the n-th thread has reached the barrier, all the waiting threads can proceed, and the barrier is reset.

#include <boost/thread/barrier.hpp>

class barrier
{
public:
    barrier(unsigned int count);
    ~barrier();

    bool wait();
};

Instances of boost::barrier are not copyable or movable.

Constructor
barrier(unsigned int count);

Effects:

Construct a barrier for count threads.

Throws:

boost::thread_resource_error if an error occurs.

Destructor
~barrier();

Precondition:

No threads are waiting on *this.

Effects:

Destroys *this.

Throws:

Nothing.

Member function wait
bool wait();

Effects:

Block until count threads have called wait on *this. When the count-th thread calls wait, all waiting threads are unblocked, and the barrier is reset.

Returns:

true for exactly one thread from each batch of waiting threads, false otherwise.

Throws:

boost::thread_resource_error if an error occurs.


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