std::condition_variable_any::wait

 

C++

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Technical Specifications

 

Thread support library

Threads
thread (C++11)
this_thread namespace
get_id (C++11) yield (C++11)	sleep_for (C++11) sleep_until (C++11)
Mutual exclusion
mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17)	timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14)
Generic lock management
lock_guard (C++11) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)	lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11)
Condition variables
condition_variable (C++11)
condition_variable_any (C++11)
notify_all_at_thread_exit (C++11)
cv_status (C++11)
Futures
promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11)	launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

 

std::condition_variable_any

Member functions

condition_variable_any::condition_variable_any

condition_variable_any::~condition_variable_any

Notification

condition_variable_any::notify_one

condition_variable_any::notify_all

Waiting

condition_variable_any::wait

condition_variable_any::wait_for

condition_variable_any::wait_until

 

template< class Lock > void wait( Lock& lock );	(1)	(since C++11)
template< class Lock, class Predicate > void wait( Lock& lock, Predicate pred );	(2)	(since C++11)

wait causes the current thread to block until the condition variable is notified or a spurious wakeup occurs, optionally looping until some predicate is satisfied.

1) Atomically releases lock, blocks the current executing thread, and adds it to the list of threads waiting on *this. The thread will be unblocked when notify_all() or notify_one() is executed. It may also be unblocked spuriously. When unblocked, regardless of the reason, lock is reacquired and wait exits. If this function exits via exception, lock is also reacquired. (until C++14)

2) Equivalent to

while (!pred()) {
    wait(lock);
}

This overload may be used to ignore spurious awakenings while waiting for a specific condition to become true. Note that before enter to this method lock must be acquired, after wait(lock) exits it is also reacquired, i.e. lock can be used as a guard to pred() access.

If these functions fail to meet the postconditions (lock is locked by the calling thread), std::terminate is called. For example, this could happen if relocking the mutex throws an exception,

(since C++14)

Parameters

lock	-	an object of type Lock that meets the BasicLockable requirements, which must be locked by the current thread
pred	-	predicate which returns ​false if the waiting should be continued. The signature of the predicate function should be equivalent to the following:  bool pred();​

Return value

(none)

Exceptions

May throw std::system_error, may also propagate exceptions thrown by lock.lock() or lock.unlock().	(until C++14)
Does not throw	(since C++14)

Example

Run this code

#include <iostream>
#include <condition_variable>
#include <thread>
#include <chrono>
 
std::condition_variable_any cv;
std::mutex cv_m; // This mutex is used for three purposes:
                 // 1) to synchronize accesses to i
                 // 2) to synchronize accesses to std::cerr
                 // 3) for the condition variable cv
int i = 0;
 
void waits()
{
    std::unique_lock<std::mutex> lk(cv_m);
    std::cerr << "Waiting... \n";
    cv.wait(lk, []{return i == 1;});
    std::cerr << "...finished waiting. i == 1\n";
}
 
void signals()
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
    {
        std::lock_guard<std::mutex> lk(cv_m);
        std::cerr << "Notifying...\n";
    }
    cv.notify_all();
 
    std::this_thread::sleep_for(std::chrono::seconds(1));
 
    {
        std::lock_guard<std::mutex> lk(cv_m);
        i = 1;
        std::cerr << "Notifying again...\n";
    }
    cv.notify_all();
}
 
int main()
{
    std::thread t1(waits), t2(waits), t3(waits), t4(signals);
    t1.join(); 
    t2.join(); 
    t3.join();
    t4.join();
}

Possible output:

Waiting...
Waiting...
Waiting...
Notifying...
Notifying again...
...finished waiting. i == 1
...finished waiting. i == 1
...finished waiting. i == 1

See also

wait_for	blocks the current thread until the condition variable is woken up or after the specified timeout duration (public member function)
wait_until	blocks the current thread until the condition variable is woken up or until specified time point has been reached (public member function)