std::{{{1}}}::wait

template< class Lock > void wait( Lock& lock );	(1)	(since C++11)
template< class Lock, class Predicate > void wait( Lock& lock, Predicate stop_waiting );	(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 (bool(stop_waiting()) == true).

1) Atomically unlocks 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.

2) Equivalent to

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

This overload may be used to ignore spurious awakenings while waiting for a specific condition to become true.

Note that lock must be acquired before entering this method, and it is reacquired after wait(lock) exits, which means that lock can be used to guard access to stop_waiting().

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.

Parameters

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

Return value

(none)

Exceptions

1) Does not throw.

2) Same as (1) but may also propagate exceptions thrown by stop_waiting.

Notes

The effects of notify_one()/notify_all() and each of the three atomic parts of wait()/wait_for()/wait_until() (unlock+wait, wakeup, and lock) take place in a single total order that can be viewed as modification order of an atomic variable: the order is specific to this individual condition variable. This makes it impossible for notify_one() to, for example, be delayed and unblock a thread that started waiting just after the call to notify_one() was made.

Example

Run this code

#include <chrono>
#include <condition_variable>
#include <iostream>
#include <thread>
 
std:: 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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2135	C++11	wait threw an exception on unlocking/relocking failure	calls std::terminate

See also

wait_for	blocks the current thread until the condition variable is awakened or after the specified timeout duration (public member function of std::{{{1}}})
wait_until	blocks the current thread until the condition variable is awakened or until specified time point has been reached (public member function of std::{{{1}}})
C documentation for cnd_wait

External links

1.	The Old New Thing article: Spurious wake-ups in Win32 condition variables.