std::condition_variable:: wait

wait causes the current thread to block until the condition variable is notified or a spurious wakeup occurs. pred can be optionally provided to detect spurious wakeup.

Right after wait returns, lock. owns_lock ( ) is true , and lock. mutex ( ) is locked by the calling thread. If these postconditions cannot be satisfied ^[1] , calls std::terminate .

If any of the following conditions is satisfied, the behavior is undefined:

• lock. owns_lock ( ) is false .
• lock. mutex ( ) is not locked by the calling thread.
• If some other threads are also waiting on * this , lock. mutex ( ) is different from the mutex unlocked by the waiting functions ( wait , wait_for and wait_until ) called on * this by those threads.

1. ↑ This can happen if the re-locking of the mutex throws an exception.

Parameters

Exceptions

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

#include <chrono>
#include <condition_variable>
#include <iostream>
#include <thread>
 
std::condition_variable 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();
}

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.

See also

External links