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.

[edit] Parameters

[edit] Exceptions

[edit] 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.

[edit] 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));
    {