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std::condition_variable::wait

From cppreference.com
 
 
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void wait( std::unique_lock<std::mutex>& lock );
(1) (since C++11)
template< class Predicate >
void wait( std::unique_lock<std::mutex>& 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. pred can be optionally provided to detect spurious wakeup.

1) Atomically calls lock.unlock() and blocks on *this.
The thread will be unblocked when notify_all() or notify_one() is executed. It may also be unblocked spuriously.
When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
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.
If pred() is ill-formed, or the return value of pred() is not convertible to bool(until C++20) decltype(pred()) does not model boolean-testable(since C++20), the program is ill-formed.

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.

Contents

[edit] Parameters

lock - an lock which must be locked by the calling thread
pred - the predicate to check whether the waiting can be completed
Type requirements
-
Predicate must meet the requirements of FunctionObject.

[edit] Exceptions

1) Throws nothing.
2) Any exception thrown by pred.

[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));
    {
        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

[edit] 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 the behavior was unclear if lock.lock() throws an exception calls std::terminate in this case

[edit] See also

blocks the current thread until the condition variable is awakened or after the specified timeout duration
(public member function) [edit]
blocks the current thread until the condition variable is awakened or until specified time point has been reached
(public member function) [edit]
C documentation for cnd_wait

[edit] External links

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