std::condition_variable::wait_until
| template< class Clock, class Duration > std::cv_status |
(1) | (since C++11) |
| template< class Clock, class Duration, class Predicate > bool wait_until( std::unique_lock<std::mutex>& lock, |
(2) | (since C++11) |
wait_until causes the current thread to block until the condition variable is notified, a specific time is reached, or a spurious wakeup occurs, optionally looping until some predicate is satisfied (bool(stop_waiting()) == true).
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, or when the absolute time point timeout_time is reached. It may also be unblocked spuriously. When unblocked, regardless of the reason, lock is reacquired and wait_until exits.while (!stop_waiting()) { if (wait_until(lock, timeout_time) == std::cv_status::timeout) { return stop_waiting(); } } return true;
Calling this function if lock.mutex() is not locked by the current thread is undefined behavior.
Calling this function if lock.mutex() is not the same mutex as the one used by all other threads that are currently waiting on the same condition variable is undefined behavior.
If these functions fail to meet the postcondition (lock.owns_lock()==true and lock.mutex() is locked by the calling thread), std::terminate is called. For example, this could happen if relocking the mutex throws an exception.
Clock must meet the Clock requirements. The program is ill-formed if std::chrono::is_clock_v<Clock> is false. (since C++20)
Parameters
| lock | - | an object of type std::unique_lock<std::mutex>, which must be locked by the current thread |
| timeout_time | - | an object of type std::chrono::time_point representing the time when to stop waiting |
| 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
timeout_time was reached, std::cv_status::no_timeout otherwise.stop_waiting still evaluates to false after the timeout_time timeout expired, otherwise true. If the timeout had already expired, evaluates and returns the result of stop_waiting.
Exceptions
stop_waiting
Notes
The standard recommends that the clock tied to timeout_time be used to measure time; that clock is not required to be a monotonic clock. There are no guarantees regarding the behavior of this function if the clock is adjusted discontinuously, but the existing implementations convert timeout_time from Clock to std::chrono::system_clock and delegate to POSIX pthread_cond_timedwait so that the wait honors adjustments to the system clock, but not to the user-provided Clock. In any case, the function also may wait for longer than until after timeout_time has been reached due to scheduling or resource contention delays.
Even if the clock in use is std::chrono::steady_clock or another monotonic clock, a system clock adjustment may induce a spurious wakeup.
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 <iostream> #include <atomic> #include <condition_variable> #include <thread> #include <chrono> using namespace std::chrono_literals; std::condition_variable cv; std::mutex cv_m; std::atomic<int> i{0}; void waits(int idx) { std::unique_lock<std::mutex> lk(cv_m); auto now = std::chrono::system_clock::now(); if(cv.wait_until(lk, now + idx*100ms, [](){return i == 1;})) std::cerr << "Thread " << idx << " finished waiting. i == " << i << '\n'; else std::cerr << "Thread " << idx << " timed out. i == " << i << '\n'; } void signals() { std::this_thread::sleep_for(120ms); std::cerr << "Notifying...\n"; cv.notify_all(); std::this_thread::sleep_for(100ms); i = 1; std::cerr << "Notifying again...\n"; cv.notify_all(); } int main() { std::thread t1(waits, 1), t2(waits, 2), t3(waits, 3), t4(signals); t1.join(); t2.join(); t3.join(); t4.join(); }
Possible output:
Thread 1 timed out. i == 0 Notifying... Thread 2 timed out. i == 0 Notifying again... Thread 3 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 2093 | C++11 | timeout-related exceptions were missing in the specification | mentioned |
| LWG 2135 | C++11 | wait_until threw an exception on unlocking/relocking failure
|
calls std::terminate |
See also
| blocks the current thread until the condition variable is woken up (public member function) | |
| blocks the current thread until the condition variable is woken up or after the specified timeout duration (public member function) |