NAME¶

std::condition_variable_any::wait_until - std::condition_variable_any::wait_until

Synopsis¶

template< class Lock, class Clock, class Duration >

std::cv_status (1) (since C++11)
wait_until( Lock& lock,

const std::chrono::time_point<Clock, Duration>& timeout_time );
template< class Lock, class Clock, class Duration, class Predicate
>

bool wait_until( Lock& lock, (2) (since C++11)
const std::chrono::time_point<Clock, Duration>& timeout_time,

Predicate stop_waiting );
template< class Lock, class Clock, class Duration, class Predicate
>

bool wait_until( Lock& lock, (3) (since C++20)
std::stop_token stoken,
const std::chrono::time_point<Clock, Duration>& timeout_time,

Predicate stop_waiting );

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).

1) Atomically releases 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.
2) Equivalent to

while (!stop_waiting()) {
if (wait_until(lock, timeout_time) == std::cv_status::timeout) {
return stop_waiting();
}
}
return true;

This overload may be used to ignore spurious wakeups.

3) An interruptible wait: registers the condition_variable_any for the duration of
wait_until(), to be notified if a stop request is made on the given stoken's
associated stop-state; it is then equivalent to

while (!stoken.stop_requested()) {
if (stop_waiting())
return true;
if (wait_until(lock, timeout_time) == std::cv_status::timeout)
return stop_waiting();
}
return stop_waiting();

If these functions fail to meet the postcondition (lock 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 Lock that meets the requirements of BasicLockable,
which must be locked by the current thread
stoken - a std::stop_token to register interruption for
timeout_time - an object of type std::chrono::time_point representing the time when
to stop waiting
predicate which returns false if the waiting should be continued
(bool(stop_waiting()) == false).

stop_waiting - The signature of the predicate function should be equivalent to the
following:

bool pred();

Return value¶

1) std::cv_status::timeout if the absolute timeout specified by timeout_time was
reached, std::cv_status::no_timeout otherwise.
2) false if the predicate 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.
3) stop_waiting(), regardless of whether the timeout was met or stop was requested.

Exceptions¶

1) Any exception thrown by clock, time point, or duration during the execution
(clocks, time points, and durations provided by the standard library never throw)
2) Same as (1) but may also propagate exceptions thrown by stop_waiting
3) Same as (2)

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¶

// Run this code

#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 mentioned
the specification
LWG 2135 C++11 wait_until threw an exception on calls std::terminate
unlocking/relocking failure

See also¶

wait 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
wait_for the specified timeout duration
(public member function)