NAME¶

std::condition_variable::wait_until - std::condition_variable::wait_until

Synopsis¶

template< class Clock, class Duration >

std::cv_status
wait_until( std::unique_lock<std::mutex>& lock, (1) (since C++11)

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

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

Predicate pred );

wait_until causes the current thread to block until the condition variable is
notified, the given time point has been reached, 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, or
abs_time is reached. It may also be unblocked spuriously.
When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
2) Equivalent to while (!pred())
if (wait_until(lock, abs_time) == std::cv_status::timeout)
return pred();
return true;.
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_until 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¶

lock - an lock which must be locked by the calling thread
abs_time - the time point where waiting expires
pred - the predicate to check whether the waiting can be completed

Type requirements¶

-
Predicate must meet the requirements of FunctionObject.

Return value¶

1) std::cv_status::timeout if abs_time has been reached, otherwise
std::cv_status::no_timeout.
2) The latest result of pred() before returning to the caller.

Exceptions¶

1) Timeout-related exceptions.
2) Timeout-related exceptions, and any exception thrown by pred.

Notes¶

The standard recommends that the clock tied to abs_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 abs_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 abs_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 <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

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 mentions these exceptions
missing in the specification
LWG 2135 C++11 the behavior was unclear if calls std::terminate in
lock.lock() throws an exception this case

See also¶

wait blocks the current thread until the condition variable is awakened
(public member function)
blocks the current thread until the condition variable is awakened or after
wait_for the specified timeout duration
(public member function)

Hidden category:¶

* Pages with unreviewed LWG DR marker