NAME¶

std::condition_variable::wait_for - std::condition_variable::wait_for

Synopsis¶

template< class Rep, class Period >

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

const std::chrono::duration<Rep, Period>& rel_time);
template< class Rep, class Period, class Predicate >

bool wait_for( std::unique_lock<std::mutex>& lock, (2) (since C++11)
const std::chrono::duration<Rep, Period>& rel_time,

Predicate stop_waiting);

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 relative timeout rel_time expires. It may also
be unblocked spuriously. When unblocked, regardless of the reason, lock is
reacquired and wait_for() exits.
2) Equivalent to return wait_until(lock, std::chrono::steady_clock::now() +
rel_time, std::move(stop_waiting));. This overload may be used to ignore spurious
awakenings by looping until some predicate is satisfied (bool(stop_waiting()) ==
true).

The standard recommends that a steady clock be used to measure the duration. This
function may block for longer than timeout_duration due to scheduling or resource
contention delays.

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.

Parameters¶

lock - an object of type std::unique_lock<std::mutex>, which must be locked
by the current thread
an object of type std::chrono::duration representing the maximum time
rel_time - to spend waiting. Note that rel_time must be small enough not to
overflow when added to std::chrono::steady_clock::now().
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 relative timeout specified by rel_time expired,
std::cv_status::no_timeout otherwise.
2) false if the predicate stop_waiting still evaluates to false after the rel_time
timeout expired, otherwise true.

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

Notes¶

Even if notified under lock, overload (1) makes no guarantees about the state of the
associated predicate when returning due to timeout.

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;
int i;

void waits(int idx)
{
std::unique_lock<std::mutex> lk(cv_m);
if(cv.wait_for(lk, 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);
{
std::lock_guard<std::mutex> lk(cv_m);
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();
}

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_for 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
wait_until until specified time point has been reached
(public member function)