Thread support library
this_thread namespace
Mutual exclusion
Generic lock management
Condition variables
Latches and barriers
template< class Lock, class Rep, class Period >

std::cv_status wait_for( Lock& lock,

                         const std::chrono::duration<Rep, Period>& rel_time);
(1) (since C++11)
template< class Lock, class Rep, class Period, class Predicate >

bool wait_for( Lock& lock,
               const std::chrono::duration<Rep, Period>& rel_time,

               Predicate pred);
(2) (since C++11)
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. If this function exits via exception, lock is also reacquired. (until C++14)
2) Equivalent to return wait_until(lock, std::chrono::steady_clock::now() + rel_time, std::move(pred));. This overload may be used to ignore spurious awakenings.

A steady clock is used to measure the duration. This function may block for longer than timeout_duration due to scheduling or resource contention delays.

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, (since C++14)


[edit] Parameters

lock - an object of type Lock that meets the BasicLockable requirements, which must be locked by the current thread
rel_time - an object of type std::chrono::duration representing the maximum time to spend waiting. Note that rel_time must be small enough not to overflow when added to std::chrono::steady_clock::now().
pred - predicate which returns ​false if the waiting should be continued.

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

 bool pred();

[edit] 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 pred still evaluates to false after the rel_time timeout expired, otherwise true.

[edit] Exceptions


May throw std::system_error, may also propagate exceptions thrown by lock.lock() or lock.unlock().

(until C++14)

Any exception thrown by clock, time_point, or duration during the execution (clocks, time points, and durations provided by the standard library never throw)

(since C++14)
2) Same as (1) but may also propagate exceptions thrown by pred

[edit] 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.

[edit] Example

#include <iostream>
#include <atomic>
#include <condition_variable>
#include <thread>
#include <chrono>
using namespace std::chrono_literals;
std::condition_variable_any 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';
        std::cerr << "Thread " << idx << " timed out. i == " << i << '\n';
void signals()
    std::cerr << "Notifying...\n";
        std::lock_guard<std::mutex> lk(cv_m);
        i = 1;
    std::cerr << "Notifying again...\n";
int main()
    std::thread t1(waits, 1), t2(waits, 2), t3(waits, 3), t4(signals);


Thread 1 timed out. i == 0
Thread 2 timed out. i == 0
Notifying again...
Thread 3 finished waiting. i == 1

[edit] See also

blocks the current thread until the condition variable is woken up
(public member function) [edit]
blocks the current thread until the condition variable is woken up or until specified time point has been reached
(public member function) [edit]