template<classLock,classRep,classPeriod>std::cv_statuswait_for(Lock&lock,conststd::chrono::duration<Rep,Period>&rel_time); (1) (since C++11)template<classLock,classRep,classPeriod,classPredicate>boolwait_for(Lock&lock,conststd::chrono::duration<Rep,Period>&rel_time,Predicatepred); (2) (since C++11)template<classLock,classRep,classPeriod,classPredicate>boolwait_for(Lock&lock,std::stop_tokenstoken,conststd::chrono::duration<Rep,Period>&rel_time,Predicatepred); (3) (since C++20)wait_for causes the current thread to block until the condition variable is notified, the given duration has been elapsed, or a spurious wakeup occurs. pred can be optionally provided to detect spurious wakeup.
1) Equivalent to returnwait_until(lock,std::chrono::steady_clock::now()+rel_time);.
2,3) Waiting for a specific condition to become true, can be used to ignore spurious awakenings.
2) Equivalent to returnwait_until(lock,std::chrono::steady_clock::now()+rel_time,std::move(pred));.
3) Registers *this for the duration of this call, to be notified if a stop request is made on stoken's associated stop-state; it is then equivalent to returnwait_until(lock,std::move(stoken),
std::chrono::steady_clock::now()+rel_time,std::move(pred));.
Right after wait_for returns, lock is locked by the calling thread. If this postcondition cannot be satisfied
, calls
.
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 stoken - a stop token to register interruption for rel_time - the maximum duration to wait pred - the predicate to check whether the waiting can be completed Type requirements -Lock must meet the requirements of
. -Predicate must meet the requirements of
. -pred() must be a valid expression, and its type and value category must meet the
requirements. Return value
1)std::cv_status::timeout if rel_time has been elapsed since the beginning of this call, otherwise std::cv_status::no_timeout.
2,3) The latest result of pred() before returning to the caller.
Exceptions
1) Timeout-related exceptions.
2,3) Timeout-related exceptions, and any exception thrown by pred.
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
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_anycv;std::mutexcv_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 cvinti=0;voidwaits(){std::unique_lock<std::mutex>lk(cv_m);std::cerr<<"Waiting... \n";cv.wait(lk,[]{returni==1;});std::cerr<<"...finished waiting. i == 1\n";}voidsignals(){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();}intmain(){std::threadt1(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
C++11 timeout-related exceptions were missing in the specification mentions these exceptions
(
) C++11 convertibility to bool was too weak to reflect the expectation of implementations requirements strengthened
C++11 the behavior was unclear if lock.lock() throws an exception calls
in this case See also
blocks the current thread until the condition variable is awakened
(public member function)
blocks the current thread until the condition variable is awakened or until specified time point has been reached
(public member function)