std::condition_variable::wait

 

C++

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Regular expressions library (C++11)

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Technical Specifications

 

Thread support library

Threads
thread (C++11)
this_thread namespace
get_id (C++11) yield (C++11)	sleep_for (C++11) sleep_until (C++11)
Mutual exclusion
mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17)	timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14)
Generic lock management
lock_guard (C++11) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)	lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11)
Condition variables
condition_variable (C++11)
condition_variable_any (C++11)
notify_all_at_thread_exit (C++11)
cv_status (C++11)
Futures
promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11)	launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

 

std::condition_variable

Member functions

condition_variable::condition_variable

condition_variable::~condition_variable

Notification

condition_variable::notify_one

condition_variable::notify_all

Waiting

condition_variable::wait

condition_variable::wait_for

condition_variable::wait_until

Native handle

condition_variable::native handle

 

void wait( std::unique_lock<std::mutex>& lock );	(1)	(since C++11)
template< class Predicate > void wait( std::unique_lock<std::mutex>& lock, Predicate pred );	(2)	(since C++11)

wait causes the current thread to block until the condition variable is notified or a spurious wakeup occurs, optionally looping until some predicate is satisfied.

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. It may also be unblocked spuriously. When unblocked, regardless of the reason, lock is reacquired and wait exits. If this function exits via exception, lock is also reacquired. (until C++14)

2) Equivalent to

while (!pred()) {
    wait(lock);
}

This overload may be used to ignore spurious awakenings while waiting for a specific condition to become true. Note that before enter to this method lock must be acquired, after wait(lock) exits it is also reacquired, i.e. lock can be used as a guard to pred() access.

If these functions fail to meet the postconditions (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,

(since C++14)

Parameters

lock	-	an object of type std::unique_lock<std::mutex>, which must be locked by the current thread
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();​

Return value

(none)

Exceptions

May throw std::system_error, may also propagate exceptions thrown by lock.lock() or lock.unlock().	(until C++14)
Does not throw	(since C++14)

Notes

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.

Example

Run this code

#include <iostream>
#include <condition_variable>
#include <thread>
#include <chrono>
 
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

See also

wait_for	blocks the current thread until the condition variable is woken up or after the specified timeout duration (public member function)
wait_until	blocks the current thread until the condition variable is woken up or until specified time point has been reached (public member function)