NAME¶

std::condition_variable::wait - std::condition_variable::wait

Synopsis¶

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

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
(bool(stop_waiting()) == true).

1) Atomically unlocks 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.
2) Equivalent to

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

This overload may be used to ignore spurious awakenings while waiting for a specific
condition to become true.

Note that lock must be acquired before entering this method, and it is reacquired
after wait(lock) exits, which means that lock can be used to guard access to
stop_waiting().

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.

Parameters¶

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

(none)

Exceptions¶

1) Does not throw
2) Same as (1) but may also propagate exceptions thrown by stop_waiting

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.

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 <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

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 2135 C++11 wait threw an exception on calls std::terminate
unlocking/relocking failure

See also¶

blocks the current thread until the condition variable is woken up or
wait_for after the specified timeout duration
(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)