# std::condition_variable_any::wait

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< cpp‎ | thread‎ | condition variable any

template< class Lock > void wait( Lock& lock ); | (1) | (since C++11)
template< class Lock, class Predicate > void wait( Lock& lock, Predicate pred ); | (2) | (since C++11)
template< class Lock, class Predicate > bool wait( Lock& lock, std::stop_token stoken, Predicate pred ); | (3) | (since C++20)

`wait` causes the current thread to block until the condition variable is notified or a spurious wakeup occurs. pred can be optionally provided to detect spurious wakeup.

1) Atomically calls lock.unlock() and blocks on *this.
The thread will be unblocked when notify_all() or notify_one() is executed. It may also be unblocked spuriously.
When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
2,3) Waiting for a specific condition to become true, can be used to ignore spurious awakenings.
2) Equivalent to
while (!pred())
wait(lock);
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
while (!stoken.stop_requested())
{
if (pred())
return true;
wait(lock);
}
return pred();

Right after `wait` returns, lock is locked by the calling thread. If this postcondition cannot be satisfied[1], calls std::terminate.

1. ↑ 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
pred | - | the predicate to check whether the waiting can be completed
Type requirements
- `Lock` must meet the requirements of BasicLockable.
- `Predicate` must meet the requirements of FunctionObject.
- pred() must be a valid expression, and its type and value category must meet the BooleanTestable requirements.

### Return value

1,2) (none)
3) The latest result of pred() before returning to the caller.

### Exceptions

1) Does not throw.
2,3) Any exception thrown by pred.

### Notes

The returned value of overload (3) indicates whether pred evaluated to true, regardless of whether there was a stop requested or not.

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

    std::condition_variable_any 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 2114 (P2167R3) | C++11 | convertibility to bool was too weak to reflect the expectation of implementations | requirements strengthened
LWG 2135 | C++11 | the behavior was unclear if lock.lock() throws an exception | calls std::terminate in this case

### See also

wait_for | blocks the current thread until the condition variable is awakened or after the specified timeout duration (public member function)
wait_until | blocks the current thread until the condition variable is awakened or until specified time point has been reached (public member function)
C documentation for cnd_wait

### External links

The Old New Thing article: Spurious wake-ups in Win32 condition variables.

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