std::condition_variable_any::wait

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.

↑ 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

External links

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

[1] This can happen if the re-locking of the mutex throws an exception.

[1]

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Member functions
condition_variable_any::condition_variable_any
condition_variable_any::~condition_variable_any
Notification
condition_variable_any::notify_one
condition_variable_any::notify_all
Waiting
condition_variable_any::wait
condition_variable_any::wait_for
condition_variable_any::wait_until

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