NAME¶

std::condition_variable_any::wait - std::condition_variable_any::wait

Synopsis¶

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

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();
If pred() is ill-formed, or
the return value of pred() is not convertible to bool
(until C++20)
decltype(pred()) does not model boolean-testable
(since C++20), the program is ill-formed.

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.

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 2135 C++11 the behavior was unclear if calls std::terminate in this
lock.lock() throws an exception case

See also¶

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

External links¶

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

Hidden category:¶

* Pages with unreviewed LWG DR marker