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 > (2) (since C++11)
void wait( std::unique_lock<std::mutex>& lock, 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) Equivalent to
while (!pred())
wait(lock);.
This overload may be used to ignore spurious awakenings while waiting for a specific
condition to become true.
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.owns_lock() is true, and lock.mutex() is locked by
the calling thread. If these postconditions cannot be satisfied^[1], calls
std::terminate.

If any of the following conditions is satisfied, the behavior is undefined:

* lock.owns_lock() is false.
* lock.mutex() is not locked by the calling thread.
* If some other threads are also waiting on *this, lock.mutex() is different from
the mutex unlocked by the waiting functions (wait, wait_for and wait_until)
called on *this by those threads.
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
pred - the predicate to check whether the waiting can be completed

Type requirements¶

-
Predicate must meet the requirements of FunctionObject.

Exceptions¶

1) Throws nothing.
2) Any exception thrown by pred.

Notes¶

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 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