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.
lock.unlock() and blocks on *this.lock.lock() (possibly blocking on the lock), then returns.while (!pred())wait(lock);
*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 towhile (!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
pred() before returning to the caller.Exceptions
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
#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
| blocks the current thread until the condition variable is awakened or after the specified timeout duration (public member function) | |
| 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. |