std::condition_variable_any::wait_for
| template< class Lock, class Rep, class Period > std::cv_status wait_for( Lock& lock, |
(1) | (since C++11) |
| template< class Lock, class Rep, class Period, class Predicate > bool wait_for( Lock& lock, const std::chrono::duration<Rep, Period>& rel_time, |
(2) | (since C++11) |
| template< class Lock, class Rep, class Period, class Predicate > bool wait_for( Lock& lock, std::stop_token stoken, |
(3) | (since C++20) |
wait_for causes the current thread to block until the condition variable is notified, the given duration has been elapsed, or a spurious wakeup occurs. pred can be optionally provided to detect spurious wakeup.
std::chrono::steady_clock::now() + rel_time, std::move(pred));.
Right after wait_for 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.
Contents |
[edit] Parameters
| lock | - | an lock which must be locked by the calling thread |
| stoken | - | a stop token to register interruption for |
| rel_time | - | the maximum duration to wait |
| 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. | ||
[edit] Return value
[edit] Exceptions
[edit] Notes
Even if notified under lock, overload (1) makes no guarantees about the state of the associated predicate when returning due to timeout.
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.
[edit] 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
[edit] 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 2093 | C++11 | timeout-related exceptions were missing in the specification | mentions these exceptions |
| 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 |
[edit] See also
| blocks the current thread until the condition variable is awakened (public member function) | |
| blocks the current thread until the condition variable is awakened or until specified time point has been reached (public member function) |
