std::condition_variable_any::wait_for - cppreference.com (original) (raw)
| template< class Lock, class Rep, class Period > std::cv_status wait_for( Lock& lock, const std::chrono::duration<Rep, Period>& rel_time ); | (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, Predicate pred ); | (2) | (since C++11) |
| template< class Lock, class Rep, class Period, class Predicate > bool wait_for( Lock& lock, std::stop_token stoken, const std::chrono::duration<Rep, Period>& rel_time, Predicate pred ); | (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.
2,3) Waiting for a specific condition to become true, can be used to ignore spurious awakenings.
- 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 return wait_until(lock, std::move(stoken),
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
2,3) The latest result of pred() before returning to the caller.
[edit] Exceptions
- Timeout-related exceptions.
2,3) Timeout-related exceptions, and any exception thrown by pred.
[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
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) [edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | blocks the current thread until the condition variable is awakened or until specified time point has been reached (public member function) [edit] |