std::condition_variable_any::wait - cppreference.com (original) (raw)
| 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.
- 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.
- Equivalent to
while (!pred())
wait(lock);
- 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();
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.
Contents
- 1 Parameters
- 2 Return value
- 3 Exceptions
- 4 Notes
- 5 Example
- 6 Defect reports
- 7 See also
- 8 External links
[edit] 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. |
[edit] Return value
1,2) (none)
- The latest result of pred() before returning to the caller.
[edit] Exceptions
- Does not throw.
2,3) Any exception thrown by pred.
[edit] 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.
[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 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 or after the specified timeout duration (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] | | |