NAME¶

std::condition_variable::wait_for - std::condition_variable::wait_for

Synopsis¶

template< class Rep, class Period >

std::cv_status wait_for( std::unique_lock<std::mutex>& lock, (1) (since C++11)

const std::chrono::duration<Rep, Period>&
rel_time );
template< class Rep, class Period, class Predicate >

bool wait_for( std::unique_lock<std::mutex>& lock, (2) (since C++11)
const std::chrono::duration<Rep, Period>& rel_time,

Predicate pred );

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.

1) Equivalent to return wait_until(lock, std::chrono::steady_clock::now() +
rel_time);.
2) Equivalent to return wait_until(lock, std::chrono::steady_clock::now() +
rel_time, std::move(pred));.
This overload may be used to ignore spurious awakenings while waiting for a specific
condition to become true.

Right after wait_for 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
rel_time - the maximum duration to wait
pred - the predicate to check whether the waiting can be completed

Type requirements¶

-
Predicate must meet the requirements of FunctionObject.

Return value¶

1) std::cv_status::timeout if rel_time has been elapsed since the beginning of this
call, otherwise std::cv_status::no_timeout.
2) The latest result of pred() before returning to the caller.

Exceptions¶

1) Timeout-related exceptions.
2) Timeout-related exceptions, and any exception thrown by pred.

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.

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 2093 C++11 timeout-related exceptions were mentions these exceptions
missing in the specification
LWG 2135 C++11 the behavior was unclear if calls std::terminate in
lock.lock() throws an exception this case

See also¶

wait blocks the current thread until the condition variable is awakened
(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)

Hidden category:¶

* Pages with unreviewed LWG DR marker