NAME¶

std::condition_variable::notify_one - std::condition_variable::notify_one

Synopsis¶

void notify_one() noexcept; (since C++11)

If any threads are waiting on *this, calling notify_one unblocks one of the waiting
threads.

Parameters¶

(none)

Return value¶

(none)

Notes¶

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.

The notifying thread does not need to hold the lock on the same mutex as the one
held by the waiting thread(s); in fact doing so is a pessimization, since the
notified thread would immediately block again, waiting for the notifying thread to
release the lock. However, some implementations (in particular many implementations
of pthreads) recognize this situation and avoid this "hurry up and wait" scenario by
transferring the waiting thread from the condition variable's queue directly to the
queue of the mutex within the notify call, without waking it up.

Notifying while under the lock may nevertheless be necessary when precise scheduling
of events is required, e.g. if the waiting thread would exit the program if the
condition is satisfied, causing destruction of the notifying thread's condition
variable. A spurious wakeup after mutex unlock but before notify would result in
notify called on a destroyed object.

Example¶

// Run this code

#include <chrono>
#include <condition_variable>
#include <iostream>
#include <thread>
using namespace std::chrono_literals;

std::condition_variable cv;
std::mutex cv_m;
int i = 0;
bool done = false;

void waits()
{
std::unique_lock<std::mutex> lk(cv_m);
std::cout << "Waiting... \n";
cv.wait(lk, []{ return i == 1; });
std::cout << "...finished waiting; i == " << i << '\n';
done = true;
}

void signals()
{
std::this_thread::sleep_for(200ms);
std::cout << "Notifying falsely...\n";
cv.notify_one(); // waiting thread is notified with i == 0.
// cv.wait wakes up, checks i, and goes back to waiting

std::unique_lock<std::mutex> lk(cv_m);
i = 1;
while (!done)
{
std::cout << "Notifying true change...\n";
lk.unlock();
cv.notify_one(); // waiting thread is notified with i == 1, cv.wait returns
std::this_thread::sleep_for(300ms);
lk.lock();
}
}

int main()
{
std::thread t1(waits), t2(signals);
t1.join();
t2.join();
}

Possible output:¶

Waiting...
Notifying falsely...
Notifying true change...
...finished waiting; i == 1

See also¶

notify_all notifies all waiting threads
(public member function)
C documentation for
cnd_signal

Hidden category:¶

* Pages with unreviewed LWG DR marker