std::condition_variable::notify_all (3) - Linux Manuals

std::condition_variable::notify_all: std::condition_variable::notify_all

NAME

std::condition_variable::notify_all - std::condition_variable::notify_all

Synopsis

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

Unblocks all threads currently waiting for *this.

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.

Example

// Run this code

  #include <iostream>
  #include <condition_variable>
  #include <thread>
  #include <chrono>

  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