C++ condition_variable::notify_all() Function

The std::condition_variable::notify_all() function in C++, is a synchronization mechanism used in multithreading. It wakes up all threads waiting on the condition variable, allowing them to recheck the associated condition.

This function is used to when a shared resource state changes, making it ready for multiple waiting threads to proceed.

Syntax

Following is the syntax for std::condition_variable::notify_all() function.

void notify_all() noexcept;

Parameters

This function does not accepts any parameter.

Return value

This function does not return anything.

Example 1

Let's look at the following example, where we are going to consider the basic usage of the notify_all() function.

#include <iostream>
#include <thread>
#include <condition_variable>
#include <mutex>
std::condition_variable a;
std::mutex b;
bool ready = false;
void x(int id) {
   std::unique_lock < std::mutex > lock(b);
   a.wait(lock, [] {
      return ready;
   });
   std::cout << id << " is proceeding.\n";
}
int main() {
   std::thread x1(x, 1);
   std::thread x2(x, 2);
   std::this_thread::sleep_for(std::chrono::seconds(1));
   {
      std::lock_guard < std::mutex > lock(b);
      ready = true;
   }
   a.notify_all();
   x1.join();
   x2.join();
   return 0;
}

Output

Output of the above code is as follows −

2 is proceeding.
1 is proceeding.