std::condition_variable is a class in c++11, it will be used by thread to wake up other(one or more) waiting thread. In other word, one or multiple threads are waiting for a specific condition, when condition is satisfy then a thread can wake up other (one or more) waiting thread.
Main API of std::condition_variable class (assume std::condition_variable obj;) :
- obj.notify_one(); //wake up only one thread
- obj.notify_all(); //wake up all waiting threads
- obj.wait(ul); //ul is unique pointer, thread waits for notification
- obj.wait_for(ul, duration); //ul is unique pointer, thread waits for notification for duration
- obj.wait_until(ul, time_point); //ul is unique pointer, thread waits for notification until time-point
Note:
- wait() API internally unlock the mutex before going to sleep that why we will be using unique lock instead of lock guard.
- wait() API internally lock the mutex after receiving notification from other thread that why we will be using unique lock instead of lock guard.
- Using lambdas, checking flag to avoid suspicious wake up call.
Example: There are two thread(Reader and Writer), Reader is waiting for Writer thread, so using condition variable Writer thread(after completion of writing) can wake up Reader thread.
Source code with example:
/*
Program: Condition Variable
Author: Alpha Master
Date: 6 Feb 2021
*/
//Header File
#include<iostream>
#include<thread>
#include<chrono>
#include<mutex>
#include<condition_variable>
//Global Variable
bool dataReady{false};
std::mutex gMtx;
std::condition_variable gConVar;
int data{0};
//Reader Thread
void ReadThread()
{
auto i{0};
while(i<5)
{
std::cout<<"!!!Waiting, Data is NOT ready"<<std::endl;
{
std::unique_lock<std::mutex> ul(gMtx);
//Using lambdas, checking flag to avoid suspicious wakeup call
//wait internally unlock the mutex so we are using unique pointer
gConVar.wait(ul, []{return dataReady;});
std::cout<<"Got the data:"<<data<<std::endl;
dataReady = false;
}
++i;
}
}
//Writer Thread
void WriteThread()
{
auto i{0};
while(i<5)
{
//Wait till, data is read by reader
while(dataReady)
{
std::this_thread::sleep_for(std::chrono::seconds(1));
}
//Now, Writer will write
{
std::lock_guard<std::mutex> lg(gMtx);
++data;
dataReady = true;
//purposefully waiting to verify the result
std::this_thread::sleep_for(std::chrono::seconds(1));
}
gConVar.notify_one();
++i;
}
}
int main()
{
std::cout<<"Conditon variable Functionality"<<std::endl;
//Thread 1 and its joinable
std::thread t1(ReadThread);
//Thread 2 and its also joinable
std::thread t2(WriteThread);
//std::cout<<"Main thread is Waiting for Child thread"<<std::endl;
t1.join();
t2.join();
return 0;
};
Output:
Conditon variable Functionality !!!Waiting, Data is NOT ready Got the data:1 !!!Waiting, Data is NOT ready Got the data:2 !!!Waiting, Data is NOT ready Got the data:3 !!!Waiting, Data is NOT ready Got the data:4 !!!Waiting, Data is NOT ready Got the data:5