Tech Sujhav : std::thread::join

join(): It basically temporarily halts the execution at the point where join() function is called till the thread associated with the calling object i.e. t_obj completes its task.

std::thread::join [Advanced] -- [Click Here]

Let’s check the below examples.

Suggestion:- it would be great, if you copy following code and paste it on any online compiler (or vi/vim) and try to run it by your-self

Case-1: join() is not called	Case-2: join() is called
#include<iostream> #include<thread> //Thread Header using namespace std; //“displayActivity” is the actual task //to be performed by our thread void displayActivity() { for(int i=0;i<10;i++) cout<<"www.techsujhav.com : "<<i<<endl; } int main() { cout<<"Going to start thread"<<endl; std::thread t_obj(displayActivity); //join() is intentionally commented //to check the behaviour of our code //t_obj.join(); cout<<"Main exit"<<endl; return 0; }	#include<iostream> #include<thread> //Thread Header using namespace std; //“displayActivity” is the actual task //to be performed by our thread void displayActivity() { for(int i=0;i<5;i++) cout<<"www.techsujhav.com : "<<i<<endl; } int main() { cout<<"Going to start thread"<<endl; std::thread t_obj(displayActivity); cout<<"Before Join"<<endl; t_obj.join(); cout<<"After Join"<<endl; cout<<"Main exit"<<endl; return 0; }
Compile: $g++ join.cpp -o join -lpthread	Compile: $g++ join.cpp -o join -lpthread
Output:$ ./join Going to start thread Main exit terminate called without an active exception Aborted	Output:$ ./join Going to start thread Before Join www.techsujhav.com : 0 www.techsujhav.com : 1 www.techsujhav.com : 2 www.techsujhav.com : 3 www.techsujhav.com : 4 After Join Main exit
Observation (1): We are not able to see any print from our task "displayActivity" assigned to thread object. Although all other prints are visible from main (). Explanation: Once our thread object t_obj is created with “displayActivity” passed in constructor it notifies to OS (Operation System) to create a thread and constructor call finished and moved ahead for next instruction in main(). OS shall take some time (may be in nano-second) to create a thread, meanwhile main() has finished its activity and exited, therefore we don’t have any print of our task “displayActivity” Observation(2): terminate called without an active exception Aborted Explanation: Will discuss in Advanced section of this post	Observation: Our code worked as expected. join() function halts execution of main and let the thread to finish assigned task. More important to observe it that join() makes main to wait at the line it(join) is called.