Tech Sujhav : std::thread::detach [Advanced]

Main idea of this section to discuss various use cases of detach(), d's and don'ts of detach function.

Let check followings open items. if you have some question then you can put them in comment section.

Question-1: What will happen if main thread exits before detached thread finished its assigned task?

Question-2: Can we rejoin a detached thread?

Question-1: What will happen if main thread exits before detached thread finished its assigned task?

Detach thread shall be stopped there itself and all acquired resources shall be freed. Let check with following example.

[Copy, Paste & Run]

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

using namespace std;

void displayActivity()
{
   for(int i=0;i<5;i++)
   {
       cout<<"www.techsujhav.com :: "<<i<<endl;
       std::this_thread::sleep_for(1ns);
   }
}

int main()
{
   {//new scope
       std::thread t1(displayActivity);
       t1.detach();
   }

   for(int i=0;i<3;i++)
   {
       cout<<"Performing Activity in main()"<<endl;

       //intentionally putting sleep of 1-neno second.
       //for better display of out example.

       std::this_thread::sleep_for(1ns);
   }
   return 0;
}

$ ./detach

Performing Activity in main()

www.techsujhav.com :: 0

Performing Activity in main()

www.techsujhav.com :: 1

Performing Activity in main()

www.techsujhav.com :: 2

Explanation:

As it is clearly visible that www.techsujhav.com :: 3 and www.techsujhav.com :: 4 were not printed because main exited before our displayActivity thread could finish its activity.

Question-2: Can we rejoin a detached thread?

No it is not possible to join a thread once we have called detach() function on that object. It will throw an exception.

As we know if we want to call join() on a thread-object object then that thread object must be ACTIVE (joinable() == true).

Let check with example

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

using namespace std;

void displayActivity()
{
   for(int i=0;i<5;i++)
   {
       cout<<"www.techsujhav.com :: "<<i<<endl;
       std::this_thread::sleep_for(1ns);
   }
}

int main()
{

   {//new scope
       std::thread t1(displayActivity);
       cout<<"Going to call detach()"<<endl;

       t1.detach();

       try
       {
           cout<<"Going to call join()"<<endl;
           t1.join();
       }
       catch(std::exception& e)
       {
           cout<<"Exception Caught :: "<<e.what()<<endl;
       }
   }
  return 0;
}

$./detach

Going to call detach()

Going to call join()

Exception Caught :: Invalid argument

www.techsujhav.com :: 0

PREV:: std::thread::detach()

Main Page

Call function with Reference Variable, Default Argument

and Pointer Variable from thread::NEXT

Your Comments /Suggestions & Questions are always welcome.

We would like to help you with best of our knowledge.

So feel free to put Questions.

2 comments:

pavanSeptember 21, 2023 at 5:56 PM
what you gave is confusing, because once a thread is detached , it will run independently from main function .

The detached thread t1 starts executing displayActivity and prints "www.techsujhav.com :: 0".
The main thread enters the loop in main, prints "Performing Activity in main()", and sleeps for 1 nanosecond (which is very short and might not have a noticeable effect).
The detached thread t1 continues running and prints "www.techsujhav.com :: 1", "www.techsujhav.com :: 2".
The main thread goes through two more iterations of the loop, printing "Performing Activity in main()" each time.
The detached thread t1 continues and prints "www.techsujhav.com :: 3", "www.techsujhav.com :: 4".
Keep in mind that the sleep duration of 1 nanosecond is extremely short, and in practice, the operating system's scheduler may not have enough granularity to make the interleaving very noticeable. You might consider using a longer sleep duration for a more visible effect.

Pages

std::thread::detach [Advanced]

2 comments: