std::thread created as detach thread terminates when main exits

Question

I am writing the following c++ code to create a detach thread :

void threadF() {
    ofstream        f("data") ;
    for(int i=1; i<=5; i++) {
            f<<"t1:::"<<i<<"\n" ;
            f.flush() ;
            sleep(1) ;
    }
    f.close() ;
}

int main() {

    thread t1(threadF) ;

    cout<<"main #1\n" ;
    sleep(2) ;
    t1.detach() ;
    cout<<"main #2\n" ;
}

When I run this code, I have the following observations :

1. main exits after 2 secs (as expected)
2. the detach child thread also seems to be running only for 2secs because the output data file contains only 2 lines.

I expected that the thread should have executed completely and that the output data file should have contained 5 lines. This is not happening

I need an explanation for this behavior.

AS per the ink What happens to a detached thread when main() exits?, does it mean that the detached thread would automatically terminate when the main exits ?

Answer 1

I need an explanation for this behavior.

When main returns, the process is terminated. If there was a thread running, there are no guarantees that it will have a chance to finish.

Furthermore, technically touching any global state after main has returned results in undefined behaviour because all static storage objects will be destroyed by the main thread.

Basically, std::thread::detach is rarely useful because you simply have to use some other way to join the thread instead. There's no standard way to "wait for all detached threads and then stop". There are platform specific ways though (pthread_exit).