Output of multiple thread executions only happens once?

Question

#include <iostream>
#include <thread>

void func() {
    std::cout << "Hello";
}

int main() {
    std::vector<std::thread> threads; 
    int n = 100;
    for (int i = 0; i < n; i++) {
         std::cout << "executing thread" << std::endl;
         threads.push_back(std::thread(func));
    }
}

My program prints "executing thread" once and it ends. What is the cause?

Please create a [Minimal, Complete, and Verifiable Example](http://stackoverflow.com/help/mcve) to show us. — Some programmer dude, Jun 07 '18 at 11:02
Probably your program ends before any threads actually starts. But yes, if you want an answer, provide an MCVE. — YSC, Jun 07 '18 at 11:03
The ***current*** problem you have is the same as in the duplicate. Once you fix that you have the problem all answers here tell you about. — Some programmer dude, Jun 07 '18 at 11:18
@UKMonkey, what does "start your threads in a function" mean? — Solomon Slow, Jun 07 '18 at 15:06

score 1 · Answer 1 · answered Jun 07 '18 at 11:05

After this loop completes the destructor of std::vector<std::thread> is invoked. std::thread destructor calls std::terminate if the thread was neither detached nor joined, like in your case.

To fix that, add the following after the loop:

for(auto& thread : threads)
    thread.join();

jspcal · Answer 2 · 2018-06-07T11:19:40.657

1

Make sure you join the threads to wait for them all to complete:

for (auto &thread : threads) {
    thread.join();
}

If the program continues after this point and doesn't exit immediately, flush the output since it may be buffered:

std::cout << std::flush;

edited Jun 07 '18 at 11:19

answered Jun 07 '18 at 11:06

jspcal

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even after adding these lines of code after the loop, my for loop is executing only once. @jspcal – rjlion795 Jun 07 '18 at 11:11
OP has `std::endl` which already does flush the stream – 463035818_is_not_an_ai Jun 07 '18 at 11:18
the question isnt about the `"Hello"` but about the `cout` in `main` which appears only once and that one has a `endl` – 463035818_is_not_an_ai Jun 07 '18 at 11:20
OPs comment below a different answer: "i guess you didn't get my question. the loop executes only once i.e "executing thread" is only printed once. This is the problem I am getting" – 463035818_is_not_an_ai Jun 07 '18 at 11:29
@user463035818: That's irrelevant. The point about needing to flush is correct. – jspcal Jun 07 '18 at 11:30

score 0 · Answer 3 · answered Jun 07 '18 at 11:10

0

Even if you don't join, it should still print "executing thread" 100 times.

Perhaps the problem is using "endl" instead of "std::endl"

answered Jun 07 '18 at 11:10

Abdul Ahad

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Some programmer dude · Answer 4 · 2018-06-07T11:15:24.407

Once the loop creating the threads is done, your program continues. And it continues to leave the main function which causes all variables defined inside the main function to go out of scope and their life-time ending. This leads to the destruction of the objects, including the vector which then leads to all the thread object in the vector being destructed.

And as noted by others, the destruction of a thread object will lead to program termination if the thread is not joined or detached.

While the other answers tell you to join the threads (which IMO is the recommended solution here) there is another possible solution: To detach the threads.

While this will lead to the std::terminate function to be called and prematurely terminate your program, this will lead to another problem, as leaving the main function ends the process and all its threads.

If you for some reason what your threads to continue to live on after the main function exits, you need to detach the threads and exit the "main thread" using an operating-system specific function. This will leave the process running with all your created threads still chugging along.

i guess you didn't get my question. the loop executes only once i.e "executing thread" is only printed once. This is the problem I am getting. — rjlion795, Jun 07 '18 at 11:13

Output of multiple thread executions only happens once?

4 Answers4