C++ std::thread and std::async tests are in progress, but an error occurs

Question

I am trying to identify the difference between std::::thread and std:async through a simple experiment.(check std::async Thread Pool functionality)

However, an error occurs during execution.

What's the problem and how can I fix it?

thread test.

void for_print(int num) {
    for (int i = 0; i < 1000; i++)
        printf("%d Thread : %d\n", num, i);
}


int main(){
    std::vector<std::thread> v_thread(2000);

    for (size_t i = 0; i < 2000; i++)
        v_thread.emplace_back(std::thread(for_print, i));

    //join()
    for (size_t i = 0; i < 2000; i++)
        v_thread[i].join();

    std::vector<std::thread>().swap(v_thread);
    return 0;
}

async test

void for_print(int num) {
    for (int i = 0; i < 1000; i++)
        printf("%d async : %d\n", num, i);
}

int main() {
    std::vector<std::future<void>> v_async(2000);

    for (size_t i = 0; i < 2000; i++)
        v_async.emplace_back(std::async(std::launch::async, for_print, i));

    for (size_t i = 0; i < 2000; i++)
        v_async[i].wait();

    std::vector<std::future<void>>().swap(v_async);
    return 0;
}

What error occurs? Quote it in full. How else can anyone confidently tell you what the problem is? — underscore_d, Jun 09 '20 at 14:30
_an error occurs_ — Which error? Please, provide a complete error message and build configuration info (compiler, flags, etc.) — Daniel Langr, Jun 09 '20 at 14:31

Daniel Langr · Accepted Answer · 2020-06-09T14:36:26.407

2

std::vector<std::thread> v_thread(2000);

for (size_t i = 0; i < 2000; i++)
   v_thread.emplace_back(std::thread(for_print, i));

for (size_t i = 0; i < 2000; i++)
   v_thread[i].join();

This is wrong. You first create a vector of 2000 "empty" thread objects, then add additional 2000 "living" threads, and finally join those first 2000 thread, which are not joinable.

The same holds in the case of std::async.

Just change

std::vector<std::thread> v_thread(2000);

to

std::vector<std::thread> v_thread;

and

std::vector<std::future<void>> v_async(2000);

to

std::vector<std::future<void>> v_async;

OT: These lines:

std::vector<std::thread>().swap(v_thread);
return 0;

at the end of main are completely superfluous.

EDIT

Also note that systems typically have some limit for the number of running threads. Creating 2000 threads might therefore be a problem also in this context.

edited Jun 09 '20 at 14:36

answered Jun 09 '20 at 14:33

Daniel Langr

22,196
3
50
93

2

Adding a `v_thread.reserve(2000);` could also be helpful. – NathanOliver Jun 09 '20 at 14:36
Thank you! I have a question. Does the thread release from memory after .join() step? – Hwan E Jun 09 '20 at 14:43
@HwanE Vector releases its memory (buffer) in its destructor, which is called automatically at the end of `main`. – Daniel Langr Jun 10 '20 at 05:55

C++ std::thread and std::async tests are in progress, but an error occurs

1 Answers1