The function called by std::async is not executed immediately? - c++

Question

#include <iostream>
#include <future>
auto gClock = clock();
char threadPool(char c) {
std::cout << "enter thread :" << c << " cost time:" << clock() - gClock << std::endl;
std::this_thread::sleep_for(std::chrono::seconds(2));
for (int i = 0; i < 10; i++)
std::cout << c;
std::cout << std::endl;
return c;
}
void fnTestAsync(){
auto begin = clock();
std::future<char> futures[10];
for (int i = 0; i < 10; ++i){
futures[i] = std::async(std::launch::async,threadPool, 'a' + i);
}
for (int i = 0; i < 10; ++i){
std::cout << futures[i].get() << " back ,cost time: " << clock() - begin << std::endl;
}
std::cout << "fnTestAsync: " << clock() - begin << std::endl;
}
int main(){
std::thread testAsync(fnTestAsync);
testAsync.detach();
std::this_thread::sleep_for(std::chrono::seconds(10));
return 0;
}
run result
I'm trying to get these 10 threads to execute together and all return immediately after a two second delay, but I output the time spent and find that it takes about 2900ms, much larger than the 2000ms I expected.
What is the cause of this increase?
How should he fix it?