What does std::thread.join() do?

问题:

By definition from C++ reference:

Blocks the current thread until the thread identified by *this finishes its execution.

So does this mean when using .join(), there's no need to mutex.lock() when that thread calls some function? I'm new to mutual exclusion and threading, so I'm kind of confused.

Note: I've found a book C++ Concurrency in Action and I am reading the book. It is very well written for a beginner on multithreading like me.

Thank you all for the help.

回答1:

You still need mutexes and conditions. Joining a thread makes one thread of execution wait for another thread to finish running. You still need mutexes to protect shared resources. It allows main() in this example to wait for all threads to finish before quitting itself.

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

using namespace std;



int global_counter = 0;
std::mutex counter_mutex;

void five_thread_fn(){
    for(int i = 0; i<5; i++){
        counter_mutex.lock();
        global_counter++;
        counter_mutex.unlock();
        std::cout << "Updated from five_thread"  << endl;
        std::this_thread::sleep_for(std::chrono::seconds(5));
    }
    //When this thread finishes we wait for it to join
}

void ten_thread_fn(){
    for(int i = 0; i<10; i++){
        counter_mutex.lock();
        global_counter++;
        counter_mutex.unlock();
        std::cout << "Updated from ten_thread"  << endl;
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }
    //When this thread finishes we wait for it to join
}
int main(int argc, char *argv[]) {
    std::cout << "starting thread ten..." << std::endl;
    std::thread ten_thread(ten_thread_fn);

    std::cout << "Running ten thread" << endl;
    std::thread five_thread(five_thread_fn);


    ten_thread.join();
    std::cout << "Ten Thread is done." << std::endl;
    five_thread.join();
    std::cout << "Five Thread is done." << std::endl;
}

Note that the output might look like this:

starting thread ten...
Running ten thread
Updated frUopmd atteend_ tfhrroema df
ive_thread
Updated from ten_thread
Updated from ten_thread
Updated from ten_thread
Updated from ten_thread
Updated from five_thread
Updated from ten_thread
Updated from ten_thread
Updated from ten_thread
Updated from ten_thread
Updated from ten_thread
Updated from five_thread
Ten Thread is done.
Updated from five_thread
Updated from five_thread
Five Thread is done.

Since std::cout is a shared resource access and use of it should also be mutex protected too.

回答2:

join() stops current thread until another one finishes. mutex stops current thread until mutex owner releases it or locks right away if it isn't locked. So these guys are quite different

回答3:

It blocks the current thread until the execution of the thread is completed on which join() is called.

If you do not specify join() or dettach() on the thread then it will result in runtime error as the main/current thread will complete its execution and the other thread created will be still running.

回答4:

std::thread.join has three functions I can think of off-hand and some others:

a) Encourages continual creating/terminating/destroying of threads, so hammering performance and increasing the probabilty of leaks, thread-runaway, memory-runaway and general loss-of-control of your app.

b) Stuffs GUI event-handlers by enforcing unwanted waits, resulting in unresponsive 'hourglass apps' that your customers will hate.

c) Causes apps to fail to shutdown because they are waiting for the termination of an unresposive, uninterruptible thread.

d) Other bad things.

I understand that you are new to multithreading, and I wish you the best with it. Also, consider that I've had a lot of Adnams Broadside tonight, but:

Join(), and it's friends in other languages like TThread.WaitFor, (Delphi), are to efficient multithreading like Windows ME was to operating systems.

Please try hard to progress and come to understand other multithreaded concepts - pools, tasks, app-lifetime threads, inter-thread comms via producer-consumer queues. In fact, almost anything except Join().