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

Question

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.

Answer 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.

Answer 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