QtConcurrentRun和互斥锁的非常简单的方法有时会给出分段错误

Question

I want to know what is wrong with this code. Sometimes I get sementation fault, sometimes not. This is a problem that I have in a greater piece of software, the idea is that just one thread at the same time is executing the method MyClass::print. Even with this simple example, it fail with segmentation fault. What is wrong in the code? How I can solve the problem?

Thanks!

#include <iostream>
#include <ctime>
#include <QMutex>
#include <QtConcurrentRun>

class MyClass : QThread {
public:
    void print(std::string str) {
        mutex.lock();
        std::cout << "In some thread: " <<  str << "\n";
        mutex.unlock();
    }

private:
    QMutex mutex;

};

int main() {

    MyClass myCl;

    for(int i=0; i < 10; i++) {
        QtConcurrent::run(&myCl, &MyClass::print,std::string("bla"));
    }

}

Answer 1

You do not join your threads after the for cycle that spawns them.

Therefore, the main() function may end before all the threads have finished. This would make myCl go out of scope, and the threads would be trying to access an object that has been destroyed (in particular, its mutex variable). Which gives you Undefined Behavior .

Things should get better if you joined your threads before exiting from main() (or find any other way of not exiting from main() as long as any of the threads is still running).