Question

Well I’m trying to set a variable to use in a thread, it works fine if I call the pthread from the main function, but if I call it from a function or a function inside a class, the variable is lost and prints garbage instead, that’s why I added the condition

if(this->pickup < 7)

so I minimized the code so I could post it here because it has all the examples I’m saying.

The output of this code below is:

Access by Class:

Hello, world! <

Access Directly:

Hello, world!, N: 6<

I would like to have the same result as in Access Directly in Access by Class, I want it to output the “, N: 6” because after all it was defined. what am I missing here?

I hope I was clear enough, thanks in advance.

(By the way, I’m using the pthread library that is available for windows)
so here is the code:

#include <stdio.h>
#include <pthread.h>
#include <iostream>
#include <conio.h>
#include <windows.h>

class C {
public:
    int pickup;

    void *hello()
    {
        std::cout << "\nHello, world!";

        if(this->pickup < 7)
        std::cout << ", N: " << this->pickup;

        std::cout << "<" << std::endl;
        printf("HI");
        return 0;
    }

    static void *hello_helper(void *context)
    {
        return ((C *)context)->hello();
    }


    void StartThread(){
        C c;
        c.pickup = 6;

        pthread_t t;
        pthread_create(&t, NULL, &C::hello_helper, &c);
    }


};



int main () {

    C c;
    std::cout << "Access by Class: \n";
    c.StartThread();
    c.pickup = 6;
    Sleep(2000);

    std::cout << "\nAccess Directly: \n";
    pthread_t t;
    pthread_create(&t, NULL, &C::hello_helper, &c);

    _getch();
    return 0;
}

Answer 1

c is destroyed when StartThread() returns, meaning hello_helper() is using a dangling pointer resulting in undefined behaviour.

Change to:

void StartThread(){
    C* c = new C();
    c->pickup = 6;

    pthread_t t;
    pthread_create(&t, NULL, &C::hello_helper, c);
}

Remember to delete the argument passed into hello_helper():

static void *hello_helper(void *context)
{
    C* c = static_cast<C*>(context);
    c->hello();
    delete c;
    return 0;
} 

EDIT:

Always deleteing the argument passed into hello_helper() would prevent passing stack allocated objects into hello_helper(). A mechanism is required to instruct hello_helper() whether it is responsible for destructing its argument.