Question

So I am pretty new to threading in general and have been experimenting with pthreads for the past couple of weeks. I have created a class that has a threaded function within itself. It works fine, until I tried to set a class property (an integer) to a value.

.h file:

#ifndef TESTCLASS_H
#define TESTCLASS_H

#include <iostream>
#include <windows.h>

using namespace std;

class testClass
{
    public:
        testClass();
        HANDLE h;
        static DWORD WINAPI mythread(LPVOID param);
        int mytestint;
        void printstuffs();
        void startThread();
};

#endif // TESTCLASS_H

.cpp file

#include "testClass.h"

testClass::testClass()
{
    cout << "Created class" << endl;
}
DWORD WINAPI testClass::mythread(LPVOID param)
{
    cout << "In thread" << endl;
    testClass* This = (testClass*)param;

    cout << "Calling class function" << endl;
    This->printstuffs();

    cout << "Thread is done" << endl;

    return NULL;
}
void testClass::printstuffs()
{
    cout << "In class function " << endl;
    mytestint = 42; // <- crashes here
    cout << "Test Int = " << mytestint << endl;
}
void testClass::startThread()
{
    h = CreateThread(NULL, 0, mythread, (LPVOID)0, 0, NULL);
    cout << "Thread started" << endl;
}

So why does it crash when I call mytestint = 42; ?

Answer 1

You’re calling mythread with a null pointer. When you cast that to This, you end up calling a function on a null object. When you do mytestint = 42, the computer sees it like this->mytestint = 42, and since this is NULL, you dereference a null pointer, and the program segfaults. You need to do something like the following:

h = CreateThread(NULL, 0, mythread, (LPVOID)this, 0, NULL);

If possible, I would also suggest migrating over to standard C++ threads introduced in C++11. Since it looks like you’re just learning multithreading it would be useful to learn the standard facilities (which are included with the latest versions of MSVC and GCC) vice vendor-specific APIs.