Question

I’m having a problem calling a thread more than once and the variables messing up. I’m new to threads, so I’m sure I’m missing something simple.

struct PARAMS
{
    time_t secondsAtStart;
};

DWORD WINAPI ProcessChange(void* parameter) {
    PARAMS* params = (PARAMS*)parameter;
    Sleep(3000);
    _tprintf(TEXT("Seconds: (%d)\n"), params->secondsAtStart);
    return 0;
}


void FileChanged(CString filename, CString action) {
    struct PARAMS *params = NULL;
    params = (struct PARAMS *)malloc(sizeof(PARAMS)+1);
    params->secondsAtStart = time(null);
    // I've also tried it this way.    
    //PARAMS params;
    //params.secondsAtStart = time(NULL);
    HANDLE hThread = CreateThread(NULL, 0, ProcessChange, &params, 0, NULL);
    // If I uncomment this, it works, but just one thread runs at a time.
    //WaitForSingleObject(hThread, INFINITE);
}

If I don’t uncomment the WaitForSingleObject, then the secondsAtStart variable gets corrupted. The end result I need is that if FileChanged gets called 3 times right after one another, I’m going to have the first two runs do nothing and the last one do the action.

Thanks,
Ben

Answer 1

Passing addresses of (or references to) local variables of a function, i.e. variables of automatic storage, to a thread causes undefined behaviour if the thread lives longer than the function.

In your code, params points to an object of dynamic storage, but the pointer itself is a local variable. You pass its address &params to the thread. This only works if by waiting for the thread to finish you guarantee the pointer lives longer than the thread. Otherwise it causes undefined behaviour, which quite naturally manifests itself in nonsensical values being printed.

Passing params instead of &params should solve the problem. (Also note that the code as written causes a memory leak; you’ll need to make sure you actually free the space allocated after the thread has finished.)