CODEWITHSUNDEEP
c++multithreadingthread-safety
user2375871
user2375871

Reputation: 13

Memory issues with threads

I'm working on a multi-threaded server application. I have this struct that I try to pass to 2 Threads:

struct params{
  SafeQueue<int> *mq;
  Database *db;
};
class Server{
  Server(Database *db){
    DWORD sthread, ethread;
    params *p;
    p = new params;
    p->db = db;
    SafeQueue<int> *msgq = new SafeQueue<int>;
    p->mq = msgq;
    cout << "Address of params: " << p << endl;
    cout << "Address of SafeQueue: " << msgq << endl;
    cout << "Starting Server...\n";
    CreateThread(NULL, 0, smtpReceiver, &p, 0, &sthread); 
    CreateThread(NULL, 0, mQueue, &p, 0, &ethread);
  }
}
DWORD WINAPI mailQueue(LPVOID lpParam){
  params *p = (params *) lpParam;
  SafeQueue<int> *msgQ = p->mq;
  cout << "Address of params: " << p << endl;
  cout << "Address of SafeQueue: " << msgQ << endl;
  cout << "Queue thread started...\n";
}

Now the issue I'm having is the pointer to SafeQueue in the mailQueue thread has the address of the params struct... See output:

Address of params: 0x23878
Address of SafeQueue: 0x212c8
Starting Server...
Address of params: 0x28fe60
Address of SafeQueue: 0x23878
Queue thread started...

Upvotes: 0

Views: 59

Answers (1)

Jonathan Wakely
Jonathan Wakely

Reputation: 171263

CreateThread(NULL, 0, mQueue, &p, 0, &ethread);
                              ^^

This should be just p

You pass a params** to the mailQueue thread then cast it to params* and dereference it, that's undefined behaviour. What happens in practice is that p->mq is the address at *p (because offsetof(params, mq) == 0) which is the value of p in the Server constructor, as you're seeing in the cout output.

To fix it, you should be passing a params* to the new thread, i.e. the variable p not its address.

Upvotes: 2

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