Question

I am having a small issue that I cannot solve. I am making a small server to redirect my syslog messages to it. It is very basic, but I would like to know what I did wrong cause I keep have the following error when I call join ():

/boost/path/shared_ptr.hpp:418: T* boost::shared_ptr< <template-parameter-1-1> >::operator->() const [with T = boost::thread]: Assertion `px != 0' failed.

The code will explain more:

class SysLogServer
{
public:

  typedef boost::shared_ptr<boost::thread>  Ptr_thread;

  bool Start ()
  {
    ...
    _thrd = Ptr_thread(new boost::thread (boost::bind(&SysLogServer::run, this)));
    if (!_thrd.get ())
      return ERROR ("Thread couldn't be instanciated.");
    ...
  }

  bool Stop ()
  {
    ...
    _thrd->join ();
    ...
  }

private:

  void run()
  {
    ...
  }

  Ptr_thread _thrd;

};

Thank you very much for your help.

PS: If there is any improvment to be more “thread safe”, tell me cause it really interests me 🙂

Edit:

Thank you for your comments, I think that the shared_ptr is indeed useless there but that it might me useful to inherit the class from boost::enable_shared_from_this to ensure that the class is not freed before the end of the thread, which should not happen.

Start() is of course called before Stop(), I perform a simple check with a state attribute. The run() method is simply accepting connections.

class SysLogServer
{
public:

  bool Start ()
  {
    ...
    _thrd = boost::thread(boost::bind(&SysLogServer::run, this)));
    ...
  }

  bool Stop ()
  {
    ...
    _thrd.join();
    ...
  }

  void run ()
  {
    std::cout << "Start running..." << std::endl; // never printed
    // Create a socket
    // Create a sockaddr_in
    // Bind them together
    while (!_serverStopped && !listen(sockfd, 5)) // on Stop(): _severStopped = true
     {
       // Get socket from accept
       // Print the received data
       // Close the socket given by accept
     }
    // close the first socket
  }

  boost::thread _thrd;
};

It works now. I used the almost same solution before with pointers, without any success and my friend SIGSEGV 🙂

Edit 2:

It didn’t work with pointers cause I was forgetting to check in Stop() that the server has been started. The Start() method fails for another reason.

Thank you for your useful advices

Answer 1

The reason for the assertion is not immediately clear from the code that you have presented here, but nonetheless, this code could be improved significantly.

You appear to be using a shared_ptr, but there does not seem to be any need. Ptr_thread could be changed to just boost::thread. This would lead to simpler, more efficient code with easier to understand object lifetimes.

The code could then be changed to:

class SysLogServer
{
public:

  bool Start ()
  {
      ...
      _thrd = boost::thread(boost::bind(&SysLogServer::run, this)));
      ...
  }

  bool Stop ()
  {
      ...
      _thrd.join();
      ...
  }

private:

    void run()
    {
        ...
    }

    boost::thread _thrd;

};

This code is still incorrect if Stop() is called before Start() has been called, which is the only obvious explanation for your original code failing.