[Boost-users] newbie thread synchronization question

Subject: [Boost-users] newbie thread synchronization question
From: Cory Riddell (cory_at_[hidden])
Date: 2009-02-26 17:26:01

I'm a total newbie with the Thread library, so please forgive me if this
is a dumb question.

Say I make a class called Shared that looks something like this:
  class Shared {
  private:
    boost::mutex monitor;
    int qty;

  public:
    Shared() : qty(0) { }
    void lock() { monitor.lock(); }
    void unlock() { monitor.unlock(); }
    int getQty() { return qty; }
    void setQty(int q) { qty = q; }
  };

If I use it in from multiple threads with code that looks like:
  // s is an instance of Shared
  s.lock();
  int q = s.getQty();
  ++q;
  s.setQty(q);
  s.unlock();

will it work reliably on a multi-core machine? When the mutex unlock()
happens, does that flush the CPU cache and write the value of s.qty to
main memory? Does the mutex lock() operation clear the CPU cache and
force a reload of s from the main memory? I'm trying to understand how
synchronization works among multiple cpus.

I've only worked on multithreaded code in Java and there the memory
model is well defined. AFAIK, that isn't the case in C++.

Cory