[boost] boost::signal thread safety (implied question: is boost::function thread safe ?)

From: Philippe Vaucher (philippe.vaucher_at_[hidden])
Date: 2007-03-01 09:23:19

Hello,

Well, the subject says it all ;)
This question comes because boost::signal's documentation says it's not
thread safe, but I have trouble finding exactly why it isn't.

Let's look the following example:

Each thread is connected to the same signal, then the signal is emmited, and
each threads receives the message.... Why would this not be thread safe ?
Of course in this code the issue would be that I don't mutex the "finished"
variable but that's another story.

#include <iostream>
#include <boost/bind.hpp>
#include <boost/thread.hpp>
#include <boost/signals.hpp>

struct thread1
{
  void operator()() { finished = false; while(!finished); }
  void slot() { finished = true; }
  bool finished;
};

struct thread2
{
  void operator()() { finished = false; while(!finished); }
  void slot() { finished = true; }
  bool finished;
};

int main()
{
    boost::signal<void ()> sig;

    thread1 t1;
    thread2 t2;

    sig.connect(boost::bind(&thread1::slot, &t1));
    sig.connect(boost::bind(&thread2::slot, &t2));

    boost::thread trd1(boost::ref(t1));
    boost::thread trd2(boost::ref(t2));

    usleep(10);

    sig();

    trd1.join();
    trd2.join();

    return 0;
}

Thank you for insight.
Philippe