Boost Mutex Scoped Lock

Question

I was reading through a Boost Mutex tutorial on drdobbs.com, and found this piece of code:

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

boost::mutex io_mutex;

void count(int id)
{
  for (int i = 0; i < 10; ++i)
  {
    boost::mutex::scoped_lock
      lock(io_mutex);
    std::cout << id << ": " <<
      i << std::endl;
  }
}

int main(int argc, char* argv[])
{
  boost::thread thrd1(
    boost::bind(&count, 1));
  boost::thread thrd2(
    boost::bind(&count, 2));
  thrd1.join();
  thrd2.join();
  return 0;
}

Now I understand the point of a Mutex is to prevent two threads from accessing the same resource at the same time, but I don't see the correlation between io_mutex and std::cout. Does this code just lock everything within the scope until the scope is finished?

Answer 1

Now I understand the point of a Mutex is to prevent two threads from accessing the same resource at the same time, but I don't see the correlation between io_mutex and std::cout.

std::cout is a global object, so you can see that as a shared resource. If you access it concurrently from several threads, those accesses must be synchronized somehow, to avoid data races and undefined behavior.

Perhaps it will be easier for you to notice that concurrent access occurs by considering that:

std::cout << x

Is actually equivalent to:

::operator << (std::cout, x)

Which means you are calling a function that operates on the std::cout object, and you are doing so from different threads at the same time. std::cout must be protected somehow. But that's not the only reason why the scoped_lock is there (keep reading).

Does this code just lock everything within the scope until the scope is finished?

Yes, it locks io_mutex until the lock object itself goes out of scope (being a typical RAII wrapper), which happens at the end of each iteration of your for loop.

Why is it needed? Well, although in C++11 individual insertions into cout are guaranteed to be thread-safe, subsequent, separate insertions may be interleaved when several threads are outputting something.

Keep in mind that each insertion through operator << is a separate function call, as if you were doing:

std::cout << id;
std::cout << ": ";
std::cout << i;
std::cout << endl;

The fact that operator << returns the stream object allows you to chain the above function calls in a single expression (as you have done in your program), but the fact that you are having several separate function calls still holds.

Now looking at the above snippet, it is more evident that the purpose of this scoped lock is to make sure that each message of the form:

<id> ": " <index> <endl>

Gets printed without its parts being interleaved with parts from other messages.

Also, in C++03 (where insertions into cout are not guaranteed to be thread-safe) , the lock will protect the cout object itself from being accessed concurrently.