[Boost-users] Threads Don't Take Turns Under Linux (Ubuntu Gutsy Gibbon)

From: Hal Vaughan (hal_at_[hidden])
Date: 2008-03-17 09:42:46

I apologize if this gets posted more than once.  I found the group on 
GMane, tried to post, joined the email list, then tried to post several 
times but still have yet to see it show up.

I'm learning C++ and found a tutorial on threads with Boost on Dr. 
Dobb's site. Â I've copied the code from this url:

 http://www.ddj.com/cpp/184401518?pgno=5

It's supposed to fill a buffer with data on one thread and remove data 
on the other thread. Â I would expect it to add data and remove at 
almost the same time but when I run it, it adds 10 items, then removes 
them, then adds ten again, and so on. Â It's never adding and removing 
in an "interleaved" fashion. Â I've also tried the same with simple 
counters where both threads count to ten, but it doesn't seem to make a 
difference. Â It seems no matter how I set it up, one thread either 
completes, or runs until it has to stop for a mutex, then the other 
thread runs. Â It seems that there is never a case of both threads 
running at the same time. Â I've never had trouble doing this in Perl 
with fork() so I'm not at all sure what could be going wrong.

As I said, it's not just the program below, it seems to be any program 
that uses more than one thread.

Am I doing something wrong or is there a setting I need to change?

I'm compiling it with "g++ -lboost_thread mthrd.cpp -o mthrd 
&& ./mthrd";

Thanks!

Hal
------
#include <boost/thread/thread.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/thread/condition.hpp>
#include <iostream>

const int BUF_SIZE = 10;
const int ITERS = 100;

boost::mutex io_mutex;

class buffer
{
public:
  typedef boost::mutex::scoped_lock
    scoped_lock;
    
  buffer()
    : p(0), c(0), full(0)
  {
  }
    
  void put(int m)
  {
    scoped_lock lock(mutex);
    if (full == BUF_SIZE)
    {
      {
        boost::mutex::scoped_lock
          lock(io_mutex);
        std::cout <<
          "Buffer is full. Waiting..."
          << std::endl;
      }
      while (full == BUF_SIZE)
        cond.wait(lock);
    }
    buf[p] = m;
    p = (p+1) % BUF_SIZE;
    ++full;
    cond.notify_one();
  }

  int get()
  {
    scoped_lock lk(mutex);
    if (full == 0)
    {
      {
        boost::mutex::scoped_lock
          lock(io_mutex);
        std::cout <<
          "Buffer is empty. Waiting..."
          << std::endl;
      }
      while (full == 0)
        cond.wait(lk);
    }
    int i = buf[c];
    c = (c+1) % BUF_SIZE;
    --full;
    cond.notify_one();
    return i;
  }
    
private:
  boost::mutex mutex;
  boost::condition cond;
  unsigned int p, c, full;
  int buf[BUF_SIZE];
};

buffer buf;

void writer()
{
  for (int n = 0; n < ITERS; ++n)
  {
    {
      boost::mutex::scoped_lock
        lock(io_mutex);
      std::cout << "sending: "
        << n << std::endl;
    }
    buf.put(n);
  }
}

void reader()
{
  for (int x = 0; x < ITERS; ++x)
  {
    int n = buf.get();
    {
      boost::mutex::scoped_lock
        lock(io_mutex);
      std::cout << "received: "
        << n << std::endl;
    }
  }
}
    
int main(int argc, char* argv[])
{
  boost::thread thrd1(&reader);
  boost::thread thrd2(&writer);
  thrd1.join();
  thrd2.join();
  return 0;
}