Question

I am doing a multhread C++ boost on Linux.

The following program still have race conditions even though I tried to use locks.

The result is 8 or 9 or 5 . it should NOT happen.

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

 boost::mutex myMutex ;
 int g = 0 ;

 void f()
 {

    //myMutex.lock();
    {
            boost::mutex::scoped_lock lock(myMutex);
            ++g;
    }
    //myMutex.unlock();
    return ;
 }
 const int threadnum = 10;
 int main()
 {
    boost::threadpool::fifo_pool tp(threadnum);
    for (int i = 0 ; i < threadnum ; ++i)
            tp.schedule(boost::bind(f));
    std::cout << g << std::endl ;
    return 0 ;
 }

Any help will be appreciated.

thanks !

Answer 1

from http://threadpool.sourceforge.net/tutorial/intro.html :

It is very important to understand that the task is only scheduled for
execution. Schedule returns immediately and there are no guarantees
about when the tasks are executed and how long the processing will
take.

You schedule 10 tasks and then immediately print the result for as many as executed by the time you reach the line

std::cout << g << std::endl ;

So, while your mutex makes sure threads increment g one at a time, you’re not waiting for them to finish before printing the result.
One way to modify your code is to wait for all tasks in the pool to finish:

boost::threadpool::fifo_pool tp(threadnum);
for (int i = 0 ; i < threadnum ; ++i)
        tp.schedule(boost::bind(f));
tp.wait(); //WAIT FOR TASKS TO EXECUTE
std::cout << g << std::endl ;
return 0 ;