Re: Efficient way to synchronize bool variables

Goran <>
Wed, 11 Nov 2009 05:24:21 -0800 (PST)
On Nov 11, 1:09 pm, Faisal <> wrote:

In my program there are boolean vaiables which are shared between a
number of threads.

In the below code m_bRunning and m_bStopped are shared variables. The
function Execute() can be called from any of the threads.I want to
synchronize the accessing of the variable.

void OnStart()
        m_bStopped = false;
        m_bRunning = true;


void OnStop()
        m_bRunning = false;


void Execute()
        if( m_bRunning )
                //Do some tasks
                if( !m_bStopped )
                        //Do some tasks
                        m_bStopped = true;


Does simply declaring the variables as volatile would solve my
Or do i need interlockedXXX functions here.

Volatile is all but useless on any hardware that has any form of CPU

InterlockedXXX are OK. I use a class similar to (warning: compiled and
tested with head-compiler):

class CThreadSafeFlag
  mutable LONG m_Flag;
  CThreadSafeFlag(bool bSet=false) : m_Flag(bSet) {}
  bool IsSet() const { return InterlockedCompareExchange(&m_Flag, 2,
2) == true; }
  void Set() { InterlockedExchange(&m_Flag, 1); }
  void Reset() { InterlockedExchange(&m_Flag, 0); }

IsSet simply uses "impossible" exchange value to avoid changing
m_Flag, but still return it's value in a thread-safe manner.

Of course, this is all completely orthogonal to any race conditions I
might produce in rest of the code.


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