Tuesday 15 February 2011

Debugging Mutex and Locks

When there are multiple Mutex's in the program, it may be required to find if a particular thread is locked far longer than necessary. This can cause problems and the output may not be what is expected.

To get round this, I took an old example from here and modified it to help me print some debugging info.

I modified the printSomething() in Singleton.h to add a sleep as follows:



  void printSomething(char *name, int count)
{

Lock guard(mutex_);
Sleep(10);
Lock guard2(mutex_);
std::cout << name << " loop " << count << std::endl;
}




and I modified the Mutex.h as follows:



//Example from http://www.relisoft.com/Win32/active.html
#if !defined _MUTEX_H_
#define _MUTEX_H_

class
Mutex
{

friend class
Lock;
public
:
Mutex () { InitializeCriticalSection (& _critSection); }
~
Mutex () { DeleteCriticalSection (& _critSection); }
private
:
void
Acquire ()
{

DWORD start = GetTickCount();
EnterCriticalSection (& _critSection);
DWORD elapsed = GetTickCount() - start;
if
(elapsed > 15)
{

//Debugging Info
std::cout<<"Debugging Info: Waited at mutex for "<<elapsed<<std::endl;
}
}

void
Release ()
{

LeaveCriticalSection (& _critSection);
}


CRITICAL_SECTION _critSection;
};


#endif



The output is as follows:
The small problem with the above approach is that if a Thread is deadlocked, we may not get the debug output as we would have to kill the process.

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