Class to get a thread safe count of previous Instances

There have been many examples,posted on CodeGuru, of how to check for a previously running instance of an application. Some used DDE, others mutexs but what struck me as the easiest suggestion was to use a 'shared section' to store a flag - accessable by every instance.

The following class uses that technique, along with a couple of the 'interlocked' functions to make it threadsafe. Instead of a simple flag, a count of running instances is kept. The count obtained by the class is only a snapshot.

To use the class simply create a static instance of it. Then check the 'Count()' method to determine how many instances of your application where running before the CPreviousInstance object was instantiated.

Code tested on WinNT 4.0 spk 4 VC6.0 spk 2

// CPreviousInstance.h

class CPreviousInstance  
{
public:
	CPreviousInstance();
	virtual ~CPreviousInstance();

	LONG Count() const
	{
		return m_previous;
	}

private:
	static LONG		s_count;
	LONG			m_previous;

	CPreviousInstance(const CPreviousInstance&);
	CPreviousInstance& operator=(const CPreviousInstance&);
};

// CPreviousInstance.cpp

// static instance count stored in a shared read/write section

#pragma data_seg("Instance")

LONG  CPreviousInstance::s_count = 0;

#pragma data_seg()
#pragma comment(linker,"/section:Instance,rws")


// Construction/Destruction

CPreviousInstance::CPreviousInstance():
m_previous(0)
{
	m_previous = ::InterlockedIncrement(&s_count);
	--m_previous;
}

CPreviousInstance::~CPreviousInstance()
{
	::InterlockedDecrement(&s_count);
}




Comments

  • Even Jeffrey Richter did neglect the crash bug.

    Posted by Legacy on 10/17/2000 12:00am

    Originally posted by: Franz

    I saw Mike Junkin's proposal in a slightly different coding published in Jeffrey Richter's Advanced Windows, an even an expert like him did not point out that this method fails when one of the instances crashes.

    This proofs to me how much source code may exist which holds 'logical' bugs and un-thought possibilities.

    Reply
  • What about a crash

    Posted by Legacy on 03/15/1999 12:00am

    Originally posted by: Mike

    Some of the other methods allow the system to handle a crash. Wouldn't your count be incorrect if one of the instances would take a dive?

    Reply
Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Organizations are increasingly gravitating toward mobile-first application development as they assess the need to revamp their application portfolios to support touch computing and mobility. Consumerization has brought higher expectations for application usability along with the mobile devices themselves. Enterprises are increasingly shifting their new application acquisitions and development efforts toward mobile platforms. With this backdrop, it is natural to expect application platform vendors to invest in …

  • Cloud computing isn't new, but it's still a hot topic in technology circles. Moving to the cloud has done great things for many businesses. When it comes to handling a basic business necessity, cloud computing has brought many improvements to overall business continuity. Using the cloud for DR makes a lot of sense for many, but it's not exactly the most simple concept. Read this eBook to find answers to all the key questions IT professionals have about cloud-based disaster recovery, and helpful info around the …

Most Popular Programming Stories

More for Developers

RSS Feeds

Thanks for your registration, follow us on our social networks to keep up-to-date