注1：最簡單辦法，建立系統核心物件（IPC中很多，如windows event/mutex...），檢查核心物件是否建立成功，如不成功app退出，成功繼續啟動。

注2：建立共享區，如

#pragma data_seg("flag_data")
int app_count = 0;
#pragma data_seg()
#pragma comment(linker,"/SECTION:flag_data,RWS")

app判斷app_count即可。

Qt共享記憶體計數方式（小題大做）

singleinstancesharedmemory.h

#ifndef SINGLEINSTANCESHAREDMEMORY_H
#define SINGLEINSTANCESHAREDMEMORY_H
#include <QSharedMemory>

//Note: Call addRef first
class SingleInstanceSharedMemory : public QSharedMemory
{
	Q_OBJECT

public:
	SingleInstanceSharedMemory(const QString &key, QObject *parent = nullptr);
	~SingleInstanceSharedMemory();
<pre name="code" class="cpp"><pre name="code" class="cpp">	int addRef();
	int releaseRef();
	bool isRunning();
 

private:

        <span style="font-family: Arial, Helvetica, sans-serif;">bool incrementIns();</span>

bool decrementIns();};#endif // SINGLEINSTANCESHAREDMEMORY_H singleinstancesharedmemory.cpp

#include "singleinstancesharedmemory.h"

SingleInstanceSharedMemory::SingleInstanceSharedMemory(const QString &key, QObject *parent/* = nullptr*/)
	: QSharedMemory(key,parent)
{
}

SingleInstanceSharedMemory::~SingleInstanceSharedMemory()
{

}
bool SingleInstanceSharedMemory::decrementIns()
{
	if(this->isAttached())
	{
		lock();
		char *data = (char*)this->data();
		*data = '0';
		unlock();
		return true;
	}
	return false;
}

bool SingleInstanceSharedMemory::incrementIns()
{
	if(this->isAttached())
	{
		lock();
		char *data = (char*)this->data();
		*data = '1';
		unlock();
		return true;
	}
	return false;
}	

bool SingleInstanceSharedMemory::isRunning()
{
	if(this->isAttached())
	{
		char ret;
		lock();
		char *data = (char*)this->data();
		ret = *data;
		unlock();
		if ('1' == ret)
		{
			return true;
		}
	}
	return false;
}

int SingleInstanceSharedMemory::addRef()
{
	 if (!create(1)) 
	 {
		 if (QSharedMemory::AlreadyExists == error())
		 {
			 //SharedMemory has already existed
			 if (attach())
			 {
				 lock();
				 char data = *((char*)this->data());
				 unlock();
				 if ('1' == data)
				 {
					 return QSharedMemory::AlreadyExists;
				 }
			 }
		 }
	 }
	if(incrementIns())
	{
		return QSharedMemory::NoError;
	}
	else
	{
		return QSharedMemory::UnknownError;
	}

}

int SingleInstanceSharedMemory::releaseRef()
{
	if(decrementIns())
	{
		return QSharedMemory::NoError;
	}
	else
	{
		return QSharedMemory::UnknownError;
	}
}
 

程式啟動時：

	SingleInstanceSharedMemory *share_memory = new SingleInstanceSharedMemory(QString("SingleInstanceApplication"));
	if(QSharedMemory::AlreadyExists == share_memory->addRef())
	{
		//ShowAppMainWindow();
		return 0; 
	} 

如果有登出，可呼叫：
   share_memory->releaseRef()