应用程序单例

注1：最简单办法，创建系统核心对象（IPC中很多，如windows event/mutex...），检查核心对象是否创建成功，如不成功app退出，成功继续启动。

注2：创建共享区，如

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

app判断app_count即可。

Qt共享内存计数方式（小题大做）

singleinstancesharedmemory.h

#ifndef SINGLEINSTANCESHAREDMEMORY_H#define SINGLEINSTANCESHAREDMEMORY_H#include <QSharedMemory>//Note: Call addRef firstclass SingleInstanceSharedMemory : public QSharedMemory{Q_OBJECTpublic: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;}}

程序启动时：

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

如果有注销，可调用：
   share_memory->releaseRef()