AppRunGuard.cpp

//
// Created by Andrey Streltsov on 11/06/15.
// Based on code from here: http://stackoverflow.com/a/28172162/365754

#include "AppRunGuard.h"
#include <qcryptographichash.h>

namespace
{

    QString generateKeyHash( const QString& key, const QString& salt )
    {
        QByteArray data;

        data.append( key.toUtf8() );
        data.append( salt.toUtf8() );
        data = QCryptographicHash::hash( data, QCryptographicHash::Sha1 ).toHex();

        return data;
    }

}


AppRunGuard::AppRunGuard( const QString& key )
        : key( key )
        , memLockKey( generateKeyHash( key, "_memLockKey" ) )
        , sharedmemKey( generateKeyHash( key, "_sharedmemKey" ) )
        , sharedMem( sharedmemKey )
        , memLock( memLockKey, 1 )
{
    QSharedMemory fix( sharedmemKey );    // Fix for *nix: http://habrahabr.ru/post/173281/
    fix.attach();
}

AppRunGuard::~AppRunGuard()
{
    release();
}

bool AppRunGuard::isAnotherRunning()
{
    if ( sharedMem.isAttached() )
        return false;

    memLock.acquire();
    const bool isRunning = sharedMem.attach();
    if ( isRunning )
        sharedMem.detach();
    memLock.release();

    return isRunning;
}

bool AppRunGuard::tryToRun()
{
    if ( isAnotherRunning() )   // Extra check
        return false;

    memLock.acquire();
    const bool result = sharedMem.create( sizeof( quint64 ) );
    memLock.release();
    if ( !result )
    {
        release();
        return false;
    }

    return true;
}

void AppRunGuard::release()
{
    memLock.acquire();
    if ( sharedMem.isAttached() )
        sharedMem.detach();
    memLock.release();
}

AppRunGuard.h

//
// Created by Andrey Streltsov on 11/06/15.
// Based on code from here: http://stackoverflow.com/a/28172162/365754

#ifndef RUNGUARD_H
#define RUNGUARD_H
#include <QString>
#include <qsharedmemory.h>
#include <qsystemsemaphore.h>


class AppRunGuard : public QObject {

public:
    AppRunGuard( const QString& key );
    virtual ~AppRunGuard();

    bool isAnotherRunning();
    bool tryToRun();
    void release();

private:
    const QString key;
    const QString memLockKey;
    const QString sharedmemKey;

    QSharedMemory sharedMem;
    QSystemSemaphore memLock;

    Q_DISABLE_COPY(AppRunGuard)
};


#endif // RUNGUARD_H

main.cpp

//
// Created by Andrey Streltsov on 11/06/15.
// Usage example

#define kAppRunUUID "your-app-instance-uuid-here"

int main(int argc, char *argv[]) {

	AppRunGuard guard(kAppRunUUID);
    if ( !guard.tryToRun() )
        return 1;
  
  QApplication app(argc, argv);
    return app.exec();
    
}