RunGuard.cpp

/**
--------------------------------------------------------------------------------
-   Module      :   runGuard.cpp
-   Description :   This file for usage with QT makes sure only a single
-                   instance of the program is run.
-   Author      :   SaZ from Stackoverflow
-                   Tim Zaman, 18-FEB-2016
--------------------------------------------------------------------------------
*/

#include "runGuard.h"
#include <QCryptographicHash>


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;
	}
}


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

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

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

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

	return isRunning;
}

bool RunGuard::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 RunGuard::release(){
	memLock.acquire();
	if ( sharedMem.isAttached() ){
		sharedMem.detach();
	}
	memLock.release();
}