QtForum.org

You can use QtSingleApplication class, but i think it is only in the commercial edition.
On MacOS you cannot launch the application several times.
On Window you need to create a mutex

You can use this in your main :

#ifdef Q_WS_WIN
HANDLE hMutex = NULL;
#endif
int code = -1;
try
{

#ifdef Q_WS_WIN
hMutex = CreateMutex (NULL,FALSE,L"Thenameoftheapllicationforexample");
if ( (hMutex == NULL) || (GetLastError() == ERROR_ALREADY_EXISTS) )
{
CloseHandle(hMutex);
return -1;
}
#endif

// Your code
// ...


}
catch(...)
{
// Your code
}

#ifdef Q_WS_WIN
CloseHandle(hMutex);
#endif
return code;

}