How to programmatically prevent linux computer from sleeping or turning on screensaver?

From a quick look at how mplayer and SDL do it, there are two things you can do to prevent the screensaver from firing up:

• Disable it for the duration of the program:
  • Using XScreenSaverSuspend
  • Using DBus, calling org.freedesktop.ScreenSaver.Inhibit
• Ping it periodically:
  • Using XResetScreenSaver
  • Using DBus, calling org.freedesktop.ScreenSaver.SimulateUserActivity

I'm using QTDBUS using that

QDBusConnection bus = QDBusConnection::sessionBus();if(bus.isConnected()){    QString services[MAX_SERVICES] = {        "org.freedesktop.ScreenSaver",        "org.gnome.SessionManager"    };    QString paths[MAX_SERVICES] = {        "/org/freedesktop/ScreenSaver",        "/org/gnome/SessionManager"    };    for(int i = 0; i < MAX_SERVICES ; i++)    {                QDBusInterface screenSaverInterface(            services[i], paths[i],services[i], bus, this);        if (!screenSaverInterface.isValid())            continue;               QDBusReply<uint> reply = screenSaverInterface.call(            "Inhibit", "YOUR_APP_NAME", "REASON");        if (reply.isValid())        {            cookieID = reply.value();            qDebug()<<"succesful"        } else {               QDBusError error =reply.error();            qDebug()<<error.message()<<error.name();           }    }}

As far as I can tell, things with xdg in the name are the way to go for cross-desktop-environment functionality. There appears to be a commandline utility called xdg-screensaver. It seems to have a bunch of screensavers hardwired and then fall back to xset s off/xset s default, so you might want to just call it when it's installed, or fall back to copying part of its logic when it's not...