lock/unlock events for in C++ app which should not depend on screen savers

We have a product, which was developed for Windows. Because of customer request we are developing it for Linux also. So being a beginner to Linux, have migrated most of it (which I loved it).

Now got stuck where I want lock/unlock events to be captured from OS ("System Event Notification Service(SENS)" in Windows). Gone through a lot about Gnome screensaver which gives D-Bus interface to find out lock/unlock events. If instead xscreensaver is running, which doesn't provide any such interface(I Guess). Is there any concrete solution for this?

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