Should I use POSIX or OS default functionalities?

[iQChange]

What should I use? Why? What are the pros/cons of using it?

[Gamer2015]

What kind of functionality do you need that is not allready included in std and boost (or qt)?
What kind of Application do you make?
(maybe the application doesn't need any cross platform compatibility?)

Without that knowledge I'd recommend you to use POSIX because it is a Standard and therefore far more platform independent than OS default functionalities.

But I have to admit that I tried to avoid both of them and so I only used POSIX once (for multithreading a few years ago) and never any default OS functionalities so I am looking forward to see answers from more experienced Softwaredevelopers.

[iQChange]

I actually use boost for a lot of things. But sometimes it's too heavy, so I'm dealing with Juce libraries.