Do I need to add these client functions in the UML? If so, how should that be done?
The answer depends on what your purpose is for the UML.
Most people use UML for a high level overview aimed at future developers and maintainers. In that case, most of those details can be left out - and it is your choice which details to leave out.
If I do this, I might add a single box labelled "I/O Functions" without detailing which ones. Or I might create a separate diagram to detail them. But I certainly won't mention each and every function in a single diagram; Too many details will only cloud the picture.
If your purpose is to mention every single detail of your project in UML, then in essence you are writing code in a graphical way. Every attempt at that has failed, including UML. So I would strongly advice to rethink why you are using UML in that case. 
I'm sure there is an official place to put those details, but in practice barely anyone cares enough to remember where.
 Unless this is a course, and your teacher requires you to do this. In that case, put those functions wherever he says you should put them.