writing
string.h is a common project for CS students ... still remember the sleepless nights of blank stare when doing our projects .....
anyways, yes that dam thing already exist. and that library is pretty simple actually (want to try
stdio.h?

)
i believe the reason is that they want you (or us) to learn how they came towards creating a library, how to "object orient" the functionalities, etc. also, it's a good practice for understanding objects, structs, classes, etc. also, i don't know about your project, but we had to struggle with arrays, pointers, and all those goodies back when we implemented our string.h
you are not reinventing the wheel because the library exist.
you are simply trying to break the library apart and reconstruct it again
Damascus: "Don't trust anyone who recommends VI."
An emacs/pico user, i assume? and btw, it's
vi, not
VI 
(unless of course you're referring to something else)
<== vi / vim user

<== still have a lot to learn about vi
have fun

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