In an assignment for college it was suggested to use the C readline function in an exercise. I have searched for its reference but still haven't found it. Does it really exist? In which header? Can you please post the link to the reference?
Readline exists in two places, libreadline and libedit (also called libeditline). Both have an identical interface. The difference is libreadline is licensed under the GPL, libedit is 3 clause BSD. Licensing is really not a concern for an assignment, at least I don't think it is. Either license allows you to use the code freely. If you link against readline, be sure to make the whole program GPL 2 or later which will satisfy whatever version of the GPL governs the system readline. It may be GPL2+ or GPL3+, depending on the age of the system. I'm not advocating either license, that's up to you.
Note, take care to install either / or and adjust linking as needed (-lreadline or -ledit or -leditline). Both are libraries and not a part of the standard C library.
Edit (afterthought):
If releasing a program to the wild, its a nice gesture to allow the user to configure it with their readline of choice. For instance: --with-readline or --with-libedit, etc. This allows a binary package that conforms to their choice of license, at least as far as readline is concerned.
Links: Readline and Edit/Editline.