Console Question

Does any know one how to use c internal function to do this job

e.g. cls in window and clear in unix

There is no standard way of doing that. So one way that works in windows will not work in unix and vice versa. Read the faq.

You may be looking at the system() call, but there are security issues regarding this method to clear the screen - predominantly the program (cls or clear) can be replaced by a malicious program - not to mention it's slow as well.