i need to write a c++ program for the simple edlin test editor.
can any one give me a hint on where i should start off
i am not sure about how the edlin text editor handles text files
and how it in general functions.

edlin is a line editor.
That means you can edit only a single line of text at a time.

To find out how it works, go to a commandprompt and type edlin followed by a filename (for a nonexistent file...).
You'll get a prompt. Type a question mark (?) and press enter, this will give you a list of commands.
Some commands will change a line of text or allow you to change it or enter it, others will work on the file.
To cancel edit mode, use Ctrl-Z (F6).

Here's a very short test session:

C:\DOCUME~1\wtg>edlin test.txt
Nieuw bestand
       1:*hello world
       2:*this is a test^Z

C:\DOCUME~1\wtg>type test.txt
hello world
this is a test