How to add the user to file ?

[yasser]

Hello ,

I have problem when I want to send the user name of a computer to file that made before ..

witch means ( I'm programming a c++ program that save the users of one machine and let them to send e-mails to other users in same machine and they can read their messages or delete it or send new message to other user to this local machine .. ) How can I know that this person who signed in has this user name and show other users ?


by the way , I'm using Linux .. and all the other users do ..


thank , I really need to know ..