Ubuntu :: Socket Programing Using TCP?

I need a code for a multi client and server

where a client need to login to server after a connection has been established if login is not sucessfull connection should be terminated I need it in c language

server asks client name and password if authenticated , a connection is established . and if client is new can also register its name

I have done it using files but its not working when asks for user name it searches a file with that username if a file is found and password with same as username it opens the username file check for the string if it matches with the string login is ok