linesw structure

[LEE at kuhub.cc.ukans.edu]

I am writing a sort of tty device driver for unix pc 3b1. I found out that 
there are linesw structure in include/sys/conf.h whose components are pointers
to functions which seems very important for writing tty device driver. The 
structure is

struct   linesw  {
                 int      (*l_open)();
                 int      (*l_close)();
                 int      (*l_read)();
                 int      (*l_write)();
                 int      (*l_ioctl)();
                 int      (*l_input)();
                 int      (*l_output)();
                 int      (*l_mdmint)();
};

What does these l_xxxx functions do? What are the parameters for these
function? Where can I get the information for these functions? Any response
will be greatly appreciated. Please, help me!!!