The utmp file records who is currently using the system. The wtmp file records
all logins and logouts. See utmp(5).
The function login() takes the supplied struct utmp ut, and writes
it to both utmp and wtmp file.
The function logout() clears the entry in the utmp file again.
More precisely, login() takes the argument ut struct, fills the
field ut->ut_type (if there is such a field) with the value
USER_PROCESS, and fills the field ut->ut_pid (if there is
such a field) with the process ID of the calling process. Then it tries to
fill the field ut->ut_line. It takes the first of stdin, stdout,
stderr that is a tty, and stores the corresponding pathname minus a possible
leading /dev/ into this field, and then writes the struct to the utmp file. On
the other hand, if no tty name was found, this field is filled with
"???" and the struct is not written to the utmp file. After this,
the struct is written to the wtmp file.
The logout() function searches the utmp file for an entry matching the
ut_line argument. If a record is found, it is updated by zeroing out
the ut_name and ut_host fields, updating the ut_tv
timestamp field and setting ut_type (if there is such a field) to