These files can grow rapidly on busy systems, daily or weekly rotation is recommended. If any of these files do not exist, it is not created. These files must be created manually and are normally maintained in either the script /etc/daily or the script /etc/weekly (See cron(8).)
#define _PATH_UTMP "/var/run/utmp"
#define _PATH_WTMP "/var/log/wtmp"
#define _PATH_LASTLOG "/var/log/lastlog"
#define UT_NAMESIZE 8
#define UT_LINESIZE 8
#define UT_HOSTSIZE 16
struct lastlog {
time_t ll_time;
char ll_line[UT_LINESIZE];
char ll_host[UT_HOSTSIZE];
};
struct utmp {
char ut_line[UT_LINESIZE];
char ut_name[UT_NAMESIZE];
char ut_host[UT_HOSTSIZE];
time_t ut_time;
};
Each time a user logs in, the login program looks up the user's UID in the file lastlog. If it is found, the timestamp of the last time the user logged in, the terminal line and the hostname are written to the standard output. (Providing the login is not quiet see login(1).) The login program then records the new login time in the file lastlog
After the new Fa lastlog record is written , the file utmp is opened and the Fa utmp record for the user inserted. This record remains there until the user logs out at which time it is deleted. The utmp file is used by the programs rwho(1), users(1), w(1), and who(1).
Next, the login program opens the file wtmp and appends the user's Fa utmp record. The same Fa utmp record, with an updated time stamp is later appended to the file when the user logs out. (See init(8).) The wtmp file is used by the programs last(1) and ac(8).
In the event of a date change, a shutdown or reboot, the following items are logged in the wtmp file.