| UTMP(5) | File Formats Manual | UTMP(5) | 
utmp, wtmp,
  lastlog —
#include <utmp.h>
<utmp.h> declares the
  structures used to record information about current users in the file
  utmp, logins and logouts in the file
  wtmp, and last logins in the file
  lastlog. The time stamps of date changes, shutdowns
  and reboots are also logged in the wtmp file.
The wtmp file can grow rapidly on busy
    systems, and is normally rotated with
    newsyslog(8).
These files must be created manually; if they do not exist, they are not created automatically.
#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(1) 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 set
    quiet; see
    login(1). The
    login(1) program then records
    the new login time in the file lastlog.
After the new lastlog record is written, the
    file utmp is opened and the
    utmp record for the user inserted. This record remains
    there until the user logs out at which time it is deleted (by clearing the
    user and host fields, and updating the timestamp field). The
    utmp file is used by the programs
    rwho(1),
    users(1),
    w(1), and
    who(1).
Next, the login(1)
    program opens the file wtmp, and appends the user's
    utmp record. When the user logs out, a
    utmp record with the tty line, an updated time stamp,
    and cleared user and host fields is appended to the file by
    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.
rebootshutdown~’ is placed in the field
      ut_line, and reboot or
      shutdown in the field
      ut_name (see
      shutdown(8) and
      reboot(8)).
    
  date|’ indicates the time
      prior to the change, and the character
      ‘{’ indicates the new time.utmp file.wtmp file.lastlog file.utmp file format appeared in
  Version 1 AT&T UNIX. The
  wtmp file format appeared in
  Version 3 AT&T UNIX. The
  lastlog file format appeared in
  3.0BSD.
| September 1, 2019 | NetBSD 9.4 |