The file
<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.
shutdown
A system reboot or shutdown has been initiated. The character ‘~' is placed in the field
ut_line, and
reboot or
shutdown in the field
ut_name (see
shutdown(8) and
reboot(8)).
date
The system time has been manually or automatically updated by
date(1). The command name
date is recorded in the field
ut_name. In the field
ut_line, the character ‘\*(Ba' indicates the time prior to the change, and the character ‘{' indicates the new time.