.\"	$OpenBSD: utmp.5,v 1.25 2015/09/10 17:55:21 schwarze Exp $
.\"	$NetBSD: utmp.5,v 1.5 1994/11/30 19:31:35 jtc Exp $
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.\"     @(#)utmp.5	8.2 (Berkeley) 3/17/94
.\"
.Dd $Mdocdate: September 10 2015 $
.Dt UTMP 5
.Os
.Sh NAME
.Nm utmp ,
.Nm wtmp ,
.Nm lastlog
.Nd login records
.Sh SYNOPSIS
.In utmp.h
.Sh DESCRIPTION
The
.In utmp.h
file declares the structures used to record information about current
users in the
.Nm utmp
file, logins and logouts in the
.Nm wtmp
file, and last logins in the
.Nm lastlog
file.
The timestamps of date changes, shutdowns, and reboots are also logged in the
.Nm wtmp
file.
.Pp
.Nm wtmp
can grow rapidly on busy systems, so daily or weekly rotation is recommended.
If any one of these files does not exist, it is not created.
They must be created manually and are maintained by
.Xr newsyslog 8 .
.Bd -literal -offset indent
#define _PATH_UTMP      "/var/run/utmp"
#define _PATH_WTMP      "/var/log/wtmp"
#define _PATH_LASTLOG   "/var/log/lastlog"

#define UT_NAMESIZE     32
#define UT_LINESIZE     8
#define UT_HOSTSIZE     256

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;
};
.Ed
.Pp
Each time a user logs in, the
.Xr login 1
program looks up the user's
.Tn UID
in the
.Nm lastlog
file.
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 (provided the login is not
.Dq quiet ;
see
.Xr login 1 ) .
The
.Xr login 1
program then records the new login time in the
.Nm lastlog
file.
.Pp
After the new
.Fa lastlog
record is written,
.\" the
.\" .Xr libutil 3
.\" routine
the
.Nm utmp
file is opened and the
.Fa utmp
record for the user is inserted.
This record remains until the user logs out at which time it is deleted.
The
.Nm utmp
file is used by the programs
.Xr users 1 ,
.Xr w 1 ,
and
.Xr who 1 .
.Pp
Next, the
.Xr login 1
program opens the
.Nm wtmp
file and appends the user's
.Fa utmp
record.
When the user logs out, a
.Fa utmp
record with the tty line, an updated timestamp, and zeroed name and host
fields is appended to the file (see
.Xr init 8 ) .
The
.Nm wtmp
file is used by the programs
.Xr last 1
and
.Xr ac 8 .
.Pp
In the event of a date change, shutdown, or reboot, the
following items are logged in the
.Nm wtmp
file:
.Pp
.Bl -tag -width shutdownxx -compact
.It Li reboot
.It Li shutdown
A system reboot or shutdown has been initiated.
A tilde
.Pq Sq \&~
character is placed in the field
.Fa ut_line ,
and
.Dq reboot
or
.Dq shutdown
in the field
.Fa ut_name
(see
.Xr shutdown 8
and
.Xr reboot 8 ) .
.Pp
.It Li date
The system time has been manually or automatically updated (see
.Xr date 1 ) .
The command name
.Xr date 1
is recorded in the field
.Fa ut_name .
In the field
.Fa ut_line ,
the
.Dq \&|
character indicates the time prior to the change and the
.Dq \&{
character indicates the new time.
.El
.Sh FILES
.Bl -tag -width /var/log/lastlog -compact
.It Pa /var/run/utmp
.It Pa /var/log/wtmp
.It Pa /var/log/lastlog
.El
.Sh SEE ALSO
.Xr last 1 ,
.Xr login 1 ,
.Xr who 1 ,
.Xr ac 8 ,
.Xr init 8 ,
.Xr newsyslog 8
.Sh HISTORY
A
.Nm utmp
and
.Nm wtmp
file format appeared in
.At v3 .
The
.Nm lastlog
file format appeared in
.Bx 3.0 .
.Sh CAVEATS
The strings in the
.Fa utmp
and
.Fa lastlog
structures are
.Em not
normal
.Sq C
strings and are thus not guaranteed to be null terminated.