V7/usr/man/man5/utmp.5

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.TH UTMP 5 
.SH NAME
utmp, wtmp \- login records
.SH SYNOPSIS
.B #include <utmp.h>
.SH DESCRIPTION
The
.I utmp
file allows one to discover information about who is currently
using UNIX.
The file is a sequence of entries with the following
structure declared in the include file:
.RS
.PP
.nf
.so /usr/include/utmp.h
.fi
.RE
.PP
This structure gives the name of the special file
associated with the user's terminal, the user's login name,
and the time of the login in the form of
.IR time (2).
.PP
The
.I wtmp
file records all logins and logouts.
Its format is exactly like
.I utmp
except that
a null user name indicates a logout on the associated
terminal.
Furthermore, the terminal name `~' indicates that the
system was rebooted at the indicated time;
the adjacent pair of entries with terminal names
`\^|\^' and `}' indicate the system-maintained time
just before and just after a
.I date
command has changed the system's idea of the time.
.PP
.I Wtmp
is maintained by
.IR login (1)
and
.IR init (8).
Neither of these programs creates the file,
so if it is removed record-keeping is turned off.
It is summarized by
.IR ac (1).
.SH FILES
/etc/utmp
.br
/usr/adm/wtmp
.SH "SEE ALSO"
login(1), init(8), who(1), ac(1)