V3/man/man5/utmp.5

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.pa 1
.he 'UTMP (V)'3/15/72'UTMP (V)'
.ti 0
NAME		/tmp/utmp  -- user information
.sp
.ti 0
DESCRIPTION	This
file allows one to discover information about who is currently
using UNIX.
The file is binary; each entry is 16(10) bytes long.
The first eight bytes contain a user's login name or
are null if the table slot is unused.
The low order byte of the next word contains the last
character of a typewriter name.
The next two words contain the user's login time.
The last word is unused.

This file resides in directory /tmp.
.sp
.ti 0
SEE ALSO	/etc/init, which maintains the file;
.br
who(I), which interprets it.