V6/usr/man/man5/utmp.5

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.sh NAME
utmp \*- user information
.sh 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.
.sh FILES
/etc/utmp
.sh "SEE ALSO"
init (VIII) and login (I), which maintain the file;
who (I), which interprets it.