V4/man/man5/utmp.5

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.th UTMP V 9/10/73
.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.
.s3
This file resides in directory /tmp.
.sh "SEE ALSO"
/etc/init, which maintains the file;
who(I), which interprets it.