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(libc.info)User Accounting Database

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The User Accounting Database
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   Most Unix-like operating systems keep track of logged in users by
maintaining a user accounting database.  This user accounting database
stores for each terminal, who has logged on, at what time, the process
ID of the user's login shell, etc., etc., but also stores information
about the run level of the system, the time of the last system reboot,
and possibly more.

   The user accounting database typically lives in `/etc/utmp',
`/var/adm/utmp' or `/var/run/utmp'.  However, these files should
*never* be accessed directly.  For reading information from and writing
information to the user accounting database, the functions described in
this section should be used.

Manipulating the Database

Scanning and modifying the user

accounting database.

XPG Functions

A standardized way for doing the same thing.

Logging In and Out

Functions from BSD that modify the user

accounting database.