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Test Your Knowledge in Computers #592
Difficulty: Medium
Suppose we want to sort a huge collection of 1 trillion, 10^12, elements. Doing so with Selection Sort or Insertion Sort would require about n^2 = (10^12)^2 = 10^24 or 1 sextillion comparisons.
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wtmp(5) [mojave man page]

UTMP(5) 						      BSD File Formats Manual							   UTMP(5)

NAME
utmp, wtmp, lastlog -- login records (DEPRECATED) SYNOPSIS
#include <utmp.h> DESCRIPTION
The interfaces in file <utmp.h> are all DEPRECATED and are only provided for compatibility with previous releases of Mac OS X. See pututxline(3) and utmpx(5) for the supported interfaces. <utmp.h> declares the structures used to record information about current users in the file utmp, logins and logouts in the file wtmp, and last logins in the file lastlog. The time stamps of date changes, shutdowns and reboots are also logged in the wtmp file. These files can grow rapidly on busy systems, daily or weekly rotation is recommended. If any of these files do not exist, it is not cre- ated. These files must be created manually and are normally maintained in either the script /etc/daily or the script /etc/weekly. (See cron(8).) #define _PATH_UTMP "/var/run/utmp" #define _PATH_WTMP "/var/log/wtmp" #define _PATH_LASTLOG "/var/log/lastlog" #define UT_NAMESIZE 8 #define UT_LINESIZE 8 #define UT_HOSTSIZE 16 struct lastlog { time_t ll_time; char ll_line[UT_LINESIZE]; char ll_host[UT_HOSTSIZE]; }; struct utmp { char ut_line[UT_LINESIZE]; char ut_name[UT_NAMESIZE]; char ut_host[UT_HOSTSIZE]; time_t ut_time; }; Each time a user logs in, the login program looks up the user's UID in the file lastlog. If it is found, the timestamp of the last time the user logged in, the terminal line and the hostname are written to the standard output. (Providing the login is not quiet, see login(1).) The login program then records the new login time in the file lastlog. After the new lastlog record is written , the file utmp is opened and the utmp record for the user inserted. This record remains there until the user logs out at which time it is deleted. The utmp file is used by the programs rwho(1), users(1), w(1), and who(1). Next, the login program opens the file wtmp, and appends the user's utmp record. The same utmp record, with an updated time stamp is later appended to the file when the user logs out. (See launchd(8).) The wtmp file is used by the programs last(1) and ac(8). In the event of a date change, a shutdown or reboot, the following items are logged in the wtmp file. reboot shutdown A system reboot or shutdown has been initiated. The character '~' is placed in the field ut_line, and reboot or shutdown in the field ut_name. (See shutdown(8) and reboot(8).) date The system time has been manually or automatically updated. (See date(1).) The command name date is recorded in the field ut_name. In the field ut_line, the character '|' indicates the time prior to the change, and the character '{' indicates the new time. FILES
(These files no longer exist in 10.5 or later.) /var/run/utmp The utmp file. /var/log/wtmp The wtmp file. /var/log/lastlog The lastlog file. SEE ALSO
last(1), login(1), who(1), ac(8), launchd(8) HISTORY
A utmp and wtmp file format appeared in Version 6 AT&T UNIX. The lastlog file format appeared in 3.0BSD. 4th Berkeley Distribution March 17, 1994 4th Berkeley Distribution

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UTMP(5) 						      BSD File Formats Manual							   UTMP(5)

NAME
utmp, wtmp, lastlog -- login records SYNOPSIS
#include <utmp.h> DESCRIPTION
The file <utmp.h> declares the structures used to record information about current users in the file utmp, logins and logouts in the file wtmp, and last logins in the file lastlog. The time stamps of date changes, shutdowns and reboots are also logged in the wtmp file. The wtmp file can grow rapidly on busy systems, and is normally rotated with newsyslog(8). These files must be created manually; if they do not exist, they are not created automatically. #define _PATH_UTMP "/var/run/utmp" #define _PATH_WTMP "/var/log/wtmp" #define _PATH_LASTLOG "/var/log/lastlog" #define UT_NAMESIZE 8 #define UT_LINESIZE 8 #define UT_HOSTSIZE 16 struct lastlog { time_t ll_time; char ll_line[UT_LINESIZE]; char ll_host[UT_HOSTSIZE]; }; struct utmp { char ut_line[UT_LINESIZE]; char ut_name[UT_NAMESIZE]; char ut_host[UT_HOSTSIZE]; time_t ut_time; }; Each time a user logs in, the login(1) program looks up the user's UID in the file lastlog. If it is found, the timestamp of the last time the user logged in, the terminal line and the hostname are written to the standard output, providing the login is not set quiet; see login(1). The login(1) program then records the new login time in the file lastlog. After the new lastlog record is written, the file utmp is opened and the utmp record for the user inserted. This record remains there until the user logs out at which time it is deleted (by clearing the user and host fields, and updating the timestamp field). The utmp file is used by the programs rwho(1), users(1), w(1), and who(1). Next, the login(1) program opens the file wtmp, and appends the user's utmp record. When the user logs out, a utmp record with the tty line, an updated time stamp, and cleared user and host fields is appended to the file by init(8). The wtmp file is used by the programs last(1) and ac(8). In the event of a date change, a shutdown or reboot, the following items are logged in the wtmp file. reboot shutdown A system reboot or shutdown has been initiated. The character '~' is placed in the field ut_line, and reboot or shutdown in the field ut_name (see shutdown(8) and reboot(8)). date The system time has been manually or automatically updated by date(1). The command name date is recorded in the field ut_name. In the field ut_line, the character '|' indicates the time prior to the change, and the character '{' indicates the new time. FILES
/var/run/utmp The utmp file. /var/log/wtmp The wtmp file. /var/log/lastlog The lastlog file. SEE ALSO
last(1), login(1), w(1), who(1), utmpx(5), ac(8), init(8), lastlogin(8), newsyslog(8) HISTORY
A utmp and wtmp file format appeared in Version 6 AT&T UNIX. The lastlog file format appeared in 3.0BSD. BSD
May 14, 2003 BSD

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