第二章編寫(xiě)who命令who命令的用

能who命令的原理、相關(guān)的系統(tǒng)調(diào)用who命令的實(shí)現(xiàn)who命令的用法who命令的功能who命令的功能who的工作流程涉及的系統(tǒng)文件utmp文件：maintains

a

full

accounting

ofthe

current

status

of

the

system$

who

/var/run/utmpwtmp文件：acts

as

a

historicalutmp$

who

/var/log/wtmp搜索聯(lián)機(jī)幫助——按關(guān)鍵字模糊搜索搜索聯(lián)機(jī)幫助——按小節(jié)精確搜索搜索聯(lián)機(jī)幫助——按小節(jié)精確搜索搜索聯(lián)機(jī)幫助——按小節(jié)精確搜索如何utmp記錄如何utmp記錄如何utmp記錄—pp.

35-36who的工作流程who1的實(shí)現(xiàn)#include#include#include#include<stdio.h><utmp.h><fcntl.h><unistd.h>/*

include

remote

machine

on

output

*/#define

SHOWHOSTint

main(){struct

utmpintintcurrent_record;

/*

read

info

into

here

*/utmpfd; /*

read

from

this

descriptor

*/reclen

=

sizeof(current_record);if

(

(utmpfd

=

open(UTMP_FILE,O_RDONLY))

==

-1

){perror(

UTMP_FILE

); /*

UTMP_FILEis

in

utmp.h

*/exit(1);}while

(

read(utmpfd,

¤t_record,

reclen)

==

reclen

)show_info(¤t_record);close(utmpfd);return

0; /*

went

ok

*/}who1的實(shí)現(xiàn)/*show

info()displays

contents

of

the

utmp

struct

in

human

readable

form*note*

these

sizes

should

not

be

hardwired*/show_info(

struct

utmp

*utbufp

){printf("%-8.8s",

utbufp->ut_name);/*

the

logname*/printf("

");/*

a

space*/printf("%-8.8s",

utbufp->ut_line);/*

the

tty*/printf("

");/*

a

space*/printf("%10ld",

utbufp->ut_time);/*

login

time*/printf("

");/*

aspace*/#ifdefSHOWHOSTprintf("(%s)",

utbufp->ut_host);/*

the

host*/#endifprintf("\n");/*

newline*/}who1的效果who1存在的問(wèn)題冗余記錄顯示時(shí)間消除冗余記錄消除冗余記錄void

show_info(

struct

utmp

*utbufp

){if

(utbufp->ut_type

!=

USER_PROCESS

)return;…..時(shí)間的轉(zhuǎn)化時(shí)間的轉(zhuǎn)化時(shí)間的轉(zhuǎn)化void

showtime(

long

timeval

){/* displays

time

in

aformat

fit

for

human

consumptionuses

ctime

to

build

a

string

then

picks

partsout

of

itNote:

%12.12s

printsastring

12chars

wide

and

LIMITSit

to12chars.

*/char

*cp; /*

to

hold

address

oftime

*/cp=

ctime(&timeval); /*

converttime

tostring

*/printf("%12.12s",

cp+4);/*

pick

12

charsfrom

pos

4

*/}#include

<time.h>printf("%10ld",

utbufp->ut_time);→showtime(

utbufp->ut_time

);who2的效果cp命令的實(shí)現(xiàn)系統(tǒng)調(diào)用creat(p.44)、write

(p.45)程序：cp1.c

(p.

46)緩沖技術(shù)提高文件讀寫(xiě)效率程序：utmplib.c