實現了linux中w ho 命令，采用了緩沖機制，一次從utmp文件中讀取16條數據，這樣可以大大提高性能。
下面是代碼，采用了緩沖機制：
??1?/*
??2??*?what?is?diffient?to?who1?is?that?we?add?a?buffer?area?in?who2.c.
??3??*??In?this?way,?the?efferency?is?improved?.
??4??*/
??5?#include?<stdio.h>
??6?#include?<utmp.h>
??7?#include?<fcntl.h>
??8?#include?<unistd.h>
??9?#include?<stdlib.h>
?10?#include?<time.h>
?11?
?12?#define?SHOWHOST??????????//include?remote?machine?on?output
?13?
?14?//-------------------------------------------------------------------------
?15?#define?NRECS???16
?16?#define?NULLUT?((struct?utmp?*)NULL)
?17?#define?UTSIZE?(sizeof(struct?utmp))
?18?
?19?static?char?utmpbuf[NRECS?*?UTSIZE];????????????//storage
?20?static?int??num_recs;???????????????????????????//num?stored
?21?static?int??cur_rec;????????????????????????????//next?to?go
?22?static?int??fd_utmp?=?-1;???????????????????????//read?from
?23?
?24?int?utmp_open(char?*filename)
?25?{
?26???fd_utmp?=?open(filename,?O_RDONLY);
?27???cur_rec?=?num_recs?=?0;
?28???return?fd_utmp;
?29?}
?30?
?31?int?utmp_reload()
?32?{
?33???int?amt_read;
?34???amt_read?=?read(fd_utmp,?utmpbuf,?NRECS?*?UTSIZE);
?35???num_recs?=?amt_read/UTSIZE;????????????????????????//how?many?did?we?get?
?36???cur_rec?=?0;???????????????????????????????????????//reset?pointer
?37???
?38???return?num_recs;
?39?}
?40?
?41?struct?utmp?*?utmp_next()
?42?{
?43???struct?utmp?*?recp;
?44???if(?fd_utmp?==?-1)?return?NULLUT;
?45???
?46???if(cur_rec?==?num_recs?&&?utmp_reload()?==?0)?return?NULLUT;
?47???
?48???recp?=?(struct?utmp?*)?&utmpbuf[cur_rec?*?UTSIZE];
?49???cur_rec++;
?50???return?recp;
?51?}
?52?
?53?void?utmp_close()
?54?{
?55???if(fd_utmp?!=?-1)?close(fd_utmp);??//don't?close?if?not?open
?56?}
?57?
?58?//-------------------------------------------------------------------------
?59?
?60?void?show_info(struct?utmp?*);
?61?void?showtime(long?timeval);
?62?
?63?int?main()
?64?{
?65???struct?utmp?*?utbufp;???????//holds?pointer?to?next?rec
?66???struct?utmp?*?utmp_next();??//return?pointer?to?next
?67???
?68???if(utmp_open(UTMP_FILE)?==?-1)
?69???{
?70?????perror(UTMP_FILE);
?71?????exit(1);
?72???}
?73???while?(?(utbufp?=?utmp_next()?)?!=?((struct?utmp?*)?NULL)?)
?74?????show_info(utbufp);
?75???
?76???utmp_close();
?77????
?78????return?0;
?79???
?80?}
?81?
?82?/*
?83??*?display?contents?of?the?utmp?struct?
?84??*/
?85?void?show_info(?struct?utmp?*?utbufp)
?86?{
?87???/*
?88????*?remove?the?blank?record
?89????*??the?entry?ut_type?in?struct?utmp?indicate?the?types
?90????*???USER_PROCESS?is?the?auser?process
?91????*/
?92???if(utbufp->?ut_type?!=?USER_PROCESS)?return;
?93????
?94???printf("%?-8.8s",?utbufp->ut_name);
?95???printf("?");
?96???printf("%?-12.12s",?utbufp->ut_line);
?97???printf("?");
?98???//printf("%101d",?utbufp->ut_time);
?99???showtime(utbufp->ut_time);
100???printf("?");
101???#ifdef?SHOWHOST
102?????printf("(%s)",?utbufp->ut_host);
103???#endif
104???printf("\n");??
105?}
106?
107?void?showtime(long?timeval)
108?{
109???/*
110????*?display?time
111????*/
112????char?*cp;
113????cp?=?ctime(&timeval);
114????printf("%12.12s",?cp+4);
115?}
116?
下面的代碼沒有采用緩沖機制，每次讀取一條記錄：
?1?#include?<stdio.h>
?2?#include?<utmp.h>
?3?#include?<fcntl.h>
?4?#include?<unistd.h>
?5?#include?<stdlib.h>
?6?#include?<time.h>
?7?
?8?#define?SHOWHOST??????????//include?remote?machine?on?output
?9?
10?void?show_info(struct?utmp?*);
11?void?showtime(long?timeval);
12?
13?int?main()
14?{
15???struct?utmp?current_record;????//read?info?into?here
16???int??????????????utmpfd;???????????//read?from?this?descriptor
17???int????????????reclen?=?sizeof(current_record);??//how?many?bytes?to?read
18???
19???if(?(utmpfd?=?open(UTMP_FILE,?O_RDONLY))?==?-1)
20???{
21?????perror(UTMP_FILE);??//UTMP_FILE?is?in?utmp.h
22?????exit(1);
23???}
24?????/*
25????*?read?data?structure?from?UTMP_FILE
26????*/
27???while(?read(utmpfd,?¤t_record,?reclen)?==?reclen)
28?????????show_info(¤t_record);
29???
30???close(utmpfd);
31???
32?
33??//?return?0;
34?}
35?
36?/*
37??*?display?contents?of?the?utmp?struct?
38??*/
39?void?show_info(?struct?utmp?*?utbufp)
40?{
41???/*
42????*?remove?the?blank?record
43????*??the?entry?ut_type?in?struct?utmp?indicate?the?types
44????*???USER_PROCESS?is?the?auser?process
45????*/
46???if(utbufp->?ut_type?!=?USER_PROCESS)?return;
47????
48???printf("%?-8.8s",?utbufp->ut_name);
49???printf("?");
50???printf("%?-12.12s",?utbufp->ut_line);
51???printf("?");
52???//printf("%101d",?utbufp->ut_time);
53???showtime(utbufp->ut_time);
54???printf("?");
55???#ifdef?SHOWHOST
56?????printf("(%s)",?utbufp->ut_host);
57???#endif
58???printf("\n");??
59?}
60?
61?void?showtime(long?timeval)
62?{
63???/*
64????*?display?time
65????*/
66????char?*cp;
67????cp?=?ctime(&timeval);
68????printf("%12.12s",?cp+4);
69?}
70?
可以對比一下性能，記錄少的情況下不明顯，但是在記錄超過1000條時就可以明顯看出來了。這個大家可以做一下實驗。
這個who命令實現的還算比較完美。