Understanding Unix/Linux Programming-who指令练习

  1 /*Apply a Buffer trick in who3.c*/
  2
  3 #include <stdio.h>
  4 #include <stdlib.h>
  5 #include <utmp.h>
  6 #include <fcntl.h>
  7 #include <unistd.h>
  8 #include <time.h>
  9
 10 #define NRECS 16
 11 #define NULLUT ((struct utmp * ) NULL )
 12 #define UTSIZE (sizeof(struct utmp))
 13
 14 #define SHOWHOST
 15
 16 static char utmpbuf[NRECS * UTSIZE] ;
 17 static int  num_recs ;
 18 static int  cur_rec ;
 19 static int fd_utmp = -1 ;
 20
 21 void show_info(struct utmp * utbufp) ;
 22 void show_time(time_t timeval);
 23
 24 int utmp_open(char *);
 25 int utmp_reload(void);
 26 struct utmp * utmp_next(void);
 27 void utmp_close(void);
 28
 29 int main(){
 30     struct utmp * utbufp ;
 31     struct utmp * utmp_next() ;
 32
 33     if( utmp_open( UTMP_FILE ) == -1 ){
 34         perror(UTMP_FILE );
 35         exit(1);
 36     }
 37
 38     while( (utbufp = utmp_next()) != ((struct utmp * ) NULL)){
 39         show_info(utbufp);
 40     }
 41
 42     utmp_close();
 43     return 0 ;
 44
 45 }
 46
 47 void show_info(struct utmp * utbufp ){
 48     if( utbufp->ut_type != USER_PROCESS)
 49         return;
 50
 51     printf("%-8.8s" , utbufp->ut_name );
 52     printf(" ");
 53     printf("%-8.8d" , utbufp->ut_type );
 54     printf(" ");
 55     printf("%-8.8s" , utbufp->ut_line);
 56     printf(" ");
 57     show_time(utbufp->ut_time);
 58
 59 #ifdef SHOWHOST
 60     printf("(%s)", utbufp->ut_host);
 61 #endif
 62     printf("\n");
 63     //printf("One Done!\n");
 64
 65 }
 66
 67 void show_time(long timeval)
 68 {
 69     char * cp ;
 70     cp = ctime(& timeval);
 71     printf("%12.12s" , cp + 4 );
 72 }
 73
 74 int utmp_open( char * filename ){
 75     fd_utmp = open(filename , O_RDONLY );
 76 }
 77
 78 struct utmp * utmp_next(void){
 79     struct utmp * recp ;
 80     if( fd_utmp == -1 ){
 81         return NULLUT ;
 82     }
 83     if( cur_rec == num_recs && utmp_reload() == 0 ){
 84         return NULLUT ;
 85     }
 86
 87     recp = (struct utmp * ) & utmpbuf[cur_rec * UTSIZE];
 88     cur_rec ++ ;
 89     return recp ;
 90 }
 91
 92 int utmp_reload(void)
 93 {
 94     int amt_read ; // file read , fd
 95     amt_read = read(fd_utmp , utmpbuf , NRECS * UTSIZE );
 96     num_recs = amt_read/UTSIZE ;
 97     cur_rec = 0 ;
 98     return num_recs ;
 99 }
100
101 void utmp_close(void){
102     if(fd_utmp != -1 ){
103         close(fd_utmp );
104     }
105 }

就不加注释啦，书本上并没有给出完整代码，我就简单完善下啦。

简单地运用了缓冲的思想。

在OpenSuse Leap42.1下编译运行通过。

来自为知笔记(Wiz)