Understanding Unix/Linux Programming-who指令练习

/*Apply a Buffer trick in who3.c*/

#include <stdio.h>
#include <stdlib.h>
#include <utmp.h>
#include <fcntl.h>
#include <unistd.h>
#include <time.h>

#define NRECS 16
#define NULLUT ((struct utmp * ) NULL )
#define UTSIZE (sizeof(struct utmp))

#define SHOWHOST    

static char utmpbuf[NRECS * UTSIZE] ;
static int  num_recs ;
static int  cur_rec ;
static int fd_utmp = -1 ;

void show_info(struct utmp * utbufp) ;
void show_time(time_t timeval);

int utmp_open(char *);
int utmp_reload(void);
struct utmp * utmp_next(void);
void utmp_close(void);

int main(){ 
    struct utmp * utbufp ;
    struct utmp * utmp_next() ;

    if( utmp_open( UTMP_FILE ) == -1 ){
        perror(UTMP_FILE );
        exit(1);
    }

    while( (utbufp = utmp_next()) != ((struct utmp * ) NULL)){
        show_info(utbufp);
    }

    utmp_close();
    return 0 ;

}

void show_info(struct utmp * utbufp ){
    if( utbufp->ut_type != USER_PROCESS)
        return;

    printf("%-8.8s" , utbufp->ut_name );
    printf(" ");
    printf("%-8.8d" , utbufp->ut_type );
    printf(" ");
    printf("%-8.8s" , utbufp->ut_line);
    printf(" ");
    show_time(utbufp->ut_time);

#ifdef SHOWHOST
    printf("(%s)", utbufp->ut_host);
#endif
    printf("
");
    //printf("One Done!
");

}

void show_time(long timeval)
{
    char * cp ;
    cp = ctime(& timeval);
    printf("%12.12s" , cp + 4 );
}

int utmp_open( char * filename ){
    fd_utmp = open(filename , O_RDONLY );
}

struct utmp * utmp_next(void){
    struct utmp * recp ;
    if( fd_utmp == -1 ){
        return NULLUT ;
    }
    if( cur_rec == num_recs && utmp_reload() == 0 ){
        return NULLUT ;
    }

    recp = (struct utmp * ) & utmpbuf[cur_rec * UTSIZE];
    cur_rec ++ ;
    return recp ;
}

int utmp_reload(void)
{
    int amt_read ; // file read , fd 
    amt_read = read(fd_utmp , utmpbuf , NRECS * UTSIZE );
    num_recs = amt_read/UTSIZE ;
    cur_rec = 0 ;
    return num_recs ;
}

void utmp_close(void){
    if(fd_utmp != -1 ){
        close(fd_utmp );
    }
}

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)