Linux下who命令模仿

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#include        <stdlib.h>#include        <stdio.h>#include        <unistd.h>#include        <utmp.h>#include        <fcntl.h>#include        <time.h>/* #define      SHOWHOST */void showtime(long);void show_info(struct utmp *);int main(){        struct utmp     utbuf;          /* read info into here */        int             utmpfd;         /* read from this descriptor */        if ( (utmpfd = open(UTMP_FILE, O_RDONLY)) == -1 ){                perror(UTMP_FILE);                exit(1);        }        while( read(utmpfd, &utbuf, sizeof(utbuf)) == sizeof(utbuf) )                show_info( &utbuf );        close(utmpfd);        return 0;}/* *      show info() *                      displays the contents of the utmp struct *                      in human readable form *                      * displays nothing if record has no user name */void show_info( struct utmp *utbufp ){        if ( utbufp->ut_type != USER_PROCESS )                return;        printf("%-8.8s", utbufp->ut_name);      /* 显示用户账号 */        printf(" ");                            /* 空格     */        printf("%-8.8s", utbufp->ut_line);      /*显示端口名      */        printf(" ");                            /*空格      */        showtime( utbufp->ut_time );            /*显示时间 */#ifdef SHOWHOST        if ( utbufp->ut_host[0] != '\0' )                printf(" (%s)", utbufp->ut_host);/* the host    */#endif        printf("\n");                          /* newline      */}void showtime( long timeval )/* *      displays time in a format fit for human consumption *      uses ctime to build a string then picks parts out of it *      Note: %12.12s prints a string 12 chars wide and LIMITS *      it to 12chars. */{        char    *cp;                    /* to hold address of time      */        cp = ctime(&timeval);           /* 把时间转化成字符串形式       */                                        /* string looks like            */                                        /* Mon Feb  4 00:46:40 EST 1991 */                                        /* 0123456789012345.            */        printf("%12.12s", cp+4 );       /* pick 12 chars from pos 4     */}
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