自己编写who命令

系统：ubuntu 12.04

1.man who

有这么几句话：

      If FILE is not specified, use /var/run/utmp.  /var/log/wtmp as FILE  is
       common.   If  ARG1  ARG2  given, -m presumed: `am i' or `mom likes' are
       usual.

who命令索取信息存在这个文档中

2.看到关键词utmp wtmp

  man -k utmp

 挑选后的结果

utmp (5)             - login records

3.察看utmp的帮助文档

   man 5 utmp

里面有utmp数据结构的详细说明

4.将文件中utmp中的信息读取出来

用 open read close 函数

5.在我的struct utmp 中没有ut_time 的直接定义

# define ut_time        ut_tv.tv_sec 

 struct
   {
     int32_t tv_sec;             /* Seconds.  */   //时间的秒数
     int32_t tv_usec;            /* Microseconds.  */
   } ut_tv;

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

 #define      SHOWHOST   //decide to show host or not

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);          //perror()
                exit(1);
        }

        while( read(utmpfd, &utbuf, sizeof(utbuf)) == sizeof(utbuf) )
                show_info( &utbuf );
        close(utmpfd);
        return 0;
}

void show_info( struct utmp *utbufp )
{
        if ( utbufp->ut_type != USER_PROCESS )  //dont show no useful infomation
                return;

        printf("%-8.8s", utbufp->ut_name);      /* the logname  */
        printf(" ");                            /* a space      */
        printf("%-8.8s", utbufp->ut_line);      /* the tty      */
        printf(" ");                            /* a space      */
        showtime( utbufp->ut_time );            /* display time */
#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);           /* convert time to string       */
                                        /* string looks like            */
                                        /* Mon Feb  4 00:46:40 EST 1991 */
                                        /* 0123456789012345.            */
        printf("%12.12s", cp+4 );       /* pick 12 chars from pos 4     */
}