编写who命令

第一个版本：

/* who1.c - a first version of the who program
 *          open, read UTMP file, and show results.
 */
#include <stdio.h>
#include <utmp.h>
#include <fcntl.h>
#include <utmp.h>
#include <stdlib.h>
#include <unistd.h>
#define SHOWHOST    /* include remote machine on output */
void show_info( struct utmp *utbufp );
int main(int argc, char *argv[])
{
    struct utmp current_record;     /* read info into here */
    int         utmpfd;             /* read from this file descriptor */
    int         reclen = sizeof(current_record);
    if ( (utmpfd = open(UTMP_FILE, O_RDONLY)) == -1 )
    {
        perror( UTMP_FILE );    /* UTMP_FILE is in utmp.h */
        exit(1);
    }
    while( read(utmpfd, ¤t_record, reclen) == reclen)
    {
        show_info(¤t_record);
    }
    close(utmpfd);
    return 0;
}
/* show_info()
 * displays contents of the utmp struct in human readable form.
 * *note* these sizes should not be hardwird.
 */
void show_info( struct utmp *utbufp )
{
    printf("%-8.8s", utbufp->ut_name);      /* the logname */
    printf("\t");
    printf("%-8.8s", utbufp->ut_line);      /* the tty */
    printf("\t");
    printf("%-10ld", utbufp->ut_time);      /* login time */
    printf("\t");
#ifdef SHOWHOST
    printf("(%s)", utbufp->ut_host);        /* the host */
#endif
    printf("\n");
}

第一个版本的效果如下：

总结：能够分栏按照格式显示4栏用户信息，但是不能区分非实际用户，不能纠正时间显示问题。

解决方法：

实际用户------在 "/usr/include /bits/utmp.h"中有相关的宏定义，能够用此来区分实际用户。

在显示信息函数 show_info() 中如此修改：

纠正时间显示问题------转换时间到human readable。

了解到 unix 保存时间为 time_t 类型，是一个type long int time_t。

那么把时间转换为 human readable 的函数式 ctime():

char* ctime(const time_t *timep)

结合我们需要显示的格式，那么正确的使用方法是这样的：

printf("%12.12s",ctime(&t)+4);

最终的源文件如下：

/* who1.c - a first version of the who program
 *          open, read UTMP file, and show results.
 */
#include <stdio.h>
#include <utmp.h>
#include <fcntl.h>
#include <utmp.h>
#include <stdlib.h>
#include <unistd.h>
#define SHOWHOST    /* include remote machine on output */
void show_info( struct utmp *utbufp );
void show_time(const time_t *timep);
int main(int argc, char *argv[])
{
    struct utmp current_record;     /* read info into here */
    int         utmpfd;             /* read from this file descriptor */
    int         reclen = sizeof(current_record);
    if ( (utmpfd = open(UTMP_FILE, O_RDONLY)) == -1 )
    {
        perror( UTMP_FILE );    /* UTMP_FILE is in utmp.h */
        exit(1);
    }
    while( read(utmpfd, ¤t_record, reclen) == reclen)
    {
        show_info(¤t_record);
    }
    close(utmpfd);
    return 0;
}
/* show_time() - transform long time to human readable.
 */
void show_time(const time_t *timep)
{
    printf("%14.14s", ctime(timep) + 4);
}
/* show_info()
 * displays contents of the utmp struct in human readable form.
 * *note* these sizes should not be hardwird.
 */
void show_info( struct utmp *utbufp )
{
    if (utbufp->ut_type != USER_PROCESS)
        return;
    printf("%-8.8s", utbufp->ut_name);      /* the logname */
    printf("\t");
    printf("%-8.8s", utbufp->ut_line);      /* the tty */
    printf("\t");
    //printf("%-10ld", utbufp->ut_time);      /* login time */
    show_time(&(utbufp->ut_time));
    printf("\t");
#ifdef SHOWHOST
    printf("(%s)", utbufp->ut_host);        /* the host */
#endif
    printf("\n");
}

最终的效果如下：

来自为知笔记(Wiz)