编写who命令－第1版

who命令可以显示出当前系统中已经登录的用户信息。

系统：Ubuntu 16.04.3 LTS (GNU/Linux 4.10.0-35-generic x86_64)
参考书：《Unix/Linux编程实践教程》
下载地址：http://download.csdn.net/download/wdsh13478896311/10213050

阅读联机帮助
$ man who

编写自己的who程序：

• 从文件中读取数据结构
• 将结构中的信息以合适的形式显示出来

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

void show_time(time_t t) {
    struct tm *tt = localtime(&t);
    printf("%d-%02d-%02d %02d:%02d", tt->tm_year+1900, tt->tm_mon+1, tt->tm_mday, tt->tm_hour, tt->tm_min);
}

void show_info(struct utmp *record) {
    if (record->ut_type != USER_PROCESS)
        return;
    printf("%-8s", record->ut_user);
    printf(" ");
    printf("%-12s", record->ut_line);
    printf(" ");
    show_time(record->ut_tv.tv_sec);
    printf(" ");
    printf("(%s)", record->ut_host);
    printf("\n");
}

int main() {
    struct utmp cur_record;
    int fd = open("/var/run/utmp", O_RDONLY);
    if (fd == -1) {
        printf("Error!\n");
        exit(1);
    }
    while (read(fd, &cur_record, sizeof(cur_record)) == sizeof(cur_record)) {
        show_info(&cur_record);
    }
    close(fd);
    return 0;
}

效果：
xx@xxx:~/xxx$ ./a.out
derc pts/8 2018-01-18 13:21 (106.120.121.131)
xx@xxx:~/xxx$ who
derc pts/8 2018-01-18 13:21 (106.120.121.131)

补充：time_t在哪里定义的(适用于Ubuntu 16.04.3 LTS)

1. /usr/include/time.h

# include 

__BEGIN_NAMESPACE_STD
/* Returned by `time'.  */
typedef __time_t time_t;

2. /usr/include/x86_64-linux-gnu/bits/types.h

__STD_TYPE __TIME_T_TYPE __time_t;      /* Seconds since the Epoch.  */

3. /usr/include/x86_64-linux-gnu/bits/typesizes.h

#define __TIME_T_TYPE           __SYSCALL_SLONG_TYPE

# define __SYSCALL_SLONG_TYPE   __SLONGWORD_TYPE

4. /usr/include/x86_64-linux-gnu/bits/types.h

#define __SLONGWORD_TYPE        long int

补充：printf格式化控制
参考：http://blog.csdn.net/xue_changkong/article/details/41945215