who的编写实例

1.who是如何工作的
who实际上是通过utmp文件来获取登陆用户的信息的
#include <utmp.h>
信息结构如下：
#define UTMP_FILE /var/adm/utmp
#define ut_name ut_user
struct utmp{
    char ut_user[32];//登录名
    char ut_id[14];
    char ut_line[32];//设备名
    short ut_type;//入口类型
    pid_t ut_pid;//进程id
    struct exit_status{
        short e_termination;
        short e_exit;
    }ut_exit;
    time_t ut_time;
    char ut_host[64];
};



2.编写实例
#include <stdio.h>
#include <utmp.h>
#include <fcntl.h>
#include <unistd.h>
#include <time.h>
#define SHOWHOST
void showtime(long);
void show_info(struct utmp*);

void show_info(struct utmp * utbufp)
{
        if(utbufp->ut_type != USER_PROCESS)
        {
                return;
        }
        printf("% -8.8s",utbufp->ut_name);
        printf(" ");
        printf("% -8.8s",utbufp->ut_line);
        printf(" ");
        showtime(utbufp->ut_time);
        printf(" ");
   #ifdef SHOWHOST
        printf("(%s)",utbufp->ut_host);
   #endif
        printf("\n");
}

void showtime(long timeval)
{
        struct tm *tvp;
        tvp = gmtime(&timeval);
        printf("%d-%d-%d %d:%d",tvp->tm_year+1990,tvp->tm_mon+1,tvp->tm_mday,tvp->tm_hour,tvp->tm_min);
}


int main()
{
        struct utmp current_record;
        int utmpfd;
        int reclen = sizeof(current_record);

        if((utmpfd = open(UTMP_FILE,O_RDONLY)) == -1)
        {
                perror(UTMP_FILE);
                _exit(1);
        }

        while(read(utmpfd,&current_record,reclen) == reclen)
        {
                show_info(&current_record);
        } 
        close(utmpfd);
        return 0;
}

3.测试实例
./who 结果如下：

root     pts/0    2102-8-5 8:51 (192.168.1.100)
root     pts/1    2102-8-5 12:35 (192.168.1.100)