实现who

学习目标

用系统调用实现who命令mywho
注意时间格式要与系统中的who一致

查看who命令的功能

使用man who查看详细内容

输入man -k utmp

输入man utmp，可以看到utmp的结构

struct utmp {
               short   ut_type;              /* Type of record */
               pid_t   ut_pid;               /* PID of login process */
               char    ut_line[UT_LINESIZE]; /* Device name of tty - "/dev/" */
               char    ut_id[4];             /* Terminal name suffix,
                                                or inittab(5) ID */
               char    ut_user[UT_NAMESIZE]; /* Username */
               char    ut_host[UT_HOSTSIZE]; /* Hostname for remote login, or
                                                kernel version for run-level
                                                messages */
               struct  exit_status ut_exit;  /* Exit status of a process
                                                marked as DEAD_PROCESS; not
                                                used by Linux init (1 */
               /* The ut_session and ut_tv fields must be the same size when
                  compiled 32- and 64-bit.  This allows data files and shared
                  memory to be shared between 32- and 64-bit applications. */
           #if __WORDSIZE == 64 && defined __WORDSIZE_COMPAT32
         int32_t ut_session;           /* Session ID (getsid(2)),
                                                used for windowing */
               struct {
                   int32_t tv_sec;           /* Seconds */
                   int32_t tv_usec;          /* Microseconds */
               } ut_tv;                      /* Time entry was made */
           #else
                long   ut_session;           /* Session ID */
                struct timeval ut_tv;        /* Time entry was made */
           #endif

               int32_t ut_addr_v6[4];        /* Internet address of remote
                                                host; IPv4 address uses
                                                just ut_addr_v6[0] */
               char __unused[20];            /* Reserved for future use */
           };

编写mywho

#include <stdio.h>
#include <stdlib.h>
#include <utmp.h>
#include <fcntl.h>
#include <unistd.h>
#include <time.h>
#define SHOWOST
void showinfo(struct utmp *utbufp);
long showtime(long timeval);
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)
                showinfo(&current_record);
        close(utmpfd);
        return 0;
}
void showinfo(struct utmp *utbufp){
        if(utbufp->ut_type!=USER_PROCESS)
                return;
        else{
                printf("%-8.8s",utbufp->ut_name);
                printf(" ");
                printf("%-8.8s",utbufp->ut_line);
                printf(" ");
                showtime(utbufp->ut_time);
                printf(" ");
                printf("(%s)",utbufp->ut_host);
                printf("\n");
        }
}
long showtime(long timeval)
{
        struct tm *cp;
        cp = gmtime(&timeval);
        printf("    ");
        printf("%d-%d-%d %d:%d ",cp->tm_year+1900,cp->tm_mon+1,cp->tm_mday,(cp->tm_hour+8)%24,cp->tm_min);
}

结果：