用I/O标准库实现who命令

 

linux的who命令实际上是能通过读取utmp文件来获得相关的信息

然后按格式输出来，很简单，主要用来了utmp结构，下面是他的描述

      struct utmp {
              short ut_type;              /* type of login */
              pid_t ut_pid;               /* PID of login process */
              char ut_line[UT_LINESIZE]; /* device name of tty - "/dev/" */
              char ut_id[4];              /* init id or abbrev. ttyname */
              char ut_user[UT_NAMESIZE]; /* user name */
              char ut_host[UT_HOSTSIZE]; /* hostname for remote login */
              struct exit_status ut_exit; /* The exit status of a process
                                             marked as DEAD_PROCESS */

              /* 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, used for windowing */
              struct {
                  int32_t tv_sec;         /* Seconds */
                  int32_t tv_usec;        /* Microseconds */
              } ut_tv;                    /* Time entry was made */
          #else
               long int ut_session;        /* Session ID, used for windowing */
               struct timeval ut_tv;       /* Time entry was made */
          #endif

              int32_t ut_addr_v6[4];       /* IP address of remote host */
              char __unused[20];           /* Reserved for future use */
          };

具体的可以查看utmp.h文件，实现就比较简单了，我就贴个源码算了，呵呵

#include <stdio.h>
#include <utmp.h>
#include <time.h>

void showtime(long);
void show_info(struct utmp*);

int
main ( )
{
    struct utmp utbuf;
    FILE* fd;
    int    rdnumber;

    if ((fd=fopen(UTMP_FILE,"r"))==NULL )
    {
        perror(UTMP_FILE);
    }

    while (fread(&utbuf,sizeof(utbuf),1,fd)==1 )
        show_info(&utbuf);
    fclose(fd);
    return 0;
}   /* ----- end of function main ----- */

 

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);
#ifdef SHOWHOST
    if(utbufp->ut_host[0]!='/0')
        printf("(%s)",utbufp->ut_host);
#endif
    printf("/n");
}   /* ----- end of function show_info ----- */

 

void showtime ( long timeval )
{
    char* cp;
    cp=ctime(&timeeval);
    printf("%12.12s",cp+4);
}   /* ----- end of function showtime ----- */