chapter 9 exercises

chapter 9 exercises

Refer back to our discussion of the utmp and wtmp files in . Why are the logout records written by the init process? Is this handled the same way for a network login?

答案：utmp和wtmp记录了所有用的的login和logout信息。对于使用terminal device进行的登陆，由于每个terminal对应一个getty，后来又exec成了login shell, 当这个用户logout时，login shell退出，init会收到SIGCHLD信号。所以init就可以写入用户登出信息。

对于使用network login的用户，他们也有login shell，但是这个login shell并不是由init直接 fork出来的getty exec得到的。而是由init fork出一个进程执行/bin/sh，然后执行/etc/rc，它spawn出一个daemon即inetd， 然后inetd fork出telnetd, telnetd进程fork出的进程执行login然后执行login shell得到的。所以network登陆的login shell的父亲是telnetd。所以网络登陆的信息由telnetd负责填写

9.2	Write a small program that calls fork and has the child create a new session. Verify that the child becomes a process group leader and that the child no longer has a controlling terminal.

。#include

#include

#include

#include

 

int main()

{

       int pid = fork();

       if( pid < 0 )

              exit(0);

       setbuf(stdout,NULL);

       if( pid == 0 )

       {

              //child

              puts("this is the child");

              pid_t sid = setsid();

              if( sid>0 )

                     printf("chid session id : %d, pid : %d, ppid: %d, pgid:%d, freground grp id:%d\n", sid, getpid(), getppid(), getpgid(getpid()), tcgetpgrp(STDIN_FILENO) );

              if( errno == ENOTTY )

                     puts("child has no tty");

              puts("chid exit");

              exit(0);

 

       }

 

       printf("father  session id : %d, pid : %d, ppid: %d, pgid:%d, freground grp id:%d\n", getsid(getpid()), getpid(), getppid(), getpgid(getpid()), tcgetpgrp(STDIN_FILENO) );

}

本程序通过调用tcgetpgrp(STDIN_FILENO)，由于该函数出错，然后产看errno，当等于ENOTTY时及表明STDIN_FILENO就不是controlling terminal。由于我们并没有进行任何redirection，因此可见这个session已经断开了与controlling terminal的连接。

除此方法，没有发现别的能够检查是否有controlling terminal的方法。

输出结果：

shaoting@desktopbj-LabSD:/home/shaoting/mytest> ./a.out

this is the child

chid session id : 6717, pid : 6717, ppid: 6716, pgid:6717, freground grp id:-1

child has no tty

chid exit

father  session id : 5086, pid : 6716, ppid: 5094, pgid:6716, freground grp id:6716

shaoting@desktopbj-LabSD:/home/shaoting/mytest>