第4阶段——制作根文件系统之分析init进程(2)

int init_main(int argc, char **argv)
{
 ... ...
 console_init();                //初始化控制台，在init_post()中只是打开了控制台设备
 ... ...
 if (argc > 1      //在init_post()中执行的”_init_process("/sbin/init");”,所以argc=1, argv=/sbin/init
        && (!strcmp(argv[1], "single") || !strcmp(argv[1], "-s") || LONE_CHAR(argv[1], '1')))
     {... ...}                   //此处省略,因为不执行
 else {       
      parse_inittab();              //argc==1,执行else,读取解析init 表(解析配置文件)
       }

.... ...                               //运行应用程序

}

#define INITTAB      "/etc/inittab"           //定义INITTAB=/etc/inittab
static void parse_inittab(void)
{
file = fopen(INITTAB, "r");  //找到INITTAB定义,显然是打开 /etc/inittab 配置文件

/* 如果/etc/inittab无法打开,则调用new_init_action进行一些默认的操作配置 */
if (file == NULL) {         
     new_init_action(CTRLALTDEL, "reboot", "");
     new_init_action(SHUTDOWN, "umount -a -r", "");
   if (ENABLE_SWAPONOFF) new_init_action(SHUTDOWN, "swapoff -a", "");
     new_init_action(RESTART, "init", "");
     new_init_action(ASKFIRST, bb_default_login_shell, "");
     new_init_action(ASKFIRST, bb_default_login_shell, VC_2);
     new_init_action(ASKFIRST, bb_default_login_shell, VC_3);
     new_init_action(ASKFIRST, bb_default_login_shell, VC_4);
     /* sysinit */
     new_init_action(SYSINIT, INIT_SCRIPT, "");
     return ;
     }

/* while一直循环解析file文件将里面的内容一行一行读出来，然后调用new_init_action进行操作*/
while (fgets(buf, INIT_BUFFS_SIZE, file) != NULL) {  
                   /* Skip leading spaces */
                   for (id = buf; *id == ' ' || *id == '\t'; id++);
                   /* Skip the line if it's a comment */
                   if (*id == '#' || *id == '\n')
                           continue;
                           ... ...
new_init_action(a->action, command, id);        //读完后调用new_init_action
     }}

new_init_action(int action, const char *command, const char *cons)
{... ...}

# define bb_dev_null "/dev/null"                 //定义bb_dev_null等于"/dev/null"
static struct init_action *init_action_list = NULL;     //定义init_action型结构体链表

static void new_init_action(int action, const char *command, const char *cons)  //函数开始
{
/*
先介绍init_action结构体,定义如下:
struct init_action {
struct init_action *next;      //指向下一个init_action结构体,用于链表
int action;                       //执行时机,用于何时执行
pid_t pid;                       //process id(进程号)
char command[INIT_BUFFS_SIZE];  //应用程序或者脚本, 就是要启动的进程。
char terminal[CONSOLE_NAME_SIZE]; //终端
};
*/

/*定义init_action型指针, *new_action:指新的结构体*/
struct init_action *new_action, *a, *last;       

/*判断cons是否 "/"开头 */
if (strcmp(cons, bb_dev_null) == 0 && (action & ASKFIRST))
           return; 

/* a和last都等于init_action_list 链表,a始终指向下一个结构体,查找是否有相同的command和termin*/
     for (a = last = init_action_list; a; a = a->next)
   {
         /*找到有相同的command和termin,则只更新action执行时机参数,并return*/
       if ((strcmp(a->command, command) == 0)&& (strcmp(a->terminal, cons) == 0))
       {a->action = action;
       return;}

     /*更新last,等于上一个init_action 结构体*/
        last = a;
   }

new_action = xzalloc(sizeof(struct init_action));  //为new_action分配内存,使它成为静态变量,不释放
if (last) {       //last!=NULL,说明init_action_list当前有内容，将链表下一个节点等于new_action
           last->next = new_action;           }
   else {      //last==NULL，说明init_action_list里面还没有内容,直接将链表等于new_action
         init_action_list = new_action;           }
strcpy(new_action->command, command);  //更新当前链表里的command
new_action->action = action;             //更新链表里的action
strcpy(new_action->terminal, cons);      //更新链表里的command
messageD(L_LOG | L_CONSOLE, "command='%s' action=%d tty='%s'\n",
new_action->command, new_action->action, new_action->terminal);
}

if (file == NULL) {         

/*ID为空, runlevels忽略, action= ctrlaltdel, process= reboot */
new_init_action(CTRLALTDEL, "reboot", "");     

/*ID为空, runlevels忽略, action= shutdown, process= umount -a -r */
new_init_action(SHUTDOWN, "umount -a -r", ""); 

/* ENABLE_SWAPONOFF 未定义,不分析*/if (ENABLE_SWAPONOFF) new_init_action(SHUTDOWN, "swapoff -a", "");  

/*ID为空, runlevels忽略, action= restart, process= init */
new_init_action(RESTART, "init", ""); 

/*ID为空, runlevels忽略, action= askfirst, process= -/bin/sh */
/* 其中bb_default_login_shell ="-/bin/sh" */
new_init_action(ASKFIRST, bb_default_login_shell, "");
 
/*ID=/dev/tty2, runlevels忽略, action= askfirst, process=-/bin/sh */
new_init_action(ASKFIRST, bb_default_login_shell, VC_2); //VC_2= "/dev/tty2"
 
/* ID=/dev/tty3, runlevels忽略, action= askfirst, process=-/bin/sh */
new_init_action(ASKFIRST, bb_default_login_shell, VC_3);  // VC_3= "/dev/tty3"
 
/* ID=/dev/tty4,runlevels忽略, action= askfirst, process=-/bin/sh */
new_init_action(ASKFIRST, bb_default_login_shell, VC_4); // VC_4= "/dev/tty3"
 
/* sysinit */
/*ID为空, runlevels忽略, action= sysinit, process= etc/init.d/rcS */

new_init_action(SYSINIT, INIT_SCRIPT, ""); //INIT_SCRIPT="etc/init.d/rcS"

return ;
}

:: ctrlaltdel:reboot          //当按下Ctrl+Alt+Delete组合键时，会执行reboot

:: shutdown:umount -a -r      //  告诉init，在系统关机的时候执行umount命令卸载所有文件系统，失败则以读模式安装

:: restart:init                //init重启时，指定执行init进程

:: askfirst: -/bin/sh          //启动-/bin/sh之前不显示，等待用户按enter键

/dev/tty2:: askfirst:-/bin/sh      //启动-/bin/sh之前在终端tty2上显示信息，并等待用户按enter键

/dev/tty3:: askfirst:-/bin/sh      //启动-/bin/sh之前在终端tty3上显示信息，并等待用户按enter键

/dev/tty4:: askfirst:-/bin/sh         //启动-/bin/sh之前在终端tty4上显示信息，并等待用户按enter键

:: askfirst:etc/init.d/rcS           //启动etc/init.d/rcS之前在终端tty4上显示信息，并等待用户按enter键

int init_main(int argc, char **argv)
{
 ... ...
 console_init();                //初始化控制台，在init_post()中只是打开了控制台设备
 ... ...

if (argc > 1      //在init_post()中执行的”_init_process("/sbin/init");”,所以argc=1, argv=/sbin/init
        && (!strcmp(argv[1], "single") || !strcmp(argv[1], "-s") || LONE_CHAR(argv[1], '1')))
{... ...}                   //此处省略,因为不执行
 else {       
                 parse_inittab();              //读取解析init 表(解析配置文件)
       }
....
  
/* First run the sysinit command */
run_actions(SYSINIT);    /*首先运行系统初始化的链表节点(SYSINIT:等待运行结束为止)*/
/* Next run anything that wants to block */
run_actions(WAIT);      //运行 action = WAIT的链表节点(WAIT:等待运行结束为止)
/* Next run anything to be run only once */
run_actions(ONCE);           //运行 action = ONCE的链表节点(ONCE:不会等待运行)
while (1) {
  run_actions(RESPAWN);    //运行 action = RESPAWN的链表节点(pid==0时才能运行) 
  run_actions(ASKFIRST); //运行action = ASKFIRST的链表节点(pid==0时才能运行,且还需要等待回车)

  sleep(1);                  //让CPU等待会儿
  wpid = wait(NULL);        //等待上面两个的子进程退出

  while (wpid > 0)              //退出后设置pid=0，然后while重新运行RESPAWN&& ASKFIRST
    { a->pid = 0;}           
      }

}

static void run_actions(int action)       //执行时机参数
{
  struct init_action *a, *tmp;
   for (a = init_action_list; a; a = tmp)            //从链表init_action_list中循环查找
  {tmp = a->next;                           //指向下一个链表的节点
     if (a->action == action)                    //找到相同名称的action节点了
    {        if (a->terminal[0] && access(a->terminal, R_OK | W_OK))
         { delete_init_action(a);}   //已经使用过该应用程序,从链表中删除

            /* SYSINIT|WAIT|CTRLALTDEL|SHUTDOWN|RESTART这些的应用程序都需要等待执行完毕 */
        else if (a->action & (SYSINIT | WAIT | CTRLALTDEL | SHUTDOWN | RESTART))
        {  waitfor(a, 0); //(0:ID号为空) 执行a节点的应用程序,然后等待它执行完毕      
          delete_init_action(a);  //然后从链表init_action_list中删除(delete)
        } 
        else if (a->action & ONCE) //action(运行时机)=ONCE时，不需要等待执行完毕
       {run(a);                    //创建子进程后即删除该节点
        delete_init_action(a);        }         
         //action(运行时机)= RESPAWN | ASKFIRST时，也不需要等待执行完毕
         else if (a->action & (RESPAWN | ASKFIRST))
       { if (a->pid == 0) {a->pid = run(a);}  }    //a->pid==0才run(a)创建子进程
    }
  }
}

static int waitfor(const struct init_action *a, pid_t pid) //*a:链表中的一个节点
{
         int runpid;
         int status, wpid;
         /*run(a):创建<process>子进程(运行应用程序)*/
         runpid = (NULL == a)? pid : run(a);  //当a==NULL,runpid=pid=0,否则runpid=run(a).
         while (1) {
                   wpid = waitpid(runpid, &status, 0);  //等待应用程序执行完毕
                   if (wpid == runpid)
                            break;
                   if (wpid == -1 && errno == ECHILD) {
                            /* we missed its termination */
                            break;
                   }

                   /* FIXME other errors should maybe trigger an error, but allow
                    * the program to continue */
         }
         return wpid;
}

static pid_t run(const struct init_action *a) //*a:链表中的一个节点   
{
 .. ...
     if (a->action & (SYSINIT | WAIT | CTRLALTDEL | SHUTDOWN | RESTART)) 
          {... ...}        //只分析RESPAWN和ASKFIRST有什么不同,所以此处省略
     if (a->action & ASKFIRST)  //action==ASKFIRST的时候
      {
         /*打印\nPlease press Enter to activate this console.(请按回车键启动控制台.)*/
       tatic const char press_enter[] ALIGN1 ="\nPlease press Enter to activate this console. ";
       char c;
       ... ...
       while (read(0, &c, 1) == 1 && c != '\n');  //一直等待用户回车
       }

BB_EXECVP(cmdpath, cmd);          //创建子进程

}

/dev/null  (为空的话就是/dev/null, 所有写到这个文件中的数据都会被丢弃掉。)