07.构建根文件系统之init进程分析

	/* Check if we are supposed to be in single user mode */
	/*linux内核启动应用程序的时候并没有给他什么参数，所以argc = 1*/
	if (argc > 1
	 && (!strcmp(argv[1], "single") || !strcmp(argv[1], "-s") || LONE_CHAR(argv[1], '1'))
	) {
		/* Start a shell on console */
		new_init_action(RESPAWN, bb_default_login_shell, "");
	} else {
		/* Not in single user mode -- see what inittab says */

		/* NOTE that if CONFIG_FEATURE_USE_INITTAB is NOT defined,
		 * then parse_inittab() simply adds in some default
		 * actions(i.e., runs INIT_SCRIPT and then starts a pair
		 * of "askfirst" shells */
		parse_inittab();
	}

static void parse_inittab(void)
{
#if ENABLE_FEATURE_USE_INITTAB
	FILE *file;
	char buf[INIT_BUFFS_SIZE], lineAsRead[INIT_BUFFS_SIZE];
	char tmpConsole[CONSOLE_NAME_SIZE];
	char *id, *runlev, *action, *command, *eol;
	const struct init_action_type *a = actions;
	/*打开配置文件*/
	/*inittab格式
	<id>:<runlevels>:<action>:<process>
	id => /dev/id,用作中断:stdin，stdout，stderr：printf，scanf，err
	runlevels：忽略
	<action>: Valid actions include: sysinit, respawn, askfirst, wait, once,
	                                 restart, ctrlaltdel, and shutdown.
	action：指示何时执行
	process：应用程序或者脚本
	*/
	file = fopen(INITTAB, "r");
	if (file == NULL) {
		/* No inittab file -- set up some default behavior 默认的配置项 */
#endif
		/* Reboot on Ctrl-Alt-Del */
		new_init_action(CTRLALTDEL, "reboot", "");
		/* Umount all filesystems on halt/reboot */
		new_init_action(SHUTDOWN, "umount -a -r", "");
		/* Swapoff on halt/reboot */
		if (ENABLE_SWAPONOFF) new_init_action(SHUTDOWN, "swapoff -a", "");
		/* Prepare to restart init when a HUP is received */
		new_init_action(RESTART, "init", "");
		/* Askfirst shell on tty1-4 */
		new_init_action(ASKFIRST, bb_default_login_shell, "");
		/*static void new_init_action(int action,const char -cmmand,const *cons)*/
		/*new_init_action(ASKFIRST, "-/bin/sh", "/dev/tty2");*/
		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;

/*new_init_action*/
static void new_init_action(int action, const char *command, const char *cons)
{
	struct init_action *new_action, *a, *last;

	if (strcmp(cons, bb_dev_null) == 0 && (action & ASKFIRST))
		return;

	/* Append to the end of the list */
	for (a = last = init_action_list; a; a = a->next) {
		/* don't enter action if it's already in the list,
		 * but do overwrite existing actions */
		if ((strcmp(a->command, command) == 0)
		 && (strcmp(a->terminal, cons) == 0)
		) {
			a->action = action;
			return;
		}
		last = a;
	}
	/*分配空间*/
	new_action = xzalloc(sizeof(struct init_action));
	if (last) {
		last->next = new_action;
	} else {
		init_action_list = new_action;
	}
	/*等于传入的参数*/
	strcpy(new_action->command, command);
	new_action->action = action;
	strcpy(new_action->terminal, cons);
	messageD(L_LOG | L_CONSOLE, "command='%s' action=%d tty='%s'\n",
		new_action->command, new_action->action, new_action->terminal);
		

/*结构体*/
struct init_action {
	struct init_action *next;
	int action;
	pid_t pid;
	char command[INIT_BUFFS_SIZE];
	char terminal[CONSOLE_NAME_SIZE];

/*从默认的new_init_action反推出默认的配置文件*/
/*
inittab格式
	<id>:<runlevels>:<action>:<process>
	id => /dev/id,用作中断:stdin，stdout，stderr：printf，scanf，err
	runlevels：忽略
	<action>: Valid actions include: sysinit, respawn, askfirst, wait, once,
	                                 restart, ctrlaltdel, and shutdown.
	action：指示何时执行
	process：应用程序或者脚本
*/
		/* Reboot on Ctrl-Alt-Del */
		/*::CTRLALTDEL:reboot*/
		new_init_action(CTRLALTDEL, "reboot", "");
		/* Umount all filesystems on halt/reboot */
		/*::SHUTDOWN:umount -a -r*/
		new_init_action(SHUTDOWN, "umount -a -r", "");
		/* Swapoff on halt/reboot */
		if (ENABLE_SWAPONOFF) new_init_action(SHUTDOWN, "swapoff -a", "");
		/* Prepare to restart init when a HUP is received */
		/*::RESTART:init*/
		new_init_action(RESTART, "init", "");
		/* Askfirst shell on tty1-4 */
		/*new_init_action(ASKFIRST, "-/bin/sh", "/dev/tty2");*/
		/*::ASKFIRST:-/bin/sh*/
		/*tty2::ASKFIRST:-/bin/sh*/
		/*tty3::ASKFIRST:-/bin/sh*/
		/*tty4::ASKFIRST:-/bin/sh*/
		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, "");

/**/
static void run_actions(int action)
{
	struct init_action *a, *tmp;

	for (a = init_action_list; a; a = tmp) {
		tmp = a->next;
		if (a->action == action) {
			/* a->terminal of "" means "init's console" */
			if (a->terminal[0] && access(a->terminal, R_OK | W_OK)) {
				delete_init_action(a);
			} else if (a->action & (SYSINIT | WAIT | CTRLALTDEL | SHUTDOWN | RESTART)) {
				waitfor(a, 0);
				delete_init_action(a);
			} else if (a->action & ONCE) {
				run(a);
				delete_init_action(a);
			} else if (a->action & (RESPAWN | ASKFIRST)) {
				/* Only run stuff with pid==0.  If they have
				 * a pid, that means it is still running */
				if (a->pid == 0) {
					a->pid = run(a);
				}
			}
		}
	}
}

};

/*信号量*/
/*当我们按下ctrlaltdel，内核会给init进程发一个信号。接收到信号后，会执行ctrlaltdel_signal*/
signal(SIGINT, ctrlaltdel_signal);
/**/
static void ctrlaltdel_signal(int sig ATTRIBUTE_UNUSED)
{
	run_actions(CTRLALTDEL);//执行这一类的应用程序
}