07.构建根文件系统之init进程分析

  根文件系统有许多命令，比如 ls cp，cd，这些命令其实也就是相当于一个应用程序。这些程序都封装在busybox中。编译busybox后就可以得到busybox，ls等命令就是到busybox的链接，执行ls等命令其实执行的是busybox程序。
  busybox应用程序中，ls对应的是ls.c，cp对应的是cp.c。ubbot目的是启动内核，内核的目的是启动应用程序。内核如何启动应用程序呢？内核启动的第一个是init进程。不同的设备有不同的驱动程序，init如何区分呢？下面我们分析程序。

init.c

	/* 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
}

  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
  
12
  
13
  
14
  
15
  
16
  
17
  
18
  
19
  
20
  
21
  
22
  
23
  
24
  
25
  
26
  
27
  
28
  
29
  
30
  
31
  
32
  
33
  
34
  
35
  
36
  
37
  
38
  
39
  
40
  
41
  
42
  
43
  
44
  
45
  
46
  
47
  
48
  
49
  
50
  
51
  
52
  
53
  
54
  
55
  
56
  
57
  
58
  
59
  
60
  
61
  
62
  
63
  
64
  
65
  
66
  
67
  
68
  
69
  
70
  
71
  
72
  
73
  
74
  
75
  
76
  
77
  
78
  
79
  
80
  
81
  
82
  
83
  
84
  
85
  
86
  
87
  
88
  
89
  
90
  
91
  
92
  
93
  
94
  
95
  
96
  
97
  
98
  
99
  
100
  
101
  
102
  
103
  
104
  
105
  
106
  
107
  
108
  
109
  
110
  
111
  
112
  
113
  
114
  
115
  
116
  
117
  
118
  
119
  
120
  
121
  
122
  
123
  
124
  
125
  
126
  
127
  
128
  
129
  
130
  
131
  
132
  
133
  
134
  
135
  
136
  
137
  
138
  
139
  
140
  
141
  
142
  
143
  
144
  
145
  
146
  
147
  
148
  
149
  
150
  
151
  
152
  
153
  
154
  
155
  
156
  
157
  
158
  
159
  
160
  
161
  
162
  
163
  
164
  
165
  
166
  
167
  
168
  
169
  
170
  
171
  
172
  
173
  
174
 

init进程流程以及涉及的项
busybox ->init_main
  parse_inittab
    file = fopen(INITTAB, “r”);
      new_init_action //创建一个结构体，填充
             //把这个结构放入init_action_list链表
run_actions(SYSINIT);
  waitfor(a, 0);//等待执行完毕应用程序
     run(a)//创建process子进程
     waitpid()runpid,&status,0);//等待结束
   delete_init_action(a);//在init_action_list链表中删除应用程序
run_actions(WAIT);
  waitfor(a, 0);//等待执行完毕应用程序
     run(a)//创建process子进程
     waitpid()runpid,&status,0);//等待结束
   delete_init_action(a);//在init_action_list链表中删除应用程序
run_actions(ONCE);
     run(a)//创建process子进程
      delete_init_action(a);//在init_action_list链表中删除应用程序
while (1) {
   run_actions(RESPAWN);//重新退出运行子进程
     if (a->pid == 0) {
      a->pid = run(a);
      }
   run_actions(ASKFIRST);
     if (a->pid == 0) {
      a->pid = run(a);
         "\nPlease press Enter to activate this console. ";
        while (read(0, &c, 1) == 1 && c != ‘\n’)//等待回车
        创建进程
      }
   wpid = wait(NULL);//等待子进程退出
   while (wpid > 0) {
a->pid = 0;//退出后，就设置pid为0
}
}
最小的根文件系统需要的项：
  /dev/console /dev/null
  init程序，即busybox
  /etc/inittab
  配置文件指定的应用程序
  库（某些函数的C库）

文章来源: blog.csdn.net，作者：嵌入式与Linux那些事，版权归原作者所有，如需转载，请联系作者。

原文链接：blog.csdn.net/qq_16933601/article/details/102942848