Android Init进程分析
之前在看android启动过程总是带着完成工作任务的目的去分析代码，但是对于一些代码的细节并不是很清楚，在这里就分析一下Init进程的执行过程。
以下框图简要描述系统进程间层次关系
Init进程是android系统起来之后启动的第一个进程，对于研究android系统的启动过程很重要。它是android系统中的始祖进程，USB守护进程(usbd)，挂载守护进程(vold)，无线接口守护进程(rild)等都是init进程的子进程。以下截图是我手机的运行进程情况，可以明显看出进程间的关系
还有一个PID=2的kthread进程，该进程用来创建内核空间的其它进程
直接根据代码来分析整个进程的执行过程。
int main(int argc, char **argv)
{
int fd_count = 0;
struct pollfd ufds[4];//存放pollfd
char *tmpdev;
char* debuggable;
char tmp[32];
int property_set_fd_init = 0;
int signal_fd_init = 0;
int keychord_fd_init = 0;
if (!strcmp(basename(argv[0]), "ueventd"))
return ueventd_main(argc, argv);//ueventd是init的软链接，执行这个进程的时候相当于执行init进程，然后根据进程名进入相应的执行流程
/* clear the umask */
umask(0);
/* Get the basic filesystem setup we need put
* together in the initramdisk on / and then we'll
* let the rc file figure out the rest.
*/
mkdir("/dev", 0755);//创建一些必要的目录并分配权限
mkdir("/proc", 0755);
mkdir("/sys", 0755);
mount("tmpfs", "/dev", "tmpfs", 0, "mode=0755");
mkdir("/dev/pts", 0755);
mkdir("/dev/socket", 0755);
mount("devpts", "/dev/pts", "devpts", 0, NULL);
mount("proc", "/proc", "proc", 0, NULL);
mount("sysfs", "/sys", "sysfs", 0, NULL);
/* We must have some place other than / to create the
* device nodes for kmsg and null, otherwise we won't
* be able to remount / read-only later on.
* Now that tmpfs is mounted on /dev, we can actually
* talk to the outside world.
*/
以上主要创建一些文件系统目录并挂载相应的文件系统，proc文件系统是重要的内核数据的接口，可以通过它读取一些系统信息还能操作内核参数
open_devnull_stdio();//重定向标准输入，输入，错误到/dev/__null__(dup2复制文件句柄，0,1,2分别代表标准输入 输出 错误) 屏蔽标准输入输出
log_init();//设置log信息输出设备/dev/__kmsg__,unlink之后其他进程无法访问，阅读源码定向到printk函数输出 初始化log系统
property_init();//初始化属性系统，这个可以以后分析
get_hardware_name(hardware, &revision);
process_kernel_cmdline();
#ifdef HAVE_SELINUX
INFO("loading selinux policy\n");
selinux_load_policy();
#endif
is_charger = !strcmp(bootmode, "charger");
INFO("property init\n");
if (!is_charger)
property_load_boot_defaults();
这里导入相应的处理函数，分析执行过程
static void import_kernel_cmdline(int in_qemu)
{
char cmdline[1024];
char *ptr;
int fd;
fd = open("/proc/cmdline", O_RDONLY);
if (fd >= 0) {
int n = read(fd, cmdline, 1023);
if (n < 0) n = 0;
/* get rid of trailing newline, it happens */
if (n > 0 && cmdline[n-1] == '\n') n--;
//读取/proc/cmdline中的信息，存放在cmdline字符数组并进行处理
cmdline[n] = 0;
close(fd);
} else {
cmdline[0] = 0;
}
ptr = cmdline;
while (ptr && *ptr) {
char *x = strchr(ptr, ' ');
if (x != 0) *x++ = 0;
import_kernel_nv(ptr, in_qemu);//根据' '间断符逐行分析文本
ptr = x;
}
/* don't expose the raw commandline to nonpriv processes */
chmod("/proc/cmdline", 0440);
}
static void import_kernel_nv(char *name, int in_qemu)
{
char *value = strchr(name, '=');
if (value == 0) {
if (!strcmp(name, "calibration"))
calibration = 1;//表示要校准还是什么？
return;
}
*value++ = 0;
if (*name == 0) return;
if (!in_qemu)
{
/* on a real device, white-list the kernel options */
if (!strcmp(name,"qemu")) {
strlcpy(qemu, value, sizeof(qemu));
} else if (!strcmp(name,"androidboot.console")) {
strlcpy(console, value, sizeof(console));
} else if (!strcmp(name,"androidboot.mode")) {
strlcpy(bootmode, value, sizeof(bootmode));//启动模式
} else if (!strcmp(name,"androidboot.serialno")) {
strlcpy(serialno, value, sizeof(serialno));
} else if (!strcmp(name,"androidboot.baseband")) {
strlcpy(baseband, value, sizeof(baseband));//基带
} else if (!strcmp(name,"androidboot.carrier")) {
strlcpy(carrier, value, sizeof(carrier));
} else if (!strcmp(name,"androidboot.bootloader")) {
strlcpy(bootloader, value, sizeof(bootloader));
} else if (!strcmp(name,"androidboot.hardware")) {
strlcpy(hardware, value, sizeof(hardware));
}//将以上设备信息存放在定义的字符数组中
} else {
/* in the emulator, export any kernel option with the
* ro.kernel. prefix */
char buff[32];
int len = snprintf( buff, sizeof(buff), "ro.kernel.%s", name );
if (len < (int)sizeof(buff)) {
property_set( buff, value );
}
}
}
get_hardware_name(hardware, &revision);
snprintf(tmp, sizeof(tmp), "/init.%s.rc", hardware);
init_parse_config_file(tmp);//分析相应硬件版本的rc文件
init.rc文件有自己相应的语法，分析rc文件也是根据对应的语法来分析，这里引入一片简单介绍init.rc语法的文章
Android init.rc脚本解析
int init_parse_config_file(const char *fn)
{
char *data;
data = read_file(fn, 0);//这里通过read_file函数将fn文件中的数据全部读取到data缓冲区中，malloc分配空间
if (!data) return -1;
//这里开始真正分析脚本中的命令
parse_config(fn, data);
DUMP();
return 0;
}
解析过程会先将init.rc文件action与service进行解析，然后插入到链表中依次执行，查看源码中对链表的定义
#ifndef _CUTILS_LIST_H_
#define _CUTILS_LIST_H_
#include <stddef.h>
#ifdef __cplusplus
extern "C" {
#endif /* __cplusplus */
//声明一个双向链表
struct listnode
{
struct listnode *next;
struct listnode *prev;
};
//计算结构体数据变量相对于结构体首地址的偏移量，这个很重要
#define node_to_item(node, container, member) \
(container *) (((char*) (node)) - offsetof(container, member))
//声明一个双向链表，并且指向自己
#define list_declare(name) \
struct listnode name = { \
.next = &name, \
.prev = &name, \
}
//遍历链表
#define list_for_each(node, list) \
for (node = (list)->next; node != (list); node = node->next)
//反向遍历链表
#define list_for_each_reverse(node, list) \
for (node = (list)->prev; node != (list); node = node->prev)
void list_init(struct listnode *list);//初始化一个双向链表
void list_add_tail(struct listnode *list, struct listnode *item);//将结点添加至双向链表尾部
void list_remove(struct listnode *item);
#define list_empty(list) ((list) == (list)->next)
#define list_head(list) ((list)->next)
#define list_tail(list) ((list)->prev)
#ifdef __cplusplus
};
#endif /* __cplusplus */
#endif
这里声明了三个链表
static list_declare(service_list); static list_declare(action_list); static list_declare(action_queue);
static void parse_config(const char *fn, char *s)
{
struct parse_state state;
char *args[INIT_PARSER_MAXARGS];//允许解析出来的命令行最多有64个参数
int nargs;
nargs = 0;
state.filename = fn;//文件名
state.line = 0;
state.ptr = s;//data
state.nexttoken = 0;
state.parse_line = parse_line_no_op;//此时解析函数是空操作
for (;;) {
switch (next_token(&state)) {//通过next_token函数来寻找字符数组中的关键标记
//这里面省略了一些字符的处理(如‘\r’, '\t', '"', ' '等)，只针对有效字符进行处理('\0', '\n'等)
//#define T_EOF 0 #define T_TEXT 1 #define T_NEWLINE 2
case T_EOF:
state.parse_line(&state, 0, 0); goto parser_done;
return;
case T_NEWLINE:
if (nargs) {
int kw = lookup_keyword(args[0]);//这里将分析第一个参数所代表的关键字
//根据字符匹配返回已定义好的宏定义
if (kw_is(kw, SECTION)) {//当关键字是on或service或import
state.parse_line(&state, 0, 0); //此时相当于什么都没做
parse_new_section(&state, kw, nargs, args);//对state.parse_line进行填充
} else {
state.parse_line(&state, nargs, args);//对于NEWLINE不是on service import的调用parse_line，而在后面的填充中 //parse_line函数即parse_line_action
//回调相应的处理函数
}
nargs = 0;
}
break;
case T_TEXT://不处理
if (nargs < INIT_PARSER_MAXARGS) {
args[nargs++] = state.text;
}
break;
}
} parser_done:
list_for_each(node, &import_list) {//文件解析结束后解析新导入的rc文件
struct import *import = node_to_item(node, struct import, list);
int ret;
//循环取出rc文件的路径
INFO("importing '%s'", import->filename);
ret = init_parse_config_file(import->filename);//重新解析rc文件
if (ret)
ERROR("could not import file '%s' from '%s'\n",
import->filename, fn);
}
}
首先查看一下keywords.h这个文件，对分析过程有帮助
#ifndef KEYWORD//防止重复定义
int do_chroot(int nargs, char **args);
int do_chdir(int nargs, char **args);
int do_class_start(int nargs, char **args);
int do_class_stop(int nargs, char **args);
int do_class_reset(int nargs, char **args);
int do_domainname(int nargs, char **args);
int do_exec(int nargs, char **args);
int do_export(int nargs, char **args);
int do_hostname(int nargs, char **args);
int do_ifup(int nargs, char **args);
int do_insmod(int nargs, char **args);
int do_mkdir(int nargs, char **args);
int do_mount_all(int nargs, char **args);
int do_mount(int nargs, char **args);
int do_restart(int nargs, char **args);
int do_restorecon(int nargs, char **args);
int do_rm(int nargs, char **args);
int do_rmdir(int nargs, char **args);
int do_setcon(int nargs, char **args);
int do_setenforce(int nargs, char **args);
int do_setkey(int nargs, char **args);
int do_setprop(int nargs, char **args);
int do_setrlimit(int nargs, char **args);
int do_setsebool(int nargs, char **args);
int do_start(int nargs, char **args);
int do_stop(int nargs, char **args);
int do_trigger(int nargs, char **args);
int do_symlink(int nargs, char **args);
int do_sysclktz(int nargs, char **args);
int do_write(int nargs, char **args);
int do_copy(int nargs, char **args);
int do_chown(int nargs, char **args);
int do_chmod(int nargs, char **args);
int do_loglevel(int nargs, char **args);
int do_load_persist_props(int nargs, char **args);
int do_wait(int nargs, char **args);
int do_ubiattach(int argc, char **args);
int do_ubidetach(int argc, char **args);
#define __MAKE_KEYWORD_ENUM__
#define KEYWORD(symbol, flags, nargs, func) K_##symbol,//#与##是宏定义的连接符
enum {
K_UNKNOWN,
#endif
KEYWORD(capability, OPTION, 0, 0)//这里返回的相当于K_capabikity
KEYWORD(chdir, COMMAND, 1, do_chdir)//K_chdir
KEYWORD(chroot, COMMAND, 1, do_chroot)//K_chroot
KEYWORD(class, OPTION, 0, 0)
KEYWORD(class_start, COMMAND, 1, do_class_start)
KEYWORD(class_stop, COMMAND, 1, do_class_stop)
KEYWORD(class_reset, COMMAND, 1, do_class_reset)
KEYWORD(console, OPTION, 0, 0)
KEYWORD(critical, OPTION, 0, 0)
KEYWORD(dalvik_recache, OPTION, 0, 0)
KEYWORD(disabled, OPTION, 0, 0)
KEYWORD(domainname, COMMAND, 1, do_domainname)
KEYWORD(exec, COMMAND, 1, do_exec)
KEYWORD(export, COMMAND, 2, do_export)
KEYWORD(group, OPTION, 0, 0)
KEYWORD(hostname, COMMAND, 1, do_hostname)
KEYWORD(ifup, COMMAND, 1, do_ifup)
KEYWORD(insmod, COMMAND, 1, do_insmod)
KEYWORD(import, SECTION, 1, 0)
KEYWORD(keycodes, OPTION, 0, 0)
KEYWORD(mkdir, COMMAND, 1, do_mkdir)
KEYWORD(mount_all, COMMAND, 1, do_mount_all)
KEYWORD(mount, COMMAND, 3, do_mount)
KEYWORD(on, SECTION, 0, 0)
KEYWORD(oneshot, OPTION, 0, 0)
KEYWORD(onrestart, OPTION, 0, 0)
KEYWORD(restart, COMMAND, 1, do_restart)
KEYWORD(restorecon, COMMAND, 1, do_restorecon)
KEYWORD(rm, COMMAND, 1, do_rm)
KEYWORD(rmdir, COMMAND, 1, do_rmdir)
KEYWORD(seclabel, OPTION, 0, 0)
KEYWORD(service, SECTION, 0, 0)
KEYWORD(setcon, COMMAND, 1, do_setcon)
KEYWORD(setenforce, COMMAND, 1, do_setenforce)
KEYWORD(setenv, OPTION, 2, 0)
KEYWORD(setkey, COMMAND, 0, do_setkey)
KEYWORD(setprop, COMMAND, 2, do_setprop)
KEYWORD(setrlimit, COMMAND, 3, do_setrlimit)
KEYWORD(setsebool, COMMAND, 1, do_setsebool)
KEYWORD(socket, OPTION, 0, 0)
KEYWORD(start, COMMAND, 1, do_start)
KEYWORD(stop, COMMAND, 1, do_stop)
KEYWORD(trigger, COMMAND, 1, do_trigger)
KEYWORD(symlink, COMMAND, 1, do_symlink)
KEYWORD(sysclktz, COMMAND, 1, do_sysclktz)
KEYWORD(user, OPTION, 0, 0)
KEYWORD(wait, COMMAND, 1, do_wait)
KEYWORD(write, COMMAND, 2, do_write)
KEYWORD(copy, COMMAND, 2, do_copy)
KEYWORD(chown, COMMAND, 2, do_chown)
KEYWORD(chmod, COMMAND, 2, do_chmod)
KEYWORD(loglevel, COMMAND, 1, do_loglevel)
KEYWORD(load_persist_props, COMMAND, 0, do_load_persist_props)
KEYWORD(ubiattach, COMMAND, 1, do_ubiattach)
KEYWORD(ubidetach, COMMAND, 1, do_ubidetach)
KEYWORD(ioprio, OPTION, 0, 0)
#ifdef __MAKE_KEYWORD_ENUM__
KEYWORD_COUNT,
};
#undef __MAKE_KEYWORD_ENUM__
#undef KEYWORD
#endif
以上通过枚举建立key-value映射，也就相当于map的功能，我们会通过查找key来执行对应的操作
再来查看init_parser.c这个文件，在其中两次include keywords.h这个文件
#include "keywords.h"//这是得到enum{K_UNKNOWN,K_capability,K_chdir,K_chroot......}
#define KEYWORD(symbol, flags, nargs, func) \
[ K_##symbol ] = { #symbol, func, nargs + 1, flags, },
struct {
const char *name;
int (*func)(int nargs, char **args);
unsigned char nargs;
unsigned char flags;
} keyword_info[KEYWORD_COUNT] = {
[ K_UNKNOWN ] = { "unknown", 0, 0, 0 },
#include "keywords.h"//之前已经include得过，此时为[ K_capability ] = { "capability", 0, 1, OPTION } //[ K_chdir ] = { "chdir", do_chdir, 2, COMMAND } //[ K_chroot ] = { "chroot", do_chroot, 3, COMMAND}
}; 此时keyword_info保存的关于键值对的结构体数组
实际上上面两次include的代码如下
int do_chroot(int nargs, char **args);
… …
enum
{
K_UNKNOWN,
K_ capability,
K_ chdir,
… …
}
#define KEYWORD(symbol, flags, nargs, func) \
[ K_##symbol ] = { #symbol, func, nargs + 1, flags, },
struct {
const char *name;
int (*func)(int nargs, char **args);
unsigned char nargs;
unsigned char flags;
} keyword_info[KEYWORD_COUNT] = {
[ K_UNKNOWN ] = { "unknown", 0, 0, 0 },
[K_ capability] = {" capability ", 0, 1, OPTION },
[K_ chdir] = {"chdir", do_chdir ,2, COMMAND},
… …
};
void parse_new_section(struct parse_state *state, int kw,
int nargs, char **args)
{
printf("[ %s %s ]\n", args[0],
nargs > 1 ? args[1] : "");
switch(kw) {
case K_service:
state->context = parse_service(state, nargs, args);
if (state->context) {
state->parse_line = parse_line_service;
return;
}
break;
case K_on:
state->context = parse_action(state, nargs, args);//分析对应的on判断 其中nargs与args对应于命令的参数个数和参数列表，类似main函数
if (state->context) {
state->parse_line = parse_line_action;//赋值给每个新行的parse_line
return;
}
break;
case K_import:
parse_import(state, nargs, args);
break;
}
state->parse_line = parse_line_no_op;
}
static void *parse_action(struct parse_state *state, int nargs, char **args)
{
struct action *act;//这里查看下面引入的几个结构体
if (nargs < 2) {
parse_error(state, "actions must have a trigger\n");
return 0;
}
if (nargs > 2) {
parse_error(state, "actions may not have extra parameters\n");
return 0;
}//限定nargs只能等于2
act = calloc(1, sizeof(*act));
act->name = args[1];
list_init(&act->commands);//初始化一个commands链表
list_add_tail(&action_list, &act->alist);//将当前act->alist结点添加到action_list链表尾部
/* XXX add to hash */
return act;
}
static void parse_line_action(struct parse_state* state, int nargs, char **args)
{
struct command *cmd;
struct action *act = state->context;
int (*func)(int nargs, char **args);
int kw, n;
if (nargs == 0) {
return;
}
kw = lookup_keyword(args[0]);
if (!kw_is(kw, COMMAND)) {//查找关键字是否为COMMAND，不是的就返回
parse_error(state, "invalid command '%s'\n", args[0]);
return;
}
n = kw_nargs(kw);
if (nargs < n) {
parse_error(state, "%s requires %d %s\n", args[0], n - 1,
n > 2 ? "arguments" : "argument");
return;
}
cmd = malloc(sizeof(*cmd) + sizeof(char*) * nargs);
cmd->func = kw_func(kw);//这个时候就有对应的处理函数
cmd->nargs = nargs;
memcpy(cmd->args, args, sizeof(char*) * nargs);
list_add_tail(&act->commands, &cmd->clist);//将新建的cmd->clist节点添加到commands尾部 } 对应的链表结构图如下
通常我们定义链表都是将结构体变量与指针放在一起，如下所示：
typedef struct DulNode{
ElemType data;
struct DulNode *prev;
struct DulNode *next;
}DulNode, *DuLinkList;
源码中这种建立链表的方式有些特别，建立只有指针的链表，将链表中的结点放在结构体中，通过求偏移量来访问结构体变量，提高了效率，值得借鉴
offsetof与container_of可以自己查阅学习
这里还涉及到一些结构体Action及对应的Command，Service也是如此
struct command
{
/* list of commands in an action */
struct listnode clist;
int (*func)(int nargs, char **args);
int nargs;
char *args[1];
};
struct action {
/* node in list of all actions */
struct listnode alist;
/* node in the queue of pending actions */
struct listnode qlist;
/* node in list of actions for a trigger */
struct listnode tlist;
unsigned hash;
const char *name;
struct listnode commands;
struct command *current;
};
struct socketinfo {
struct socketinfo *next;//这里用到单链表形式的结构体指针，用来管理多个socket
const char *name;
const char *type;
uid_t uid;
gid_t gid;
int perm;
};
struct svcenvinfo {
struct svcenvinfo *next;//这里用到单链表形式的结构体指针，管理多个env
const char *name;
const char *value;
};
struct service {
/* list of all services */
struct listnode slist;//链表结点
const char *name;//服务名
const char *classname;//class名 如class main等
unsigned flags;//标志
pid_t pid;//分配的进程号
time_t time_started; /* time of last start *///service启动的时间
time_t time_crashed; /* first crash within inspection window *///崩溃过程时间
int nr_crashed; /* number of times crashed within window *///崩溃次数
uid_t uid;//分配的用户id
gid_t gid;//分配的组id
gid_t supp_gids[NR_SVC_SUPP_GIDS];
size_t nr_supp_gids;
#ifdef HAVE_SELINUX
char *seclabel;
#endif
struct socketinfo *sockets;//socket信息结构体
struct svcenvinfo *envvars;//环境变量结构体
struct action onrestart; /* Actions to execute on restart. *///restart时需执行的action
/* keycodes for triggering this service via /dev/keychord */
int *keycodes;
int nkeycodes;
int keychord_id;
int ioprio_class;
int ioprio_pri;
int nargs;
/* "MUST BE AT THE END OF THE STRUCT" */
char *args[1];
}; /* ^-------'args' MUST be at the end of this struct! */
查看分析service的源码
static void *parse_service(struct parse_state *state, int nargs, char **args)
{
struct service *svc;
if (nargs < 3) {//判断服务参数个数
parse_error(state, "services must have a name and a program\n");
return 0;
}
if (!valid_name(args[1])) {//判断服务名是否有效
parse_error(state, "invalid service name '%s'\n", args[1]);
return 0;
}
svc = service_find_by_name(args[1]);//判断是否已经定义
if (svc) {
parse_error(state, "ignored duplicate definition of service '%s'\n", args[1]);
return 0;
}
nargs -= 2;
svc = calloc(1, sizeof(*svc) + sizeof(char*) * nargs);
if (!svc) {
parse_error(state, "out of memory\n");
return 0;
}
svc->name = args[1];
svc->classname = "default";
memcpy(svc->args, args + 2, sizeof(char*) * nargs);
svc->args[nargs] = 0;
svc->nargs = nargs;
svc->onrestart.name = "onrestart";
list_init(&svc->onrestart.commands);//初始化一个action onrestart的commands双向链表
list_add_tail(&service_list, &svc->slist);//将当前svc->slist结点添加至service_list链表
return svc;
}
static void parse_line_service(struct parse_state *state, int nargs, char **args)
{
struct service *svc = state->context;
struct command *cmd;
int i, kw, kw_nargs;
if (nargs == 0) {
return;
}
svc->ioprio_class = IoSchedClass_NONE;
kw = lookup_keyword(args[0]);
switch (kw) {
case K_capability:
break;
case K_class:
if (nargs != 2) {
parse_error(state, "class option requires a classname\n");
} else {
svc->classname = args[1];
}
break;
case K_console:
svc->flags |= SVC_CONSOLE;//设置flags为SVC_CONSOLE
break;
case K_disabled:
svc->flags |= SVC_DISABLED;//设置flags
svc->flags |= SVC_RC_DISABLED;
break;
case K_ioprio:
if (nargs != 3) {
parse_error(state, "ioprio optin usage: ioprio <rt|be|idle> <ioprio 0-7>\n");
} else {
svc->ioprio_pri = strtoul(args[2], 0, 8);
if (svc->ioprio_pri < 0 || svc->ioprio_pri > 7) {
parse_error(state, "priority value must be range 0 - 7\n");
break;
}
if (!strcmp(args[1], "rt")) {
svc->ioprio_class = IoSchedClass_RT;
} else if (!strcmp(args[1], "be")) {
svc->ioprio_class = IoSchedClass_BE;
} else if (!strcmp(args[1], "idle")) {
svc->ioprio_class = IoSchedClass_IDLE;
} else {
parse_error(state, "ioprio option usage: ioprio <rt|be|idle> <0-7>\n");
}
}
break;
case K_group:
if (nargs < 2) {
parse_error(state, "group option requires a group id\n");
} else if (nargs > NR_SVC_SUPP_GIDS + 2) {
parse_error(state, "group option accepts at most %d supp. groups\n",
NR_SVC_SUPP_GIDS);
} else {
int n;
svc->gid = decode_uid(args[1]);
for (n = 2; n < nargs; n++) {
svc->supp_gids[n-2] = decode_uid(args[n]);
}
svc->nr_supp_gids = n - 2;
}
break;
case K_keycodes:
if (nargs < 2) {
parse_error(state, "keycodes option requires atleast one keycode\n");
} else {
svc->keycodes = malloc((nargs - 1) * sizeof(svc->keycodes[0]));
if (!svc->keycodes) {
parse_error(state, "could not allocate keycodes\n");
} else {
svc->nkeycodes = nargs - 1;
for (i = 1; i < nargs; i++) {
svc->keycodes[i - 1] = atoi(args[i]);
}
}
}
break;
case K_oneshot:
svc->flags |= SVC_ONESHOT;
break;
case K_onrestart:
nargs--;
args++;
kw = lookup_keyword(args[0]);
if (!kw_is(kw, COMMAND)) {
parse_error(state, "invalid command '%s'\n", args[0]);
break;
}
kw_nargs = kw_nargs(kw);
if (nargs < kw_nargs) {
parse_error(state, "%s requires %d %s\n", args[0], kw_nargs - 1,
kw_nargs > 2 ? "arguments" : "argument");
break;
}
cmd = malloc(sizeof(*cmd) + sizeof(char*) * nargs);
cmd->func = kw_func(kw);
cmd->nargs = nargs;
memcpy(cmd->args, args, sizeof(char*) * nargs);
list_add_tail(&svc->onrestart.commands, &cmd->clist);
break;
case K_critical:
svc->flags |= SVC_CRITICAL;
break;
case K_dalvik_recache:
svc->flags |= SVC_DALVIK_RECACHE;
break;
case K_setenv: { /* name value */
struct svcenvinfo *ei;
if (nargs < 2) {
parse_error(state, "setenv option requires name and value arguments\n");
break;
}
ei = calloc(1, sizeof(*ei));
if (!ei) {
parse_error(state, "out of memory\n");
break;
}
ei->name = args[1];
ei->value = args[2];
ei->next = svc->envvars;//单链表操作
svc->envvars = ei;//单链表操作
break;
}
case K_socket: {/* name type perm [ uid gid ] */
struct socketinfo *si;
if (nargs < 4) {
parse_error(state, "socket option requires name, type, perm arguments\n");
break;
}
if (strcmp(args[2],"dgram") && strcmp(args[2],"stream")
&& strcmp(args[2],"seqpacket")) {
parse_error(state, "socket type must be 'dgram', 'stream' or 'seqpacket'\n");
break;
}
si = calloc(1, sizeof(*si));
if (!si) {
parse_error(state, "out of memory\n");
break;
}
si->name = args[1];
si->type = args[2];
si->perm = strtoul(args[3], 0, 8);
if (nargs > 4)
si->uid = decode_uid(args[4]);
if (nargs > 5)
si->gid = decode_uid(args[5]);
si->next = svc->sockets;//这种插入方式是逆序插入
svc->sockets = si;//将新链表表头赋值给sockets
break;
}
case K_user:
if (nargs != 2) {
parse_error(state, "user option requires a user id\n");
} else {
svc->uid = decode_uid(args[1]);
}
break;
case K_seclabel:
#ifdef HAVE_SELINUX
if (nargs != 2) {
parse_error(state, "seclabel option requires a label string\n");
} else {
svc->seclabel = args[1];
}
#endif
break;
default:
parse_error(state, "invalid option '%s'\n", args[0]);
}
}
同理，根据service解析代码我们也能画出service_list的结构图，不过比较特殊的是service option项比较多 情况各异 有很大出入
从以上代码可以看出，paser_action主要解析一个Action刚开始的情况并添加到action_list链表，paser_line_action则解析Action中的command并添加到command链表
service的解析函数亦同理
最后分析import加入的新rc文件，init.rc文件便解析完成，并将所有的action和service分别添加到action_list和service_list链表
跟随代码，下面执行这些函数，这里可能有些疑惑，上面明显声明了三个链表，但是一直都没有涉及到action_queue这个链表。
action_for_each_trigger("early-init",action_add_queue_tail);
queue_builtin_action(wait_for_coldboot_done_action, "wait_for_coldboot_done");
分析这两个个函数看到底做了什么处理，其中wait_for_coldboot_done_action是一个执行函数
void action_for_each_trigger(const char *trigger,
void (*func)(struct action *act))
{
struct listnode *node;
struct action *act;
list_for_each(node, &action_list) {//遍历已经完整的action_list链表，查找early-init action
act = node_to_item(node, struct action, alist);
if (!strcmp(act->name, trigger)) {
func(act);//执行action_add_queue_tail
}
}
}
void action_add_queue_tail(struct action *act)
{
list_add_tail(&action_queue, &act->qlist);//将early-init action中的qlist结点添加到action_queue链表中(这里开始涉及到action_queue链表)
} void queue_builtin_action(int (*func)(int nargs, char **args), char *name)
{
struct action *act;
struct command *cmd;
act = calloc(1, sizeof(*act));//首先新建一个action
act->name = name;
list_init(&act->commands);//初始化commands链表
cmd = calloc(1, sizeof(*cmd));//新建一个command结构体
cmd->func = func;
cmd->args[0] = name;
list_add_tail(&act->commands, &cmd->clist);//将cmd->clist结点添加到commands链表尾部
list_add_tail(&action_list, &act->alist);//将act->alist结点添加到上面的action_list尾部
action_add_queue_tail(act);//将这个action添加到action_queue链表尾部
}
从以上代码分析可看出action_for_each_trigger函数实现查找action_list中的action，并将其添加到action_queue尾部
queue_builtin_action则是新建一个action，将其分别添加到action_list和action_queue链表尾部
action_for_each_trigger("early-init", action_add_queue_tail);
queue_builtin_action(wait_for_coldboot_done_action, "wait_for_coldboot_done");
queue_builtin_action(keychord_init_action, "keychord_init");
queue_builtin_action(console_init_action, "console_init");
/* execute all the boot actions to get us started */
action_for_each_trigger("init", action_add_queue_tail);
/* skip mounting filesystems in charger mode */
if (!is_charger) {
action_for_each_trigger("early-fs", action_add_queue_tail);
action_for_each_trigger("fs", action_add_queue_tail);
action_for_each_trigger("post-fs", action_add_queue_tail);
action_for_each_trigger("post-fs-data", action_add_queue_tail);
}
queue_builtin_action(property_service_init_action, "property_service_init");
queue_builtin_action(signal_init_action, "signal_init");
queue_builtin_action(check_startup_action, "check_startup");
if (is_charger) {
action_for_each_trigger("charger", action_add_queue_tail);
} else {
action_for_each_trigger("early-boot", action_add_queue_tail);
queue_builtin_action(ubootenv_init_action, "ubootenv_init");
action_for_each_trigger("boot", action_add_queue_tail);
}
/* run all property triggers based on current state of the properties */
queue_builtin_action(queue_property_triggers_action, "queue_property_triggers");
#if BOOTCHART
queue_builtin_action(bootchart_init_action, "bootchart_init");
#endif
从以上的代码实现的功能都是类似的
接着阅读init.c后面的源码
for(;;) {
int nr, i, timeout = -1;
execute_one_command();//从链表中取出结点相应执行然后remove
//分析过这个函数，在这里还有个疑问，该函数都是从action队列中去结点执行，但是系统的service是怎么执行的
//难道service链表不可能只注册不执行
//这里注意on boot section中最后一个command(class_start default),最终调用do_class_start
static struct command *get_first_command(struct action *act)
{
struct listnode *node;
node = list_head(&act->commands);
if (!node || list_empty(&act->commands))
return NULL;
return node_to_item(node, struct command, clist);
} static struct command *get_next_command(struct action *act, struct command *cmd)
{
struct listnode *node;
node = cmd->clist.next;
if (!node)
return NULL;
if (node == &act->commands)
return NULL;
return node_to_item(node, struct command, clist);
}
static int is_last_command(struct action *act, struct command *cmd)
{
return (list_tail(&act->commands) == &cmd->clist);//判断cmd->clist结点是否为act->commands链表最后一个
}
void execute_one_command(void)
{
int ret;
//第一次执行cur_action是action结构体指针，cur_command是command结构体指针，都为null
if (!cur_action || !cur_command || is_last_command(cur_action, cur_command)) {
cur_action = action_remove_queue_head();//从非空action_queue链表中取出头部结点并移除
cur_command = NULL;
if (!cur_action)//cur_action为null时返回
return;
INFO("processing action %p (%s)\n", cur_action, cur_action->name);
cur_command = get_first_command(cur_action);//从cur_action中取出第一个command
} else {
cur_command = get_next_command(cur_action, cur_command);//依次取出后面的command
}
if (!cur_command)//cur_command为null时返回
return;
ret = cur_command->func(cur_command->nargs, cur_command->args);//这里才开始执行command操作
INFO("command '%s' r=%d\n", cur_command->args[0], ret);
}
当执行完一个action中的所有command后，通过for(;;)再依次执行action_queue中的下一个action
因此action_queue链表中的顺序就是系统真正的执行顺序，如图所示
到这里，大体上弄清楚了init的执行过程，但是这里有个疑问，所有的action都已经执行完毕，根本没有涉及到service
查看init.rc文件我们可以看到在on boot这个action中对应的command为
class_start core
class_start main
根据之前的parse_line_action我们可以跟踪到do_class_start函数
int do_class_start(int nargs, char **args)
{
/* Starting a class does not start services
* which are explicitly disabled. They must
* be started individually.
*/
service_for_each_class(args[1], service_start_if_not_disabled);
return 0;
}
void service_for_each_class(const char *classname,
void (*func)(struct service *svc))
{
struct listnode *node;
struct service *svc;
list_for_each(node, &service_list) {//遍历service_list链表
svc = node_to_item(node, struct service, slist);//从service_list链表中返回对应的service结构体
if (!strcmp(svc->classname, classname)) {//比较classname是否为core,main等
func(svc);
}
}
} 以上的classname为parse_line_service函数中解析的，即service中class一项
static void service_start_if_not_disabled(struct service *svc)
{
if (!(svc->flags & SVC_DISABLED)) {//判断svc的flags是否为DISABLED
service_start(svc, NULL);
}
}
void service_start(struct service *svc, const char *dynamic_args)
{
struct stat s;
pid_t pid;
int needs_console;
int n;
/* starting a service removes it from the disabled or reset
* state and immediately takes it out of the restarting
* state if it was in there
*/
svc->flags &= (~(SVC_DISABLED|SVC_RESTARTING|SVC_RESET));
svc->time_started = 0;
/* running processes require no additional work -- if
* they're in the process of exiting, we've ensured
* that they will immediately restart on exit, unless
* they are ONESHOT
*/
if (svc->flags & SVC_RUNNING) {
return;
}
needs_console = (svc->flags & SVC_CONSOLE) ? 1 : 0;
if (needs_console && (!have_console)) {
ERROR("service '%s' requires console\n", svc->name);
svc->flags |= SVC_DISABLED;
return;
}
if (stat(svc->args[0], &s) != 0) {
ERROR("cannot find '%s', disabling '%s'\n", svc->args[0], svc->name);
svc->flags |= SVC_DISABLED;
return;
}
if ((!(svc->flags & SVC_ONESHOT)) && dynamic_args) {
ERROR("service '%s' must be one-shot to use dynamic args, disabling\n",
svc->args[0]);
svc->flags |= SVC_DISABLED;
return;
}
NOTICE("starting '%s'\n", svc->name);
//以上主要设置该服务的一些标志
pid = fork();//通过fork()创建子进程
if (pid == 0) {//此为子进程
struct socketinfo *si;
struct svcenvinfo *ei;
char tmp[32];
int fd, sz;
if (properties_inited()) {
get_property_workspace(&fd, &sz);//获取属性系统句柄
sprintf(tmp, "%d,%d", dup(fd), sz);
add_environment("ANDROID_PROPERTY_WORKSPACE", tmp);
}
for (ei = svc->envvars; ei; ei = ei->next)
add_environment(ei->name, ei->value);//为该服务添加环境变量
for (si = svc->sockets; si; si = si->next) {
int socket_type = (
!strcmp(si->type, "stream") ? SOCK_STREAM :
(!strcmp(si->type, "dgram") ? SOCK_DGRAM : SOCK_SEQPACKET));
int s = create_socket(si->name, socket_type,//创建通信socket，相当于每个service都创建socket，用来与其它进程通信
si->perm, si->uid, si->gid);
if (s >= 0) {
publish_socket(si->name, s);//将句柄s添加到环境变量中，该环境变量为ANDROID_SOCKET_XXX
}
}
if (svc->ioprio_class != IoSchedClass_NONE) {
if (android_set_ioprio(getpid(), svc->ioprio_class, svc->ioprio_pri)) {//设置pid
ERROR("Failed to set pid %d ioprio = %d,%d: %s\n",
getpid(), svc->ioprio_class, svc->ioprio_pri, strerror(errno));
}
}
if (needs_console) {
setsid();
open_console();//打开控制台
} else {
zap_stdio();
}
#if 0
for (n = 0; svc->args[n]; n++) {
INFO("args[%d] = '%s'\n", n, svc->args[n]);
}
for (n = 0; ENV[n]; n++) {
INFO("env[%d] = '%s'\n", n, ENV[n]);
}
#endif
//配置进程id和组
setpgid(0, getpid());
/* as requested, set our gid, supplemental gids, and uid */
if (svc->gid) {
if (setgid(svc->gid) != 0) {
ERROR("setgid failed: %s\n", strerror(errno));
_exit(127);
}
}
if (svc->nr_supp_gids) {
if (setgroups(svc->nr_supp_gids, svc->supp_gids) != 0) {
ERROR("setgroups failed: %s\n", strerror(errno));
_exit(127);
}
}
if (svc->uid) {
if (setuid(svc->uid) != 0) {
ERROR("setuid failed: %s\n", strerror(errno));
_exit(127);
}
}
if (!dynamic_args) {
if (execve(svc->args[0], (char**) svc->args, (char**) ENV) < 0) {
ERROR("cannot execve('%s'): %s\n", svc->args[0], strerror(errno));
}
} else {
char *arg_ptrs[INIT_PARSER_MAXARGS+1];
int arg_idx = svc->nargs;
char *tmp = strdup(dynamic_args);
char *next = tmp;
char *bword;
/* Copy the static arguments */
memcpy(arg_ptrs, svc->args, (svc->nargs * sizeof(char *)));
while((bword = strsep(&next, " "))) {
arg_ptrs[arg_idx++] = bword;
if (arg_idx == INIT_PARSER_MAXARGS)
break;
}
arg_ptrs[arg_idx] = '\0';
execve(svc->args[0], (char**) arg_ptrs, (char**) ENV);//执行新进程调用的函数
}
_exit(127);
}
if (pid < 0) {//fork()错误
ERROR("failed to start '%s'\n", svc->name);
svc->pid = 0;
return;
}
svc->time_started = gettime();
svc->pid = pid;
svc->flags |= SVC_RUNNING;
if (properties_inited())
notify_service_state(svc->name, "running");//设置服务运行状态
}
从上面可以看出service的运行过程，同理，service_list链表也得到执行
execute_one_command函数在for循环中一直查找action_queue链表中是否为空，不为空的情况下就移除队首的结点并执行，否则就直接返回
restart_processes();//判断是否有进程需要重启
if (!property_set_fd_init && get_property_set_fd() > 0) {//系统属性
ufds[fd_count].fd = get_property_set_fd();//获取property系统fd
ufds[fd_count].events = POLLIN;
ufds[fd_count].revents = 0;
fd_count++;
property_set_fd_init = 1;//设置标志，下一次循环不会执行
}
if (!signal_fd_init && get_signal_fd() > 0) {//进程间通信
ufds[fd_count].fd = get_signal_fd();//获取子进程信号处理fd
ufds[fd_count].events = POLLIN;
ufds[fd_count].revents = 0;
fd_count++;
signal_fd_init = 1;
}
if (!keychord_fd_init && get_keychord_fd() > 0) {//组合键检测(系统刷机按键等)
ufds[fd_count].fd = get_keychord_fd();//获取组合键fd
ufds[fd_count].events = POLLIN;
ufds[fd_count].revents = 0;
fd_count++;
keychord_fd_init = 1;
}
if (process_needs_restart) {
timeout = (process_needs_restart - gettime()) * 1000;
if (timeout < 0)
timeout = 0;
}
if (!action_queue_empty() || cur_action)
timeout = 0;
#if BOOTCHART
if (bootchart_count > 0) {
if (timeout < 0 || timeout > BOOTCHART_POLLING_MS)
timeout = BOOTCHART_POLLING_MS;
if (bootchart_step() < 0 || --bootchart_count == 0) {
bootchart_finish();
bootchart_count = 0;
}
}
#endif
nr = poll(ufds, fd_count, timeout);
if (nr <= 0)
continue;
for (i = 0; i < fd_count; i++) {
if (ufds[i].revents == POLLIN) {
if (ufds[i].fd == get_property_set_fd())
handle_property_set_fd();
else if (ufds[i].fd == get_keychord_fd())
handle_keychord();
else if (ufds[i].fd == get_signal_fd())
handle_signal();
}
}
}
return 0;
}
static void restart_processes()
{
process_needs_restart = 0;
service_for_each_flags(SVC_RESTARTING,
restart_service_if_needed);
}
void service_for_each_flags(unsigned matchflags,
void (*func)(struct service *svc))
{
struct listnode *node;
struct service *svc;
list_for_each(node, &service_list) {//遍历service_list链表
svc = node_to_item(node, struct service, slist);
if (svc->flags & matchflags) {//判断服务标志是否为RESTARTING，成立就回调函数执行
func(svc);
}
}
}
static void restart_service_if_needed(struct service *svc)
{
time_t next_start_time = svc->time_started + 5;
if (next_start_time <= gettime()) {//当前时间不小于启动时间就重启该服务
svc->flags &= (~SVC_RESTARTING);//清除RESTARTING标志
service_start(svc, NULL);
return;
}
if ((next_start_time < process_needs_restart) ||
(process_needs_restart == 0)) {
process_needs_restart = next_start_time;
}
}
这样一个服务就会被重启，但是死亡的服务它的标志是怎么样被设置成RESTARTING，这里有疑惑
从后面的代码可以看出，inti采用I/O多路服用才监听3个句柄的情况，当可读时做相应的处理
在多路服用中，如果timeout==0 poll就不阻塞;如果timeout>0,poll只有当等待时间超时或有事件发生时才返回;如果timeout==-1就有事件发生才会返回
if (process_needs_restart) {
timeout = (process_needs_restart - gettime()) * 1000;//设置等待时间
if (timeout < 0)
timeout = 0;
}
if (!action_queue_empty() || cur_action)//如果action_queue和cur_action都不为空，timeout设为0,此时不阻塞，相当于此时不执行poll操作
timeout = 0;
#if BOOTCHART
if (bootchart_count > 0) {
if (timeout < 0 || timeout > BOOTCHART_POLLING_MS)
timeout = BOOTCHART_POLLING_MS;
if (bootchart_step() < 0 || --bootchart_count == 0) {
bootchart_finish();
bootchart_count = 0;
}
}
#endif
nr = poll(ufds, fd_count, timeout);
if (nr <= 0)//如果超时等待，就不执行后面的处理，直接跳到for循环开始处，执行action或者重启service
continue;
for (i = 0; i < fd_count; i++) {
if (ufds[i].revents == POLLIN) {
if (ufds[i].fd == get_property_set_fd())
handle_property_set_fd();
else if (ufds[i].fd == get_keychord_fd())
handle_keychord();
else if (ufds[i].fd == get_signal_fd())
handle_signal();
}
}
}
这里主要处理三大事件：属性设置事件，按键组合事件，子进程信号事件。前两者这里不做讨论，按键组合事件也只有在调试模式下才处理
从上面的分析中我们知道service是init调用fork创建的子进程，在Linux进程间通信中，可以通过SIGCHLD信号来通知子进程的状态
在之前的action_queue已经进行signal_init初始化
void signal_init(void)
{
int s[2];
struct sigaction act;
act.sa_handler = sigchld_handler;//信号处理函数
act.sa_flags = SA_NOCLDSTOP;
act.sa_mask = 0;
act.sa_restorer = NULL;
sigaction(SIGCHLD, &act, 0);//安装SIGCHLD信号处理器
/* create a signalling mechanism for the sigchld handler */
if (socketpair(AF_UNIX, SOCK_STREAM, 0, s) == 0) {//用于init进程中双端之间通信
signal_fd = s[0];//发送端socket fd
signal_recv_fd = s[1];//接收端socket fd，并且被注册到poll系统监听
fcntl(s[0], F_SETFD, FD_CLOEXEC);//设置fd的属性
fcntl(s[0], F_SETFL, O_NONBLOCK);//非阻塞
fcntl(s[1], F_SETFD, FD_CLOEXEC);
fcntl(s[1], F_SETFL, O_NONBLOCK);
}
handle_signal();
}
static void sigchld_handler(int s)
{
write(signal_fd, &s, 1);
}
void handle_signal(void)
{
char tmp[32];
/* we got a SIGCHLD - reap and restart as needed */
read(signal_recv_fd, tmp, sizeof(tmp));//接收发送过来的数据
while (!wait_for_one_process(0))//一直执行wait_for_one_process，直到返回非0
;
}
套接字可用于网络通信，也可以用于本机内的进程通信。由于本机内进程的IP地址都相同，因此只需要进程号来确定通信的双方。非网络通信套接字在Android系统中应用很多。Linux环境下使用socketpair函数创造一对未命名的、相互连接的UNIX域套接字。
在init进程中初始化signal_init，在init子进程死亡后会向init进程发送SIGCHLD信号，在init进程中已经注册该信号处理器sigchld_handler，在该函数中会向signal_fd发送信号的编号，而在另一端则接收这个数据，由于signal_recv_fd已注册在poll中，因此会调用handle_signal进行处理
static int wait_for_one_process(int block)
{
pid_t pid;
int status;
struct service *svc;
struct socketinfo *si;
time_t now;
struct listnode *node;
struct command *cmd;
//waitpid函数停止当前进程，等待子进程的结束，-1表示等待任何子进程，WNOHANG表示返回该进程的id，status为返回状态
while ( (pid = waitpid(-1, &status, block ? 0 : WNOHANG)) == -1 && errno == EINTR );
if (pid <= 0) return -1;//进程号不可能为负数，此时while循环退出
INFO("waitpid returned pid %d, status = %08x\n", pid, status);
svc = service_find_by_pid(pid);//通过pid从service_list中查找结点
if (!svc) {
ERROR("untracked pid %d exited\n", pid);
return 0;
}
NOTICE("process '%s', pid %d exited\n", svc->name, pid);
//判断service是否为oneshot，如果是代表只运行一次，则不需要再重启
if (!(svc->flags & SVC_ONESHOT)) {//如果service不为oneshot则需要重新启动，先杀死该服务创建的所有子进程
kill(-pid, SIGKILL);
NOTICE("process '%s' killing any children in process group\n", svc->name);
}
/* remove any sockets we may have created */
for (si = svc->sockets; si; si = si->next) {
char tmp[128];
snprintf(tmp, sizeof(tmp), ANDROID_SOCKET_DIR"/%s", si->name);
unlink(tmp);//释放该服务占用的所有socket资源
}
svc->pid = 0;//设置进程号为0
svc->flags &= (~SVC_RUNNING);//清除状态标志RUNNING
/* oneshot processes go into the disabled state on exit */
if (svc->flags & SVC_ONESHOT) {//如果设置了状态ONESHOT，则不需要重启，设置为DISABLED
svc->flags |= SVC_DISABLED;
}
/* disabled and reset processes do not get restarted automatically */
if (svc->flags & (SVC_DISABLED | SVC_RESET) ) {//如果状态设置了DISABLED或者RESET，则不需要重启
notify_service_state(svc->name, "stopped");//设置状态属性值为stopped
return 0;
}
now = gettime();
if (svc->flags & SVC_CRITICAL) {//如果service标志为CRITICAL
if (svc->time_crashed + CRITICAL_CRASH_WINDOW >= now) {//如果崩溃时间超过4分钟
if (++svc->nr_crashed > CRITICAL_CRASH_THRESHOLD) {//如果崩溃次数超过4次
ERROR("critical process '%s' exited %d times in %d minutes; "
"rebooting into recovery mode\n", svc->name,
CRITICAL_CRASH_THRESHOLD, CRITICAL_CRASH_WINDOW / 60);
android_reboot(ANDROID_RB_RESTART2, 0, "recovery");//重启手机
return 0;
}
} else {//重置状态值
svc->time_crashed = now;
svc->nr_crashed = 1;
}
}else if (svc->flags & SVC_DALVIK_RECACHE) {
if (svc->time_started + RECACHE_ENABLE_PHASE >= now) {
ERROR("recacheabl process '%s' exited at(%lu) ,start(%lu)",
svc->name, now, svc->time_started);
system("/system/xbin/busybox rm /data/dalvik-cache/*");
//android_reboot(ANDROID_RB_RESTART, 0, 0);
}
}
svc->flags |= SVC_RESTARTING;//设置service标志为RESTARTING，待restart_processes()函数重启该服务
/* Execute all onrestart commands for this service. */
list_for_each(node, &svc->onrestart.commands) {
cmd = node_to_item(node, struct command, clist);
cmd->func(cmd->nargs, cmd->args);
}
notify_service_state(svc->name, "restarting");//修改状态属性值为restarting
return 0;
}
init进程进入死循环，监听三大事件，并查询action_queue与service_list链表，是否有action需要执行，是否有service需要重启，并进行处理。
至此，分析完毕。
1.三大链表action_list,action_queue,service_list, action_queue才是真正用来查询执行的，因此它决定执行顺序
2.注意源码中node_to_item，该宏通过链表节点返回结构体引用
3.service_list中的service是何时才被执行，怎样执行的
4.init进程死循环中，execute_one_command()和restart_processes()函数是怎么执行action和重启service_list服务的，尤其是对service重启的处理