Android启动详解-----init

init是一个进程，它是linux系统中用户空间的第一个进程，其进程PID是1，父进程为linux系统内核的0号进程。所以其被赋予很多极其重要的职责,linux内核初始化完成后就开始执行它。Android基于Linux内核，所以启动的第一个进程也是init

对于init进程的功能分为4部分：

分析和运行所有的init.rc文件;生成设备驱动节点; （通过rc文件创建）处理子进程的终止(signal方式);提供属性服务。

init的代码在Android源码路径\system\core\init\init.c

int main(int argc, char **argv)
{
int fd_count = 0;
struct pollfd ufds[4];
char *tmpdev;
char* debuggable;
char tmp[32];
int property_set_fd_init = 0;
int signal_fd_init = 0;
int keychord_fd_init = 0;
bool is_charger = false;

if (!strcmp(basename(argv[0]), "ueventd"))
return ueventd_main(argc, argv);

if (!strcmp(basename(argv[0]), "watchdogd"))
return watchdogd_main(argc, argv);

/* clear the umask */
umask(0);

/*下面的代码开始建立各种用户空间的目录，如/dev、/proc、/sys等,并设置权限
，挂接文件系统
*/
mkdir("/dev", 0755);
mkdir("/proc", 0755);
mkdir("/sys", 0755);

mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "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);

/*检测/dev/.booting文件是否可读写和创建*/
close(open("/dev/.booting", O_WRONLY | O_CREAT, 0000));

open_devnull_stdio(); //分别是将 stdin,stdout和stderr先初始化到/dev/__null__，这样用printf或其他打印的，都输出不了
klog_init(); //将init进程中的log，打印到内核printk的log buffer中(初始化kernel log)
memlog_init(); //初始化内存Log
property_init(); //初始化属性
#ifdef TARGET_BOARD_PLATFORM_RK3288
rk_set_cpu();
#endif
rk_parse_cpu();
//获取硬件名
get_hardware_name(hardware, &revision);
//处理内核命令行
process_kernel_cmdline();

union selinux_callback cb;
cb.func_log = klog_write;
selinux_set_callback(SELINUX_CB_LOG, cb);

cb.func_audit = audit_callback;
selinux_set_callback(SELINUX_CB_AUDIT, cb);

selinux_initialize();
/* These directories were necessarily created before initial policy load
* and therefore need their security context restored to the proper value.
* This must happen before /dev is populated by ueventd.
*/
restorecon("/dev");
restorecon("/dev/socket");
restorecon("/dev/__properties__");
restorecon_recursive("/sys");

is_charger = !strcmp(bootmode, "charger");

INFO("property init\n");
if (!is_charger)
property_load_boot_defaults();

INFO("reading config file\n");
//解析init.rc文件的内容
init_parse_config_file("/init.rc");
//执行rc文件中触发器为early-init的语句
act
4000
ion_for_each_trigger("early-init", action_add_queue_tail);

queue_builtin_action(wait_for_coldboot_done_action, "wait_for_coldboot_done");
queue_builtin_action(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");
queue_builtin_action(keychord_init_action, "keychord_init");
/*
向init进程中的一个待执行action队列增加了一个名称等于“console_init”的action。这个action对应的执行函数为console_init_action
console_init_action:加载启动动画，如果动画打开失败，则在屏幕上打印： A N D R O I D字样
*/
queue_builtin_action(console_init_action, "console_init");

//触发在init脚本文件中名字为init的action，并且执行其commands，其实是：on init
action_for_each_trigger("init", action_add_queue_tail);

//在charger模式下略过mount文件系统的工作
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);
}

/* Repeat mix_hwrng_into_linux_rng in case /dev/hw_random or /dev/random
* wasn't ready immediately after wait_for_coldboot_done
*/
queue_builtin_action(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");

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);
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
//进入无限循环，建立init的子进程（init是所有进程的父进程）
for(;;) {
int nr, i, timeout = -1;

execute_one_command();
restart_processes(); //重启那些已经死去的进程

// 用来监听属性设置服务的事件
if (!property_set_fd_init && get_property_set_fd() > 0) {
ufds[fd_count].fd = get_property_set_fd();
ufds[fd_count].events = POLLIN;
ufds[fd_count].revents = 0;
fd_count++;
property_set_fd_init = 1;
}
if (!signal_fd_init && get_signal_fd() > 0) {
ufds[fd_count].fd = get_signal_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) {
ufds[fd_count].fd = get_keychord_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;
/* bootchart是一个性能统计工具，用于搜集硬件和系统的信息，并将其写入磁盘，以便其
他程序使用
*/
#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
//等待下一个命令的提交
nr = poll(ufds, fd_count, timeout);
if (nr <= 0)
continue;

//处理具体的消息
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();
}
}
}
memlog_close();
return 0;
}