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
    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(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;
}