Android系统启动流程之Init说明

要写学习总结了，先将资料整理一下，以备用。

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;
//uevent 热插拔，内核和用户空间的通信机制，netlink实现。
    if (!strcmp(basename(argv[0]), "ueventd"))
        return ueventd_main(argc, argv);
//strcmp返回0为相等，大于返回大于0，小于返回小于0.此处为电子狗
    if (!strcmp(basename(argv[0]), "watchdogd"))
        return watchdogd_main(argc, argv);
//赋予最大的权限，即初始化权限，权限的定义为文件666-umask的值。文件夹777-umask的值。
    /* clear the umask */
    umask(0);
//创建各种目录
        /* 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);
    mkdir("/proc", 0755);
    mkdir("/sys", 0755);
//虚拟内存文件系统，用于存放一些启动中的临时文件，于外界交互
    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);
//查看.booting是否为可读写和创建
        /* indicate that booting is in progress to background fw loaders, etc */
    close(open("/dev/.booting", O_WRONLY | O_CREAT, 0000));

        /* 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. //貌似与tmpfs有关 */

    open_devnull_stdio();
//Klog日志初始化，内核级的log日志
    klog_init();
//初始化属性
    property_init();
//获取硬件设备名称，此机器名为trace
    get_hardware_name(hardware, &revision);
//处理内核命令
    process_kernel_cmdline();
//SElinux是linux最杰出的新安全控制子系统，是2.6版本的linux内核中提供强制访问控制（MAC）系统。包含在内核中。
    union selinux_callback cb;
    cb.func_log = log_callback;
    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命令，恢复SElinux文件属性，即恢复文件的安全上下文。
    restorecon("/dev");
    restorecon("/dev/socket");
    restorecon("/dev/__properties__");
    restorecon_recursive("/sys");
//是否充电？
    is_charger = !strcmp(bootmode, "charger");
//当前启动信息描述，属性初始化
    INFO("property init\n");
    property_load_boot_defaults();

    INFO("reading config file\n");
//重点，分析init.rc文件内容
    init_parse_config_file("/init.rc");
//行为触发器，early-init阶段
    action_for_each_trigger("early-init", action_add_queue_tail);
//执行各种创建行为
    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");
    queue_builtin_action(console_init_action, "console_init");
//执行boot action，即init阶段
    /* execute all the boot actions to get us started */
    action_for_each_trigger("init", 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 */
//再执行一遍mix-hwrng...防止/dev/hw-random或/dev/random（虚假随机的字符流）无法及时响应
    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");
//充电模式下，会略过mount文件系统，开启核心服务
    /* Don't mount filesystems or start core system services if in charger mode. */
    if (is_charger) {
        action_for_each_trigger("charger", action_add_queue_tail);
    } else {
        action_for_each_trigger("late-init", action_add_queue_tail);
//因为充电，所以延后初始化
    }
//基于当前的属性配置状态，启动属性触发器。
    /* run all property triggers based on current state of the properties */
    queue_builtin_action(queue_property_triggers_action, "queue_property_triggers");

//bootchart工具，能对系统的性能进行分析，并生成系统启动过程的图表，提供一些有价值的信息
#if BOOTCHART
    queue_builtin_action(bootchart_init_action, "bootchart_init");
#endif
//进入无限循环
    for(;;) {
        int nr, i, timeout = -1;
//执行命令，重启死去的进程
        execute_one_command();
        restart_processes();
//此处的几个if判断语句，2的源码在循环体的外面。现在移入到里面。每次循环都要判断，初始化状态，设备是否有效
//监听来自于属性服务器事件，socket信号事件，keychord设备事件
        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;

        for (i = 0; i < fd_count; i++) {
//处理Uevent，属性服务，keychord事件
//以前是==判断，改成&。能否提速？
            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;
}