Android的init进程启动过程

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是系统内核的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。

一、在分析init的核心代码之前，可以初步了解init的主要工作：
  1. Android系统有很多属性，init提供了一个property_service（属性服务）来管理它们。
  2. 处理配置文件的命令（主要是init.rc文件），包括处理各种Action。
  3. 性能分析（使用bootchart工具）。
  4. 无限循环执行command。

二、代码路径：\system\core\init\init.c

  以下是main函数，包含对各部分的注解

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);  // 创建用户空间的目录，例如/dev,/proc,/sys等。  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文件是否可读写和可创建。  close(open("/dev/.booting", O_WRONLY | O_CREAT, 0000));  // 将标准输入、输出、错误输出重定向到/dev/__null__。  open_devnull_stdio();  // 将init的日志输出设备设置为/dev/__kmsg__。  klog_init();  // 初始化和属性相关的资源  property_init();  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");  // 分析/init.rc文件的内容  init_parse_config_file("/init.rc");    // 解析完init.rc配置文件后，会得到一系列的Action动作。  // init将动作的执行时间划分为四个阶段：early-init,init,early-boot,boot  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");  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);  }  /* 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();    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;#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;}

总结：
本文分析了Android里的init进程的启动过程，从中可以知道init做的主要工作包括对init.rc的解析，property机制的实现，service支撑的实现。