fwrite

  // /init/main.cpp
  
  #include "builtins.h"
  #include "first_stage_init.h"
  #include "init.h"
  #include "selinux.h"
  #include "subcontext.h"
  #include "ueventd.h"
  
  using namespace android::init;
  
  ... omitted parts
  
  int main(int argc, char** argv) {    
      ... omitted parts
  
      // Boost prio which will be restored later
      setpriority(PRIO_PROCESS, 0, -20);
  
      if (!strcmp(basename(argv[0]), "ueventd")) {    // ueventd is responsible for creating device nodes, permissions, anonymous shared memory, etc.
          return ueventd_main(argc, argv);
      }
  
      if (argc > 1) {
          if (!strcmp(argv[1], "subcontext")) {
              // Initialize Logger
              android::base::InitLogging(argv, &android::base::KernelLogger);
              const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();
              return SubcontextMain(argc, argv, &function_map);
          }
          // SELinux security-related
          if (!strcmp(argv[1], "selinux_setup")) {
              return SetupSelinux(argv);
          }
  
          // Init process (second stage)
          if (!strcmp(argv[1], "second_stage")) {
              return SecondStageMain(argc, argv);
          }
      }
  
      // Start init process (first stage)
      return FirstStageMain(argc, argv);
  }

  // first_stage_init.cpp
  
  int FirstStageMain(int argc, char** argv) {
  
      if (REBOOT_BOOTLOADER_ON_PANIC) {
          InstallRebootSignalHandlers();    // Restart Boot Loader on init error
      }
  
      // Record boot time
      boot_clock::time_point start_time = boot_clock::now();
      // Record list of errors
      std::vector<std::pair<std::string, int>> errors;
  
      
  #define CHECKCALL(x) \
      if ((x) != 0) errors.emplace_back(#x " failed", errno);
  
      // Clear umask, files and folders created afterwards will have full permissions
      umask(0);
      CHECKCALL(clearenv());
      CHECKCALL(setenv("PATH", _PATH_DEFPATH, 1));
      // 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.
      // mount is used to mount the tmpfs file system
      CHECKCALL(mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755"));
  
      // Create folders and assign permissions
      CHECKCALL(mkdir("/dev/pts", 0755));
      CHECKCALL(mkdir("/dev/socket", 0755));
      CHECKCALL(mkdir("/dev/dm-user", 0755));
  
      CHECKCALL(mount("devpts", "/dev/pts", "devpts", 0, NULL));
  
  #define MAKE_STR(x) __STRING(x)
      // Mount proc
      CHECKCALL(mount("proc", "/proc", "proc", 0, "hidepid=2,gid=" MAKE_STR(AID_READPROC)));
  #undef MAKE_STR
      // Don't expose the raw command line to unprivileged processes.
      CHECKCALL(chmod("/proc/cmdline", 0440));
      std::string cmdline;
      android::base::ReadFileToString("/proc/cmdline", &cmdline);
      // Don't expose the raw bootconfig to unprivileged processes.
      chmod("/proc/bootconfig", 0440);
      std::string bootconfig;
      android::base::ReadFileToString("/proc/bootconfig", &bootconfig);
  
      // 8.0 added user groups
      gid_t groups[] = {AID_READPROC};
      // Add groups to the current process's device
      CHECKCALL(setgroups(arraysize(groups), groups));
      
      // Mount sys, using sysfs to manage documents
      CHECKCALL(mount("sysfs", "/sys", "sysfs", 0, NULL));
      CHECKCALL(mount("selinuxfs", "/sys/fs/selinux", "selinuxfs", 0, NULL));
  
      // Create nodes (mknod) for output Log
      CHECKCALL(mknod("/dev/kmsg", S_IFCHR | 0600, makedev(1, 11)));
      if constexpr (WORLD_WRITABLE_KMSG) {
          CHECKCALL(mknod("/dev/kmsg_debug", S_IFCHR | 0622, makedev(1, 11)));
      }
      CHECKCALL(mknod("/dev/random", S_IFCHR | 0666, makedev(1, 8)));
      CHECKCALL(mknod("/dev/urandom", S_IFCHR | 0666, makedev(1, 9)));
      // This is needed for log wrapper, which gets called before ueventd runs.
      CHECKCALL(mknod("/dev/ptmx", S_IFCHR | 0666, makedev(5, 2)));
      CHECKCALL(mknod("/dev/null", S_IFCHR | 0666, makedev(1, 3)));
      
      // See storage config details at http://source.android.com/devices/storage/
      // Mount /mnt/{vendor,product}, using tmpfs to manage
      CHECKCALL(mount("tmpfs", "/mnt", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
                      "mode=0755,uid=0,gid=1000"));
      
      // /mnt/vendor is used to mount vendor
      CHECKCALL(mkdir("/mnt/vendor", 0755));
      // /mnt/product is used to mount product-specific partitions that cannot be
      // part of the product partition, e.g., because they are mounted read-write.
      CHECKCALL(mkdir("/mnt/product", 0755));
      // /debug_ramdisk is used to preserve additional files from the debug ramdisk
      CHECKCALL(mount("tmpfs", "/debug_ramdisk", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
                      "mode=0755,uid=0,gid=0"));
      // /second_stage_resources is used to preserve files from first to second
      // stage init
      CHECKCALL(mount("tmpfs", kSecondStageRes, "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
                      "mode=0755,uid=0,gid=0"))
  #undef CHECKCALL
      SetStdioToDevNull(argv);
    
      InitKernelLogging(argv);
  
      // List error messages
      if (!errors.empty()) {
          for (const auto& [error_string, error_errno] : errors) {
              LOG(ERROR) << error_string << " " << strerror(error_errno);
          }
          LOG(FATAL) << "Init encountered errors starting first stage, aborting";
      }
  
      ... omitted parts
  
      return 1;
  }

  // first_stage_init.cpp
  
  int FirstStageMain(int argc, char** argv) {
      
      ... omitted parts
          
      // Mount specific partition devices
      if (!DoFirstStageMount(!created_devices)) {
          LOG(FATAL) << "Failed to mount required partitions early ...";
      }
      
      ... omitted parts
  }

  // first_stage_init.cpp
  
  int FirstStageMain(int argc, char** argv) {
  
      ... omitted parts
  
      struct stat new_root_info;
      if (stat("/", &new_root_info) != 0) {
          PLOG(ERROR) << "Could not stat(\"/\"), not freeing ramdisk";
          old_root_dir.reset();
      }
      if (old_root_dir && old_root_info.st_dev != new_root_info.st_dev) {
          FreeRamdisk(old_root_dir.get(), old_root_info.st_dev);
      }
      
      // Initialize security framework: Android Verified Boot & AVB mainly used to prevent system files from being tampered with
      // Also prevents system rollback functionality
      SetInitAvbVersionInRecovery();
      setenv(kEnvFirstStageStartedAt, std::to_string(start_time.time_since_epoch().count()).c_str(),
             1);
  
      ... omitted parts
  
  }

  // first_stage_init.cpp
  
  int FirstStageMain(int argc, char** argv) {
  
      ... omitted parts
  
      // Set init process parameters
      const char* path = "/system/bin/init";
      const char* args[] = {path, "selinux_setup", nullptr};
      // Binder open (log message)
      auto fd = open("/dev/kmsg", O_WRONLY | O_CLOEXEC);
      dup2(fd, STDOUT_FILENO);
      dup2(fd, STDERR_FILENO);
      close(fd);
  
      // Pass the above args
      // Start the init process, passing the parameter "selinux_setup"
      execv(path, const_cast<char**>(args));        // Start the init process's SecondStageMain method via execv
  
  }

  // /init/main.cpp file
  
  #include "builtins.h"
  #include "first_stage_init.h"
  #include "init.h"
  #include "selinux.h"
  #include "subcontext.h"
  #include "ueventd.h"
  
  using namespace android::init;
  
  ... omitted parts
  
  // Upon re-entry, argv contains the string selinux_setup
  int main(int argc, char** argv) {    
      ... omitted parts
  
      // Boost prio which will be restored later
      setpriority(PRIO_PROCESS, 0, -20);
  
      // Check input parameters
      if (!strcmp(basename(argv[0]), "ueventd")) {    // ueventd is responsible for creating device nodes, permissions, anonymous shared memory, etc.
          return ueventd_main(argc, argv);
      }
  
      // When the number of parameters > 1
      if (argc > 1) {
          if (!strcmp(argv[1], "subcontext")) {
              // Initialize Log
              android::base::InitLogging(argv, &android::base::KernelLogger);
              const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();
              return SubcontextMain(argc, argv, &function_map);
          }
          // SELinux security-related
          if (!strcmp(argv[1], "selinux_setup")) {
              return SetupSelinux(argv);
          }
  
          // Init process (second stage)
          if (!strcmp(argv[1], "second_stage")) {
              return SecondStageMain(argc, argv);
          }
      }
  
      // Start init process (first stage)
      return FirstStageMain(argc, argv);
  }

  // /init/selinux.cpp
  
  int SetupSelinux(char** argv) {
  
      SetStdioToDevNull(argv);
  
      // 1. Initialize core Log
      InitKernelLogging(argv);
      if (REBOOT_BOOTLOADER_ON_PANIC) {
          InstallRebootSignalHandlers();
      }
      boot_clock::time_point start_time = boot_clock::now();
  
      // 2. Mount missing system blocks
      MountMissingSystemPartitions();
      SelinuxSetupKernelLogging();
      LOG(INFO) << "Opening SELinux policy";
      PrepareApexSepolicy();
  
      // Read the policy before potentially killing snapuserd.
      std::string policy;
      // Read policy
      ReadPolicy(&policy);
  
      CleanupApexSepolicy();
      auto snapuserd_helper = SnapuserdSelinuxHelper::CreateIfNeeded();
      if (snapuserd_helper) {
          // Kill the old snapused to avoid audit messages. After this we cannot
          // read from /system (or other dynamic partitions) until we call
          // FinishTransition().
          snapuserd_helper->StartTransition();
      }
  
      // Load SE policy
      LoadSelinuxPolicy(policy);
  
      ... omitted parts
  
      const char* path = "/system/bin/init";
      const char* args[] = {path, "second_stage", nullptr};
  
      // Restart the process
      execv(path, const_cast<char**>(args));
  
      return 1;
  }

  // /init/main.cpp file
  
  #include "builtins.h"
  #include "first_stage_init.h"
  #include "init.h"
  #include "selinux.h"
  #include "subcontext.h"
  #include "ueventd.h"
  
  using namespace android::init;
  
  ... omitted parts
  
  // Upon re-entry, argv contains the string second_stage
  int main(int argc, char** argv) {    
      ... omitted parts
  
      // Boost prio which will be restored later
      setpriority(PRIO_PROCESS, 0, -20);
  
      // Check input parameters
      if (!strcmp(basename(argv[0]), "ueventd")) {   
          return ueventd_main(argc, argv);
      }
  
      if (argc > 1) {
          if (!strcmp(argv[1], "subcontext")) {
              // Initialize Log
              android::base::InitLogging(argv, &android::base::KernelLogger);
              const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();
              return SubcontextMain(argc, argv, &function_map);
          }
          // SELinux security-related
          if (!strcmp(argv[1], "selinux_setup")) {
              return SetupSelinux(argv);
          }
  
          // Init process (second stage)
          if (!strcmp(argv[1], "second_stage")) {
              return SecondStageMain(argc, argv);
          }
      }
  
      // Start init process (first stage)
      return FirstStageMain(argc, argv);
  }

  // init/init.cpp
  
  int SecondStageMain(int argc, char** argv) {
      if (REBOOT_BOOTLOADER_ON_PANIC) {
          InstallRebootSignalHandlers();        // Restart Boot Loader on init failure
      }
  
      // Boot time
      boot_clock::time_point start_time = boot_clock::now();
  
      trigger_shutdown = [](const std::string& command) { shutdown_state.TriggerShutdown(command); };
  
      // Redirect standard input, output, and error to the null device file `/dev/null`
      SetStdioToDevNull(argv);
  
      // Initialize kernel log system
      InitKernelLogging(argv);
      
      // Initialize system $PATH
      if (setenv("PATH", _PATH_DEFPATH, 1) != 0) {
          PLOG(FATAL) << "Could not set $PATH to '" << _PATH_DEFPATH << "' in second stage";
      }
  
      // The init process does not depend on other processes, so it should not crash,
      // thus the function to handle signals is set to null here.
      {
          // sigaction is for signals
          struct sigaction action = {.sa_flags = SA_RESTART};
          action.sa_handler = [](int) {};
          // Ignore SIGPIPE signal
          sigaction(SIGPIPE, &action, nullptr);    // Set SIGPIPE to null
      }
      // Set init and its forked children's oom_adj.
      if (auto result =
                  WriteFile("/proc/1/oom_score_adj", StringPrintf("%d", DEFAULT_OOM_SCORE_ADJUST));
          !result.ok()) {
          LOG(ERROR) << "Unable to write " << DEFAULT_OOM_SCORE_ADJUST
                     << " to /proc/1/oom_score_adj: " << result.error();
      }
      // Set up a session keyring that all processes will have access to. It
      // will hold things like FBE encryption keys. No process should override
      // its session keyring.
      // Call syscall to set related parameters
      keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 1);
  
      // Indicate that booting is in progress to background fw loaders, etc.
      // Create /dev/.booting file, marking that booting is in progress
      close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));
  
      ... omitted parts
  
      PropertyInit();        // Implemented in init/property_service.cpp
  
      ... omitted parts
  
      return 0;
  }

  // /init/init.cpp
  
  int SecondStageMain(int argc, char** argv) {
      ... omitted parts
  
      // Clear previously written environment variables from system properties
      // Clean up our environment.
      unsetenv("INIT_AVB_VERSION");
      unsetenv("INIT_FORCE_DEBUGGABLE");
  
      ... omitted parts
  }

  // /init/init.cpp
  
  int SecondStageMain(int argc, char** argv) {
      ... omitted parts
  
      // Now set up SELinux for the second stage.
      SelinuxSetupKernelLogging();        // Complete SELinux work for the second stage
      SelabelInitialize();                // Register some processors
      SelinuxRestoreContext();            // SELinux stored Context
  
      ... omitted parts
  }

  // /init/init.cpp
  
  int SecondStageMain(int argc, char** argv) {
  
      ... omitted parts
  
      Epoll epoll;
      // epoll.Open() creates child process I/O listener
      if (auto result = epoll.Open(); !result.ok()) {
          PLOG(FATAL) << result.error();
      }
  
      ... omitted parts
  
  }

  // /init/init.cpp
  
  int SecondStageMain(int argc, char** argv) {
  
      ... omitted parts
  
      // Create handler to process child process termination signals, register a signal to epoll listener (if there are changes, epoll will notify)
      // epoll will receive notifications first, then read data using socket
          
      // To prevent child processes of the init process from becoming zombie processes
      InstallSignalFdHandler(&epoll);
      InstallInitNotifier(&epoll);
  
      ... omitted parts
  
  }

  // /init/init.cpp
  static int property_fd = -1;
  
  int SecondStageMain(int argc, char** argv) {
  
      ... omitted parts
  
      // Start other property services
      StartPropertyService(&property_fd);
  
      unsetenv("INIT_AVB_VERSION");    // Remove environment variable INIT_AVB_VERSION 
      fs_mgr_vendor_overlay_mount_all();
      export_oem_lock_status();        // Ultimately decides the value of "ro.boot.flash.locked"
  
      MountHandler mount_handler(&epoll);
  
      // Set UDC Controller for USB storage (sys/class/udc)
      SetUsbController();
  
      ... omitted parts
  }

  // /init/init.cpp
  
  int SecondStageMain(int argc, char** argv) {
  
      ... omitted parts
  
      // Create Function & Cmd Map
      const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();
  
      // Save function_map object in Action
      // Action::set_function_map, `::` is equivalent to static Function
      Action::set_function_map(&function_map);
      if (!SetupMountNamespaces()) {
          PLOG(FATAL) << "SetupMountNamespaces failed";
      }
  
      ... omitted parts
  }

  // /init/init.cpp
  
  int SecondStageMain(int argc, char** argv) {
  
      ... omitted parts
      
      // The object will be used to obtain analysis results
      ActionManager& am = ActionManager::GetInstance();    
      ServiceList& sm = ServiceList::GetInstance();
  
      // Parse scripts
      LoadBootScripts(am, sm);
  
      ... omitted parts
  
  }
  
  // Parse .rc scripts
  static void LoadBootScripts(ActionManager& action_manager, 
                              ServiceList& service_list) {
      // Create parser
      Parser parser = CreateParser(action_manager, service_list);
  
      // Pre-set to parse ro.boot.init_rc
      std::string bootscript = GetProperty("ro.boot.init_rc", "");
  
      if (bootscript.empty()) {
          // Parse specified init.rc file
          parser.ParseConfig("/system/etc/init/hw/init.rc");
  
          if (!parser.ParseConfig("/system/etc/init")) {
              late_import_paths.emplace_back("/system/etc/init");
          }
  
          parser.ParseConfig("/system_ext/etc/init");
          if (!parser.ParseConfig("/vendor/etc/init")) {
              late_import_paths.emplace_back("/vendor/etc/init");
          }
          if (!parser.ParseConfig("/odm/etc/init")) {
              late_import_paths.emplace_back("/odm/etc/init");
          }
          if (!parser.ParseConfig("/product/etc/init")) {
              late_import_paths.emplace_back("/product/etc/init");
          }
      } else {
          parser.ParseConfig(bootscript);
      }
  }

  // /init/init.cpp
  
  int SecondStageMain(int argc, char** argv) {
  
      ... omitted parts
  
      // ActionManager parsing completed
      am.QueueBuiltinAction(SetupCgroupsAction, "SetupCgroups");
      am.QueueBuiltinAction(SetKptrRestrictAction, "SetKptrRestrict");
      am.QueueBuiltinAction(TestPerfEventSelinuxAction, "TestPerfEventSelinux");
      am.QueueBuiltinAction(ConnectEarlyStageSnapuserdAction, "ConnectEarlyStageSnapuserd");
      
      // Trigger label early-init
      am.QueueEventTrigger("early-init");
      // Queue an action that waits for coldboot done so we know ueventd has set up all of /dev...
      am.QueueBuiltinAction(wait_for_coldboot_done_action, "wait_for_coldboot_done");
      // ... so that we can start queuing up actions that require stuff from /dev.
      am.QueueBuiltinAction(SetMmapRndBitsAction, "SetMmapRndBits");
      Keychords keychords;
      am.QueueBuiltinAction(
              [&epoll, &keychords](const BuiltinArguments& args) -> Result<void> {
                  for (const auto& svc : ServiceList::GetInstance()) {
                      keychords.Register(svc->keycodes());
                  }
                  keychords.Start(&epoll, HandleKeychord);
                  return {};
              },
              "KeychordInit");
      // Trigger all the boot actions to get us started.
      am.QueueEventTrigger("init");
      // Don't mount filesystems or start core system services in charger mode.
      std::string bootmode = GetProperty("ro.bootmode", "");
      if (bootmode == "charger") {
          am.QueueEventTrigger("charger");
      } else {
          am.QueueEventTrigger("late-init");
      }
      // Run all property triggers based on current state of the properties.
      am.QueueBuiltinAction(queue_property_triggers_action, "queue_property_triggers");
  
      ... omitted parts
  }

  // /init/init.cpp
  
  int SecondStageMain(int argc, char** argv) {
  
      ... omitted parts
  
      while(true) {
          // By default, sleep until something happens.
          auto epoll_timeout = std::optional<std::chrono::milliseconds>{};
  
          // Determine if a shutdown command has been received
          auto shutdown_command = shutdown_state.CheckShutdown();
          
          if (shutdown_command) {
              HandlePowerctlMessage(*shutdown_command);
              shutdown_state.set_do_shutdown(false);
          }
  
          ... omitted parts
  
          // No need to wait && Service is not running || Service has completed running
          if (!(prop_waiter_state.MightBeWaiting() || Service::is_exec_service_running())) {
              am.ExecuteOneCommand();    // Execute one cmd
          }
  
          ... omitted parts
      }
  
  }