init 启动

init 进程是android启动的第一个进程。作为第一个启动的进程，他需要创建Zygote进程，属性服务等。
源码路径为:/system/core/init/init.cpp 启动换过程
1.启动电源以及系统启动
2.引导程序BootLoader
3.Linux内核启动
4.init进程启动

init进程的入口函数

/system/care/init/init.cpp

int main(int argc, char** argv) {
    if (!strcmp(basename(argv[0]), "ueventd")) {
        return ueventd_main(argc, argv);
    }

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

    if (argc > 1 && !strcmp(argv[1], "subcontext")) {
        InitKernelLogging(argv);
        const BuiltinFunctionMap function_map;
        return SubcontextMain(argc, argv, &function_map);
    }

    if (REBOOT_BOOTLOADER_ON_PANIC) {
        InstallRebootSignalHandlers();
    }

    bool is_first_stage = (getenv("INIT_SECOND_STAGE") == nullptr);
     //是否是第一次启动 ，如果是第一次启动就挂载目录，tmpfs，devpts，proc，sysfs 和selinuxfs 5种文件系统
    if (is_first_stage) {
        boot_clock::time_point start_time = boot_clock::now();
        umask(0);

        clearenv();
        setenv("PATH", _PATH_DEFPATH, 1);
        mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755");
        mkdir("/dev/pts", 0755);
        mkdir("/dev/socket", 0755);
        mount("devpts", "/dev/pts", "devpts", 0, NULL);
        #define MAKE_STR(x) __STRING(x)
        mount("proc", "/proc", "proc", 0, "hidepid=2,gid=" MAKE_STR(AID_READPROC));
        chmod("/proc/cmdline", 0440);
        gid_t groups[] = { AID_READPROC };
        setgroups(arraysize(groups), groups);
        mount("sysfs", "/sys", "sysfs", 0, NULL);
        mount("selinuxfs", "/sys/fs/selinux", "selinuxfs", 0, NULL);

        mknod("/dev/kmsg", S_IFCHR | 0600, makedev(1, 11));

        if constexpr (WORLD_WRITABLE_KMSG) {
            mknod("/dev/kmsg_debug", S_IFCHR | 0622, makedev(1, 11));
        }

        mknod("/dev/random", S_IFCHR | 0666, makedev(1, 8));
        mknod("/dev/urandom", S_IFCHR | 0666, makedev(1, 9));

        mount("tmpfs", "/mnt", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
              "mode=0755,uid=0,gid=1000");
        mkdir("/mnt/vendor", 0755);
        InitKernelLogging(argv);
        LOG(INFO) << "init first stage started!";
        if (!DoFirstStageMount()) {
            LOG(FATAL) << "Failed to mount required partitions early ...";
        }

        SetInitAvbVersionInRecovery();
        global_seccomp();

        SelinuxSetupKernelLogging();
        SelinuxInitialize();
        if (selinux_android_restorecon("/init", 0) == -1) {
            PLOG(FATAL) << "restorecon failed of /init failed";
        }

        setenv("INIT_SECOND_STAGE", "true", 1);

        static constexpr uint32_t kNanosecondsPerMillisecond = 1e6;
        uint64_t start_ms = start_time.time_since_epoch().count() / kNanosecondsPerMillisecond;
        setenv("INIT_STARTED_AT", std::to_string(start_ms).c_str(), 1);

        char* path = argv[0];
        char* args[] = { path, nullptr };
        execv(path, args);
        PLOG(FATAL) << "execv(\"" << path << "\") failed";
    }

    //第二次初始化
    InitKernelLogging(argv);
    LOG(INFO) << "init second stage started!";


    keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 1);

    close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));

    property_init();  //对属性初始化

    process_kernel_dt();
    process_kernel_cmdline();
    
    export_kernel_boot_props();

    property_set("ro.boottime.init", getenv("INIT_STARTED_AT"));
    property_set("ro.boottime.init.selinux", getenv("INIT_SELINUX_TOOK"));

    const char* avb_version = getenv("INIT_AVB_VERSION");
    if (avb_version) property_set("ro.boot.avb_version", avb_version);

    unsetenv("INIT_SECOND_STAGE");
    unsetenv("INIT_STARTED_AT");
    unsetenv("INIT_SELINUX_TOOK");
    unsetenv("INIT_AVB_VERSION");

    SelinuxSetupKernelLogging();
    SelabelInitialize();
    SelinuxRestoreContext();

    epoll_fd = epoll_create1(EPOLL_CLOEXEC);
    if (epoll_fd == -1) {
        PLOG(FATAL) << "epoll_create1 failed";
    }

    //用于设置子进程信号处理函数，如果子进程异常退出，init进程会调用该函数中设定 的信号处理函数来处理
    sigchld_handler_init();

    if (!IsRebootCapable()) {
        InstallSigtermHandler();
    }

    property_load_boot_defaults();
    export_oem_lock_status();
    start_property_service();//启动属性服务，
    set_usb_controller();

    const BuiltinFunctionMap function_map;
    Action::set_function_map(&function_map);

    subcontexts = InitializeSubcontexts();

    ActionManager& am = ActionManager::GetInstance();
    ServiceList& sm = ServiceList::GetInstance();
    Parser& parser = Parser::GetInstance();

    parser.AddSectionParser("service", std::make_unique<ServiceParser>(&sm));
    parser.AddSectionParser("on", std::make_unique<ActionParser>(&am));
    parser.AddSectionParser("import", std::make_unique<ImportParser>(&parser));
    std::string bootscript = GetProperty("ro.boot.init_rc", "");
    if (bootscript.empty()) {
        //解析init脚本
        parser.ParseConfig("/init.rc");
        parser.set_is_system_etc_init_loaded(
                parser.ParseConfig("/system/etc/init"));
        parser.set_is_vendor_etc_init_loaded(
                parser.ParseConfig("/vendor/etc/init"));
        parser.set_is_odm_etc_init_loaded(parser.ParseConfig("/odm/etc/init"));
    } else {
        parser.ParseConfig(bootscript);
        parser.set_is_system_etc_init_loaded(true);
        parser.set_is_vendor_etc_init_loaded(true);
        parser.set_is_odm_etc_init_loaded(true);
    }
    LoadBootScripts(am, sm);

    if (false) DumpState();

    am.QueueEventTrigger("early-init");


    am.QueueBuiltinAction(wait_for_coldboot_done_action, "wait_for_coldboot_done");
    am.QueueBuiltinAction(MixHwrngIntoLinuxRngAction, "MixHwrngIntoLinuxRng");
    am.QueueBuiltinAction(SetMmapRndBitsAction, "SetMmapRndBits");
    am.QueueBuiltinAction(SetKptrRestrictAction, "SetKptrRestrict");
    am.QueueBuiltinAction(keychord_init_action, "keychord_init");
    am.QueueBuiltinAction(console_init_action, "console_init");

    am.QueueEventTrigger("init");

    // Repeat mix_hwrng_into_linux_rng in case /dev/hw_random or /dev/random
    // wasn't ready immediately after wait_for_coldboot_done
    am.QueueBuiltinAction(MixHwrngIntoLinuxRngAction, "MixHwrngIntoLinuxRng");

    // 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");

    while (true) {
        // By default, sleep until something happens.
        int epoll_timeout_ms = -1;

        if (do_shutdown && !shutting_down) {
            do_shutdown = false;
            if (HandlePowerctlMessage(shutdown_command)) {
                shutting_down = true;
            }
        }

        if (!(waiting_for_prop || Service::is_exec_service_running())) {
        //遍历内部执行每个action中携带的command对应的执行函数
            am.ExecuteOneCommand();
        }
        if (!(waiting_for_prop || Service::is_exec_service_running())) {
            if (!shutting_down) {
            //重启死去的进程
                auto next_process_restart_time = RestartProcesses();

                if (next_process_restart_time) {
                    epoll_timeout_ms = std::chrono::ceil<std::chrono::milliseconds>(
                                           *next_process_restart_time - boot_clock::now())
                                           .count();
                    if (epoll_timeout_ms < 0) epoll_timeout_ms = 0;
                }
            }

            // If there's more work to do, wake up again immediately.
            if (am.HasMoreCommands()) epoll_timeout_ms = 0;
        }

        epoll_event ev;
        int nr = TEMP_FAILURE_RETRY(epoll_wait(epoll_fd, &ev, 1, epoll_timeout_ms));
        if (nr == -1) {
            PLOG(ERROR) << "epoll_wait failed";
        } else if (nr == 1) {
            ((void (*)()) ev.data.ptr)();
        }
    }

    return 0;
}

1.首先init的main函数或挂载系统运行时的目录，只有系统运行时才会存在。

2.在property_init 函数中是对属性的初始化
3.start_property_service启动属性服务

4.parser.parseConfig("init.rc") 解析init.rc脚本

解析.rc文件路径为:
/system/core/init/init_parser.cpp

void Parser::ParseData(const std::string& filename, const std::string& data) {
    //TODO: Use a parser with const input and remove this copy
    std::vector<char> data_copy(data.begin(), data.end());
    data_copy.push_back('\0');

    parse_state state;
    state.line = 0;
    state.ptr = &data_copy[0];
    state.nexttoken = 0;

    SectionParser* section_parser = nullptr;
    std::vector<std::string> args;

    for (;;) {
        switch (next_token(&state)) {
        case T_EOF:
            if (section_parser) {
                section_parser->EndSection();
            }
            return;
        case T_NEWLINE:
            state.line++;
            if (args.empty()) {
                break;
            }
            // If we have a line matching a prefix we recognize, call its callback and unset any
            // current section parsers.  This is meant for /sys/ and /dev/ line entries for uevent.
            for (const auto& [prefix, callback] : line_callbacks_) {
                if (android::base::StartsWith(args[0], prefix.c_str())) {
                    if (section_parser) section_parser->EndSection();

                    std::string ret_err;
                    if (!callback(std::move(args), &ret_err)) {
                        LOG(ERROR) << filename << ": " << state.line << ": " << ret_err;
                    }
                    section_parser = nullptr;
                    break;
                }
            }
            if (section_parsers_.count(args[0])) {
                if (section_parser) {
                    section_parser->EndSection();
                }
                section_parser = section_parsers_[args[0]].get();
                std::string ret_err;
                if (!section_parser->ParseSection(std::move(args), filename, state.line, &ret_err)) {
                    LOG(ERROR) << filename << ": " << state.line << ": " << ret_err;
                    section_parser = nullptr;
                }
            } else if (section_parser) {
                std::string ret_err;
                if (!section_parser->ParseLineSection(std::move(args), state.line, &ret_err)) {
                    LOG(ERROR) << filename << ": " << state.line << ": " << ret_err;
                }
            }
            args.clear();
            break;
        case T_TEXT:
            args.emplace_back(state.text);
            break;
        }
    }
}

.rc路径为:
/system/core/rootdir/init.rc

on init
    sysclktz 0

    # Mix device-specific information into the entropy pool
    copy /proc/cmdline /dev/urandom
    copy /default.prop /dev/urandom

    # Backward compatibility.
    symlink /system/etc /etc
    symlink /sys/kernel/debug /d

    # Link /vendor to /system/vendor for devices without a vendor partition.
    symlink /system/vendor /vendor

.rc文件说明

.rc文件的包含五种类型的语句:
Command(命令)
Service(服务)
service <name> <pathname>[]* //service 的名字><执行程序路径><传递参数> <option> //option是service的修饰词 影响什么时候，如何启动service Option
Import Action
on boot 和 on init 是action类型的语句 on <trigger> [&& <trigger>]* <command> 触发之后要执行的命令

1.Action

适用于相应某个命令，把符合条件的命令，依次加入到执行队列尾部

2.Commands命令

把所属的事情发生时一个一个执行

3.Service(服务)

Service 其实可以执行程序，可以在特定情况下被ini重启

服务

解析service语句

system/core/init/service.cpp

bool ServiceParser::ParseSection(std::vector<std::string>&& args, const std::string& filename,
                                 int line, std::string* err) {
    if (args.size() < 3) {  //判断是否有name可以运行
        *err = "services must have a name and a program";
        return false;
    }

    const std::string& name = args[1];
    if (!IsValidName(name)) { //检查name是否有效
        *err = StringPrintf("invalid service name '%s'", name.c_str());
        return false;
    }

    Service* old_service = service_manager_->FindServiceByName(name);
    if (old_service) {
        *err = "ignored duplicate definition of service '" + name + "'";
        return false;
    }

    std::vector<std::string> str_args(args.begin() + 2, args.end());
    service_ = std::make_unique<Service>(name, str_args);///构造一个service对象
    return true;
}

bool ServiceParser::ParseLineSection(std::vector<std::string>&& args, int line, std::string* err) {
    return service_ ? service_->ParseLine(std::move(args), err) : false;
}


bool Service::ParseLine(const std::vector<std::string>& args, std::string* err) {
    static const OptionParserMap parser_map;
    auto parser = parser_map.FindFunction(args, err);

    if (!parser) {
        return false;
    }

    return (this->*parser)(args, err);
}

//存储一个map，根据传入的参数调用对应的函数.