本篇文章主要介绍 Android 开发中的部分知识点，通过阅读本篇文章，您将收获以下内容:
一、启动流程概述
二、Android启动分析
三、init 进程启动分析
四、init 启动脚本分析
五、init 进程分析
六、init 脚本执行
七、init 进程守护
八、init rc 脚本启动Zygote
九、启动分析小结
Android启动流程跟 linux启动类似，大致分为如下五个阶段。
Android系统启动大致过程如下：
Android 启动过程
由于水平有限，无法深入了理解驱动层代码，本文主要对 Android上层启动流程进行分析。
Uboot启动Kernel完成系统设置后，会首先在系统中寻找init.rc文件,并启动init进程。
Android 启动分析
Init进程是Android启动的第一个进程，进程号为1，是Android的系统启动的核心进程，主要用来创建Zygote、属性服务等。 init.cpp 中的main 函数，是init进程的入口函数，源码主要存在systemcoreinit目录下。
常见init.xxx.rc 进程如下：
常见init.xxx.rc 进程
init 进程主要作用
init 进程主要作用
/system/core/init 部分内容如下：
/system/core/init 部分内容
main 函数主要做的事情
1.创建挂载启动所需的文件系统（tmpfs、 devpts、 proc、 sysfs、 selinuxfs等）。
2.初始化并启动属性服务
3.解析init.rc 脚本配置文件，并启动Zygote 进程。
init.cpp main 函数实现代码如下：
int main(int argc, char** argv) {
... ...
if (!strcmp(basename(argv[0]), "watchdogd")) {
//启动看门狗函数
return watchdogd_main(argc, argv);
}
... ...
//启动第一阶段
if (is_first_stage) {
boot_clock::time_point start_time = boot_clock::now();
// 清理 umask.
umask(0);
clearenv();
setenv("PATH", _PATH_DEFPATH, 1);
// 在RAM内存上获取基本的文件系统，剩余的被 rc 文件所用
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));
// 非特权应用不能使用 Android 命令行
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 staging areas for devices managed by vold
// See storage config details at http://source.android.com/devices/storage/
mount("tmpfs", "/mnt", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
"mode=0755,uid=0,gid=1000");
//创建可供读写的 vendor目录
mkdir("/mnt/vendor", 0755);
// 在/dev目录下挂载好 tmpfs 以及 kmsg
// 这样就可以初始化 /kernel Log 系统，供用户打印log
InitKernelLogging(argv);
LOG(INFO) << "init first stage started!";
if (!DoFirstStageMount()) {
LOG(FATAL) << "Failed to mount required partitions early ...";
}
SetInitAvbVersionInRecovery();
// Enable seccomp if global boot option was passed (otherwise it is enabled in zygote).
global_seccomp();
// 优先加载selinux log系统， 紧接着初始化selinux
SelinuxSetupKernelLogging();
SelinuxInitialize();
// 添加 selinux 是否启动成功的log
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);
// execv() only returns if an error hAppened, in which case we
// panic and never fall through this conditional.
PLOG(FATAL) << "execv("" << path << "") failed";
}
//启动第二阶段
InitKernelLogging(argv);
LOG(INFO) << "init second stage started!";
// 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.
keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 1);
// Indicate that booting is in progress to background fw loaders, etc.
close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));
//初始化属性
property_init();
// If arguments are passed both on the command line and in DT,
// properties set in DT always have priority over the command-line ones.
process_kernel_dt();
process_kernel_cmdline();
// Propagate the kernel variables to internal variables
// used by init as well as the current required properties.
export_kernel_boot_props();
// Make the time that init started available for bootstat to log.
property_set("ro.boottime.init", getenv("INIT_STARTED_AT"));
property_set("ro.boottime.init.selinux", getenv("INIT_SELINUX_TOOK"));
// Set libavb version for Framework-only OTA match in Treble build.
const char* avb_version = getenv("INIT_AVB_VERSION");
if (avb_version) property_set("ro.boot.avb_version", avb_version);
// 清空设置的环境变量
unsetenv("INIT_SECOND_STAGE");
unsetenv("INIT_STARTED_AT");
unsetenv("INIT_SELINUX_TOOK");
unsetenv("INIT_AVB_VERSION");
// 设置第二阶段的selinux
SelinuxSetupKernelLogging();
SelabelInitialize();
SelinuxRestoreContext();
//创建 epoll 句柄
epoll_fd = epoll_create1(EPOLL_CLOEXEC);
if (epoll_fd == -1) {
PLOG(FATAL) << "epoll_create1 failed";
}
//设置 子进程处理函数
sigchld_handler_init();
if (!IsRebootCapable()) {
// If init does not have the CAP_SYS_BOOT capability, it is running in a container.
// In that case, receiving SIGTERM will cause the system to shut down.
InstallSigtermHandler();
}
LoadRscRoProps();
property_load_boot_defaults();
export_oem_lock_status();
//启动属性服务
start_property_service();
//为USB存储设置udc Contorller, sys/class/udc
set_usb_controller();
const BuiltinFunctionMap function_map;
Action::set_function_map(&function_map);
subcontexts = InitializeSubcontexts();
ActionManager& am = ActionManager::GetInstance();
ServiceList& sm = ServiceList::GetInstance();
LoadBootScripts(am, sm);
// Turning this on and letting the INFO logging be discarded adds 0.2s to
// Nexus 9 boot time, so it's disabled by default.
if (false) DumpState();
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(MixHwrngIntoLinuxRngAction, "MixHwrngIntoLinuxRng");
am.QueueBuiltinAction(SetMmapRndBitsAction, "SetMmapRndBits");
am.QueueBuiltinAction(SetKptrRestrictAction, "SetKptrRestrict");
am.QueueBuiltinAction(keychord_init_action, "keychord_init");
am.QueueBuiltinAction(console_init_action, "console_init");
// Trigger all the boot actions to get us started.
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())) {
am.ExecuteOneCommand();
}
if (!(waiting_for_prop || Service::is_exec_service_running())) {
if (!shutting_down) {
auto next_process_restart_time = RestartProcesses();
// If there's a process that needs restarting, wake up in time for that.
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;
}
基于MTK 平台 init.cpp 源码分析
基于MTK 平台 init.cpp 主要作用
init.rc 路径 一般在system/core/rootdir下，init脚本是有Android 初始化语言编写。
Android Init Language 语句类型：
init 进程分析
init.rc on
init.rc services
init.rc import
init 进程分析
init 解析脚本分析
init 事件列表
init 事件结构
init 进程解析和执行
整理事件列表
init 构建事件
Service 事件分类
init 进程执行命令和启动服务
init进程处理消息事件
init守护进程
init 处理 prop 消息分析
init 守护服务分析
Zygote 的 classname 为main.
init.rc文件配置代码如下：
... ...
on nonencrypted
class_start main
class_start late_start
on property:sys.init_log_level=*
loglevel ${sys.init_log_level}
... ...
启动分析小结
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