linux内核初始化成功后是如何过渡到android初始化的

Android用的linux内核，以完成OS该有的功能，例如，文件系统，网络，内存管理，进程调度，驱动等，向下管理硬件资源向上提供系统调用。另一些Android特有驱动也放在内核之中。
当linux内核启动完成后，便进行Android的初始化工作。

内核端

内核是在main.c中进行初始化,从kernel_init开始

static int __init kernel_init(void * unused)
{lock_kernel();/** init can run on any cpu.*/set_cpus_allowed_ptr(current, CPU_MASK_ALL_PTR);/** Tell the world that we're going to be the grim* reaper of innocent orphaned children.** We don't want people to have to make incorrect* assumptions about where in the task array this* can be found.*/init_pid_ns.child_reaper = current;cad_pid = task_pid(current);smp_prepare_cpus(setup_max_cpus);do_pre_smp_initcalls();start_boot_trace();smp_init();sched_init_smp();cpuset_init_smp();do_basic_setup();/** check if there is an early userspace init.  If yes, let it do all* the work*/if (!ramdisk_execute_command)ramdisk_execute_command = "/init";if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {ramdisk_execute_command = NULL;prepare_namespace();}/** Ok, we have completed the initial bootup, and* we're essentially up and running. Get rid of the* initmem segments and start the user-mode stuff..*/init_post();return 0;
}

在main.c 函数init_post()开始执行Android相关代码

	if (ramdisk_execute_command) {run_init_process(ramdisk_execute_command);printk(KERN_WARNING "Failed to execute %s\n",ramdisk_execute_command);}/** We try each of these until one succeeds.** The Bourne shell can be used instead of init if we are* trying to recover a really broken machine.*/if (execute_command) {run_init_process(execute_command);printk(KERN_WARNING "Failed to execute %s.  Attempting ""defaults...\n", execute_command);}run_init_process("/sbin/init");run_init_process("/etc/init");run_init_process("/bin/init");run_init_process("/bin/sh");

run_init_process便是加载执行文件。可以看到有六处出现了这个函数，但是执行的是第一个，
也就是run_init_process(ramdisk_execute_command), 变量ramdisk_execute_command在kernel_init执行时被赋值为’/init’, 也就是说，将会执行根文件目录下的init进程，而这个进程正是Android的初始化进程.

Android端

在根目录下可以看到init是个软链接，真正的init文件在/system/bin下面
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进入目录，真身就在里面

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加载执行后，就是大家耳熟能详的init进程了，开始进入Android的世界。

int main(int argc, char** argv) {
#if __has_feature(address_sanitizer)__asan_set_error_report_callback(AsanReportCallback);
#endifif (!strcmp(basename(argv[0]), "ueventd")) {return ueventd_main(argc, argv);}if (argc > 1) {if (!strcmp(argv[1], "subcontext")) {android::base::InitLogging(argv, &android::base::KernelLogger);const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();return SubcontextMain(argc, argv, &function_map);}if (!strcmp(argv[1], "selinux_setup")) {return SetupSelinux(argv);}if (!strcmp(argv[1], "second_stage")) {return SecondStageMain(argc, argv);}}return FirstStageMain(argc, argv);
}

init可执行文件的命名

在源码/system/core/init目录下查看mk和bp文件，在文件Android.mk中

LOCAL_MODULE := init_first_stage
LOCAL_MODULE_STEM := init

在文件Android.bp中

cc_binary {name: "init_second_stage",recovery_available: true,stem: "init",defaults: ["init_defaults"],static_libs: ["libinit"],required: ["e2fsdroid","init.rc","mke2fs","sload_f2fs","make_f2fs","ueventd.rc",],srcs: ["main.cpp"],symlinks: ["ueventd"],target: {recovery: {cflags: ["-DRECOVERY"],exclude_shared_libs: ["libbinder","libutils",],},},
}