一.架构概述
长按电源启动机器手机CPU第一条指令会指向操作系统所在位置，BootLoader (系统启动加载器)，用以初始化硬件设备，建立内存空间的映像图，为最终调用系统内核准备好环境。 在 Android 里没有硬盘，而是 ROM，它类似于硬盘存放操作系统，用户程序等。 ROM 跟硬盘一样也会划分为不同的区域，用于放置不同的程序。当 Linux 内核启动后会初始化各种软硬件环境，加载驱动程序，挂载根文件系统，Linux 内核加载的准备完毕后就开始加载一些特定的程序(进程)了。第一个加载的就是 init 进程。
二.启动源码流程 init.cpp main函数
int main(int argc, char** argv) {
umask(0);
add_environment("PATH", _PATH_DEFPATH);
klog_init();
klog_set_level(KLOG_NOTICE_LEVEL);
property_init();
epoll_fd = epoll_create1(EPOLL_CLOEXEC);
signal_handler_init();
start_property_service();
init_parse_config_file("/init.rc");
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");
action_for_each_trigger("init", action_add_queue_tail);
queue_builtin_action(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");
queue_builtin_action(queue_property_triggers_action, "queue_property_triggers");
while (true) {
if (!waiting_for_exec) {
execute_one_command();
restart_processes();
int timeout = -1;
if (process_needs_restart) {
timeout = (process_needs_restart - gettime()) * 1000;
if (timeout < 0)
timeout = 0;
if (!action_queue_empty() || cur_action) {
timeout = 0;
bootchart_sample(&timeout);
epoll_event ev;
int nr = TEMP_FAILURE_RETRY(epoll_wait(epoll_fd, &ev, 1, timeout));
if (nr == -1) {
ERROR("epoll_wait failed: %s\n", strerror(errno));
} else if (nr == 1) {
((void (*)()) ev.data.ptr)();
return 0;
2.1 property_init
void property_init() {
//只初始化一次
if (property_area_initialized) {
return
property_area_initialized = true
//2.1.1 通过mmap开启共享内存位置/dev/properties 大小(128 * 1024)
if (__system_property_area_init()) {
return
//属性服务参数存储到静态workspace结构体重
pa_workspace.size = 0
//创建PROP_FILENAME句柄
pa_workspace.fd = open(PROP_FILENAME, O_RDONLY | O_NOFOLLOW | O_CLOEXEC)
if (pa_workspace.fd == -1) {
ERROR("Failed to open %s: %s\n", PROP_FILENAME, strerror(errno))
return
2.2__system_property_area_init
static int map_prop_area_rw()
//property_filename=/dev/properties
const int fd = open(property_filename,
O_RDWR | O_CREAT | O_NOFOLLOW | O_CLOEXEC | O_EXCL, 0444)
//PA_SIZE (128 * 1024)
pa_size = PA_SIZE
pa_data_size = pa_size - sizeof(prop_area)
compat_mode = false
//mmap开启虚拟内存地址
void *const memory_area = mmap(NULL, pa_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)
if (memory_area == MAP_FAILED) {
close(fd)
return -1
//内存的首地址保存在__system_property_area__
prop_area *pa = new(memory_area) prop_area(PROP_AREA_MAGIC, PROP_AREA_VERSION)
__system_property_area__ = pa
close(fd)
return 0
3.1 signal_handler_init
void signal_handler_init() {
int s[2];
if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0, s) == -1) {
ERROR("socketpair failed: %s\n", strerror(errno));
exit(1);
signal_write_fd = s[0];
signal_read_fd = s[1];
struct sigaction act;
memset(&act, 0, sizeof(act));
act.sa_handler = SIGCHLD_handler;
act.sa_flags = SA_NOCLDSTOP;
sigaction(SIGCHLD, &act, 0);
reap_any_outstanding_children();
3.3注册
register_epoll_handler(signal_read_fd, handle_signal);
3.2reap_any_outstanding_children()
static void reap_any_outstanding_children() {
while (wait_for_one_process()) {
static bool wait_for_one_process() {
int status;
pid_t pid = TEMP_FAILURE_RETRY(waitpid(-1, &status, WNOHANG));
if (pid == 0) {
return false;
} else if (pid == -1) {
ERROR("waitpid failed: %s\n", strerror(errno));
return false;
service* svc = service_find_by_pid(pid);
std::string name;
if (svc) {
name = android::base::StringPrintf("Service '%s' (pid %d)", svc->name, pid);
} else {
name = android::base::StringPrintf("Untracked pid %d", pid);
if (!svc) {
return true;
if (!(svc->flags & SVC_ONESHOT) || (svc->flags & SVC_RESTART)) {
NOTICE("Service '%s' (pid %d) killing any children in process group\n", svc->name, pid);
kill(-pid, SIGKILL);
for (socketinfo* si = svc->sockets; si; si = si->next) {
char tmp[128];
snprintf(tmp, sizeof(tmp), ANDROID_SOCKET_DIR"/%s", si->name);
unlink(tmp);
if (svc->flags & SVC_EXEC) {
INFO("SVC_EXEC pid %d finished...\n", svc->pid);
waiting_for_exec = false;
list_remove(&svc->slist);
free(svc->name);
free(svc);
return true;
svc->pid = 0;
svc->flags &= (~SVC_RUNNING);
if ((svc->flags & SVC_ONESHOT) && !(svc->flags & SVC_RESTART)) {
svc->flags |= SVC_DISABLED;
if (svc->flags & (SVC_DISABLED | SVC_RESET)) {
svc->NotifyStateChange("stopped");
return true;
time_t now = gettime();
if ((svc->flags & SVC_CRITICAL) && !(svc->flags & SVC_RESTART)) {
if (svc->time_crashed + CRITICAL_CRASH_WINDOW >= now) {
if (++svc->nr_crashed > CRITICAL_CRASH_THRESHOLD) {
android_reboot(ANDROID_RB_RESTART2, 0, "recovery");
return true;
} else {
svc->time_crashed = now;
svc->nr_crashed = 1;
svc->flags &= (~SVC_RESTART);
svc->flags |= SVC_RESTARTING;
struct listnode* node;
list_for_each(node, &svc->onrestart.commands) {
command* cmd = node_to_item(node, struct command, clist);
cmd->func(cmd->nargs, cmd->args);
svc->NotifyStateChange("restarting");
return true;
3.3register_epoll_handler(signal_read_fd, handle_signal);
void register_epoll_handler(int fd, void (*fn)()) {
epoll_event ev;
ev.events = EPOLLIN;
ev.data.ptr = reinterpret_cast<void*>(fn);
if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, fd, &ev) == -1) {
ERROR("epoll_ctl failed: %s\n", strerror(errno));
3.4 handle_signal和SIGCHLD_handler
static void handle_signal() {
char buf[32];
read(signal_read_fd, buf, sizeof(buf));
reap_any_outstanding_children();
static void SIGCHLD_handler(int) {
if (TEMP_FAILURE_RETRY(write(signal_write_fd, "1", 1)) == -1) {
ERROR("write(signal_write_fd) failed: %s\n", strerror(errno));
4.start_property_service
start_property_service();
void start_property_service() {
property_set_fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
0666, 0, 0, NULL);
if (property_set_fd == -1) {
ERROR("start_property_service socket creation failed: %s\n", strerror(errno));
exit(1);
listen(property_set_fd, 8);
register_epoll_handler(property_set_fd, handle_property_set_fd);
4.1register_epoll_handler
void register_epoll_handler(int fd, void (*fn)()) {
epoll_event ev;
ev.events = EPOLLIN;
ev.data.ptr = reinterpret_cast<void*>(fn);
if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, fd, &ev) == -1) {
ERROR("epoll_ctl failed: %s\n", strerror(errno));
4.2 handle_property_set_fd
static void handle_property_set_fd()
prop_msg msg;
int s;
int r;
struct ucred cr;
struct sockaddr_un addr;
socklen_t addr_size = sizeof(addr);
socklen_t cr_size = sizeof(cr);
char * source_ctx = NULL;
struct pollfd ufds[1];
const int timeout_ms = 2 * 1000;
int nr;
if ((s = accept(property_set_fd, (struct sockaddr *) &addr, &addr_size)) < 0) {
return;
if (getsockopt(s, SOL_SOCKET, SO_PEERCRED, &cr, &cr_size) < 0) {
close(s);
ERROR("Unable to receive socket options\n");
return;
ufds[0].fd = s;
ufds[0].events = POLLIN;
ufds[0].revents = 0;
nr = TEMP_FAILURE_RETRY(poll(ufds, 1, timeout_ms));
if (nr == 0) {
ERROR("sys_prop: timeout waiting for uid=%d to send property message.\n", cr.uid);
close(s);
return;
} else if (nr < 0) {
ERROR("sys_prop: error waiting for uid=%d to send property message: %s\n", cr.uid, strerror(errno));
close(s);
return;
r = TEMP_FAILURE_RETRY(recv(s, &msg, sizeof(msg), MSG_DONTWAIT));
if(r != sizeof(prop_msg)) {
ERROR("sys_prop: mis-match msg size received: %d expected: %zu: %s\n",
r, sizeof(prop_msg), strerror(errno));
close(s);
return;
switch(msg.cmd) {
case PROP_MSG_SETPROP:
msg.name[PROP_NAME_MAX-1] = 0;
msg.value[PROP_VALUE_MAX-1] = 0;
if (!is_legal_property_name(msg.name, strlen(msg.name))) {
ERROR("sys_prop: illegal property name. Got: "%s"\n", msg.name);
close(s);
return;
getpeercon(s, &source_ctx);
if(memcmp(msg.name,"ctl.",4) == 0) {
close(s);
if (check_control_mac_perms(msg.value, source_ctx)) {
handle_control_message((char*) msg.name + 4, (char*) msg.value);
} else {
ERROR("sys_prop: Unable to %s service ctl [%s] uid:%d gid:%d pid:%d\n",
msg.name + 4, msg.value, cr.uid, cr.gid, cr.pid);
} else {
if (check_perms(msg.name, source_ctx)) {
property_set((char*) msg.name, (char*) msg.value);
} else {
ERROR("sys_prop: permission denied uid:%d name:%s\n",
cr.uid, msg.name);
close(s);
freecon(source_ctx);
break;
default:
close(s);
break;
4.3handle_control_message
void handle_control_message(const char *msg, const char *arg)
if (!strcmp(msg,"start")) {
msg_start(arg);
} else if (!strcmp(msg,"stop")) {
msg_stop(arg);
} else if (!strcmp(msg,"restart")) {
msg_restart(arg);
} else {
ERROR("unknown control msg '%s'\n", msg);
\
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