友情提示：师傅说先了解概念，贴的代码都没做分析，只是学习笔记，想看源码解析书上有详细解释。如果以后工作需要，我也会深入学习的。
init是Linux系统中用户空间的第一个进程，而Android是基于Linux内核的，所以init也是Android系统中用户空间的第一个进程，它的进程号是1。
init有两个最重要的职责。
- init进程负责创建系统中的几个关键进程。
- Android系统有很多属性，于是init提供了一个property service（属性服务）来管理它们。
system/core/init/init.c，init进程的入口函数是main
int main(int argc, char **argv)
{
int device_fd = -1;
int property_set_fd = -1;
int signal_recv_fd = -1;
int keychord_fd = -1;
int fd_count;
int s[2];
int fd;
struct sigaction act;
char tmp[PROP_VALUE_MAX];
struct pollfd ufds[4];
char *tmpdev;
char* debuggable;
//设置子进程退出的信号处理函数
act.sa_handler = sigchld_handler;
act.sa_flags = SA_NOCLDSTOP;
act.sa_mask = 0;
act.sa_restorer = NULL;
sigaction(SIGCHLD, &act, 0);
/* clear the umask */
umask(0);
//创建文件夹，挂载设备
mkdir("/dev", 0755);
mkdir("/proc", 0755);
mkdir("/sys", 0755);
mount("tmpfs", "/dev", "tmpfs", 0, "mode=0755");
mkdir("/dev/pts", 0755);
mkdir("/dev/socket", 0755);
mount("devpts", "/dev/pts", "devpts", 0, NULL);
mount("proc", "/proc", "proc", 0, NULL);
mount("sysfs", "/sys", "sysfs", 0, NULL);
/* We must have some place other than / to create the
* device nodes for kmsg and null, otherwise we won't
* be able to remount / read-only later on.
* Now that tmpfs is mounted on /dev, we can actually
* talk to the outside world.
*/
//重定向标准输入/输出/dev/_null_
open_devnull_stdio();
log_init();
INFO("reading config file\n");
parse_config_file("/init.rc");
/* pull the kernel commandline and ramdisk properties file in */
qemu_init();
import_kernel_cmdline(0);
get_hardware_name();
snprintf(tmp, sizeof(tmp), "/init.%s.rc", hardware);
parse_config_file(tmp);
action_for_each_trigger("early-init", action_add_queue_tail);
drain_action_queue();
INFO("device init\n");
device_fd = device_init();
property_init();
// only listen for keychords if ro.debuggable is true
keychord_fd = open_keychord();
if (console[0]) {
snprintf(tmp, sizeof(tmp), "/dev/%s", console);
console_name = strdup(tmp);
}
fd = open(console_name, O_RDWR);
if (fd >= 0)
have_console = 1;
close(fd);
if( load_565rle_image(INIT_IMAGE_FILE) ) {
fd = open("/dev/tty0", O_WRONLY);
if (fd >= 0) {
const char *msg;
msg = "\n"
"\n"
"\n"
"\n"
"\n"
"\n"
"\n" // console is 40 cols x 30 lines
"\n"
"\n"
"\n"
"\n"
"\n"
"\n"
"\n"
" A N D R O I D ";
write(fd, msg, strlen(msg));
close(fd);
}
}
if (qemu[0])
import_kernel_cmdline(1);
if (!strcmp(bootmode,"factory"))
property_set("ro.factorytest", "1");
else if (!strcmp(bootmode,"factory2"))
property_set("ro.factorytest", "2");
else
property_set("ro.factorytest", "0");
property_set("ro.serialno", serialno[0] ? serialno : "");
property_set("ro.bootmode", bootmode[0] ? bootmode : "unknown");
property_set("ro.baseband", baseband[0] ? baseband : "unknown");
property_set("ro.carrier", carrier[0] ? carrier : "unknown");
property_set("ro.bootloader", bootloader[0] ? bootloader : "unknown");
property_set("ro.hardware", hardware);
snprintf(tmp, PROP_VALUE_MAX, "%d", revision);
property_set("ro.revision", tmp);
/* execute all the boot actions to get us started */
action_for_each_trigger("init", action_add_queue_tail);
drain_action_queue();
/* read any property files on system or data and
* fire up the property service. This must happen
* after the ro.foo properties are set above so
* that /data/local.prop cannot interfere with them.
*/
property_set_fd = start_property_service();
/* create a signalling mechanism for the sigchld handler */
if (socketpair(AF_UNIX, SOCK_STREAM, 0, s) == 0) {
signal_fd = s[0];
signal_recv_fd = s[1];
fcntl(s[0], F_SETFD, FD_CLOEXEC);
fcntl(s[0], F_SETFL, O_NONBLOCK);
fcntl(s[1], F_SETFD, FD_CLOEXEC);
fcntl(s[1], F_SETFL, O_NONBLOCK);
}
/* make sure we actually have all the pieces we need */
if ((device_fd < 0) ||
(property_set_fd < 0) ||
(signal_recv_fd < 0)) {
ERROR("init startup failure\n");
return 1;
}
/* execute all the boot actions to get us started */
action_for_each_trigger("early-boot", action_add_queue_tail);
action_for_each_trigger("boot", action_add_queue_tail);
drain_action_queue();
/* run all property triggers based on current state of the properties */
queue_all_property_triggers();
drain_action_queue();
/* enable property triggers */
property_triggers_enabled = 1;
ufds[0].fd = device_fd;
ufds[0].events = POLLIN;
ufds[1].fd = property_set_fd;
ufds[1].events = POLLIN;
ufds[2].fd = signal_recv_fd;
ufds[2].events = POLLIN;
fd_count = 3;
if (keychord_fd > 0) {
ufds[3].fd = keychord_fd;
ufds[3].events = POLLIN;
fd_count++;
} else {
ufds[3].events = 0;
ufds[3].revents = 0;
}
#if BOOTCHART
bootchart_count = bootchart_init();
if (bootchart_count < 0) {
ERROR("bootcharting init failure\n");
} else if (bootchart_count > 0) {
NOTICE("bootcharting started (period=%d ms)\n", bootchart_count*BOOTCHART_POLLING_MS);
} else {
NOTICE("bootcharting ignored\n");
}
#endif
for(;;) {
int nr, i, timeout = -1;
for (i = 0; i < fd_count; i++)
ufds[i].revents = 0;
drain_action_queue();
restart_processes();
if (process_needs_restart) {
timeout = (process_needs_restart - gettime()) * 1000;
if (timeout < 0)
timeout = 0;
}
#if BOOTCHART
if (bootchart_count > 0) {
if (timeout < 0 || timeout > BOOTCHART_POLLING_MS)
timeout = BOOTCHART_POLLING_MS;
if (bootchart_step() < 0 || --bootchart_count == 0) {
bootchart_finish();
bootchart_count = 0;
}
}
#endif
nr = poll(ufds, fd_count, timeout);
if (nr <= 0)
continue;
if (ufds[2].revents == POLLIN) {
/* we got a SIGCHLD - reap and restart as needed */
read(signal_recv_fd, tmp, sizeof(tmp));
while (!wait_for_one_process(0))
;
continue;
}
if (ufds[0].revents == POLLIN)
handle_device_fd(device_fd);
if (ufds[1].revents == POLLIN)
handle_property_set_fd(property_set_fd);
if (ufds[3].revents == POLLIN)
handle_keychord(keychord_fd);
}
return 0;
}
init的工作流程可精简为四个部分：
- 解析两个配置文件（init.rc，与硬件平台相关的配置文件）
- 执行各个阶段的动作
- 调用property_init初始化属性相关的资源，并通过property_start_service启动属性服务
- init进入一个无限循环，并且等待一些事情的发生。
init.rc解析
system/core/init/parser.c
int parse_config_file(const char *fn)
{
char *data;
data = read_file(fn, 0); //读取配置文件内容，这个文件时init.rc
if (!data) return -1;
parse_config(fn, data); //解析在parse_config函数完成
DUMP();
return 0;
}
static void parse_config(const char *fn, char *s)
{
struct parse_state state;
char *args[SVC_MAXARGS];
int nargs;
nargs = 0;
state.filename = fn;
state.line = 1;
state.ptr = s;
state.nexttoken = 0;
state.parse_line = parse_line_no_op; //设置解析函数，不同的内容用不同的解析函数
for (;;) {
switch (next_token(&state)) {
case T_EOF:
state.parse_line(&state, 0, 0);
return;
case T_NEWLINE:
if (nargs) {
int kw = lookup_keyword(args[0]); //得到关键字类型
if (kw_is(kw, SECTION)) { //判断关键字类型是不是SECTION
state.parse_line(&state, 0, 0);
parse_new_section(&state, kw, nargs, args); //解析这个SECTION
} else {
state.parse_line(&state, nargs, args);
}
nargs = 0;
}
break;
case T_TEXT:
if (nargs < SVC_MAXARGS) {
args[nargs++] = state.text;
}
break;
}
}
}
parse_config首先会配置文件中的section，然后针对不同的section使用不同的解析函数来解析。
属性服务
Android平台的属性服务（property service）类似Windows平台的注册表。系统重启或应用程序重启，还能根据之前设置的属性，进行初始化。与属性服务有关的代码有两行：
- property_init()；
- property_set_fd = start_property_service();
属性服务初始化
创建存储空间
system/core/init/property_init.c
static int init_property_area(void)
{
prop_area *pa;
if(pa_info_array)
return -1;
if(init_workspace(&pa_workspace, PA_SIZE))
return -1;
fcntl(pa_workspace.fd, F_SETFD, FD_CLOEXEC);
pa_info_array = (void*) (((char*) pa_workspace.data) + PA_INFO_START);
pa = pa_workspace.data;
memset(pa, 0, PA_SIZE);
pa->magic = PROP_AREA_MAGIC;
pa->version = PROP_AREA_VERSION;
/* plug into the lib property services */
__system_property_area__ = pa;
return 0;
}
虽然属性区域是由init进程创建的，但Android系统希望其他进程也能读取这块内存里的东西。这需要两个工作：
- 把属性区域创建在共享内存上，而共享内存是可以跨进程的。
- 利用gcc的constructor属性，这个属性指明了一个函数（__libc_prenit），这个函数内部将完成共享内存到本地进程的映射工作
客户端进程获取存储空间
bionic/libc/bionic/system_properties.c
int __system_properties_init(void)
{
prop_area *pa;
int s, fd;
unsigned sz;
char *env;
if(__system_property_area__ != ((void*) &dummy_props)) {
return 0;
}
env = getenv("ANDROID_PROPERTY_WORKSPACE");
if (!env) {
return -1;
}
fd = atoi(env);
env = strchr(env, ',');
if (!env) {
return -1;
}
sz = atoi(env + 1);
pa = mmap(0, sz, PROT_READ, MAP_SHARED, fd, 0);
if(pa == MAP_FAILED) {
return -1;
}
if((pa->magic != PROP_AREA_MAGIC) || (pa->version != PROP_AREA_VERSION)) {
munmap(pa, sz);
return -1;
}
__system_property_area__ = pa;
return 0;
}
客户端进程可以直接读取属性空间，但没有权限设置属性。
属性服务器
启动属性服务器
init进程会启动一个属性服务器，而客户端只能通过与属性服务器交互来设置属性。
system/core/init/property_service.c
int start_property_service(void)
{
int fd;
load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE);
/* Read persistent properties after all default values have been loaded. */
load_persistent_properties();
fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM, 0666, 0, 0);
if(fd < 0) return -1;
fcntl(fd, F_SETFD, FD_CLOEXEC);
fcntl(fd, F_SETFL, O_NONBLOCK);
listen(fd, 8);
return fd;
}
属性服务创建了一个用来接收请求的socket。
处理设置属性请求
验收请求的地方在init进程中，当属性服务器收到客户端请求时，init会调用handle_property_set_fd进行处理。
system/core/init/property_service.c
void handle_property_set_fd(int fd)
{
prop_msg msg;
int s;
int r;
int res;
struct ucred cr;
struct sockaddr_un addr;
socklen_t addr_size = sizeof(addr);
socklen_t cr_size = sizeof(cr);
if ((s = accept(fd, (struct sockaddr *) &addr, &addr_size)) < 0) {
return;
}
/* Check socket options here */
if (getsockopt(s, SOL_SOCKET, SO_PEERCRED, &cr, &cr_size) < 0) {
close(s);
ERROR("Unable to recieve socket options\n");
return;
}
r = recv(s, &msg, sizeof(msg), 0);
close(s);
if(r != sizeof(prop_msg)) {
ERROR("sys_prop: mis-match msg size recieved: %d expected: %d\n",
r, sizeof(prop_msg));
return;
}
switch(msg.cmd) {
case PROP_MSG_SETPROP:
msg.name[PROP_NAME_MAX-1] = 0;
msg.value[PROP_VALUE_MAX-1] = 0;
if(memcmp(msg.name,"ctl.",4) == 0) {
if (check_control_perms(msg.value, cr.uid, cr.gid)) {
handle_control_message((char*) msg.name + 4, (char*) msg.value);
} else {
ERROR("sys_prop: Unable to %s service ctl [%s] uid: %d pid:%d\n",
msg.name + 4, msg.value, cr.uid, cr.pid);
}
} else {
if (check_perms(msg.name, cr.uid, cr.gid)) {
property_set((char*) msg.name, (char*) msg.value);
} else {
ERROR("sys_prop: permission denied uid:%d name:%s\n",
cr.uid, msg.name);
}
}
break;
default:
break;
}
}
当客户端的权限满足要求时，init会调用property_set进行相关处理。
system/core/init/property_service.c
int property_set(const char *name, const char *value)
{
prop_area *pa;
prop_info *pi;
int namelen = strlen(name);
int valuelen = strlen(value);
if(namelen >= PROP_NAME_MAX) return -1;
if(valuelen >= PROP_VALUE_MAX) return -1;
if(namelen < 1) return -1;
pi = (prop_info*) __system_property_find(name);
if(pi != 0) {
/* ro.* properties may NEVER be modified once set */
if(!strncmp(name, "ro.", 3)) return -1;
pa = __system_property_area__;
update_prop_info(pi, value, valuelen);
pa->serial++;
__futex_wake(&pa->serial, INT32_MAX);
} else {
pa = __system_property_area__;
if(pa->count == PA_COUNT_MAX) return -1;
pi = pa_info_array + pa->count;
pi->serial = (valuelen << 24);
memcpy(pi->name, name, namelen + 1);
memcpy(pi->value, value, valuelen + 1);
pa->toc[pa->count] =
(namelen << 24) | (((unsigned) pi) - ((unsigned) pa));
pa->count++;
pa->serial++;
__futex_wake(&pa->serial, INT32_MAX);
}
/* If name starts with "net." treat as a DNS property. */
if (strncmp("net.", name, strlen("net.")) == 0) {
if (strcmp("net.change", name) == 0) {
return 0;
}
/*
* The 'net.change' property is a special property used track when any
* 'net.*' property name is updated. It is _ONLY_ updated here. Its value
* contains the last updated 'net.*' property.
*/
property_set("net.change", name);
} else if (persistent_properties_loaded &&
strncmp("persist.", name, strlen("persist.")) == 0) {
/*
* Don't write properties to disk until after we have read all default properties
* to prevent them from being overwritten by default values.
*/
write_persistent_property(name, value);
}
property_changed(name, value);
return 0;
}
客户端发送请求
客户端通过property_set发送请求，property_set由libcutils库提供
system/core/libcutils/properties.c
int property_set(const char *key, const char *value)
{
prop_msg msg;
unsigned resp;
if(key == 0) return -1;
if(value == 0) value = "";
if(strlen(key) >= PROP_NAME_MAX) return -1;
if(strlen(value) >= PROP_VALUE_MAX) return -1;
msg.cmd = PROP_MSG_SETPROP;
strcpy((char*) msg.name, key);
strcpy((char*) msg.value, value);
return send_prop_msg(&msg);
}
static int send_prop_msg(prop_msg *msg)
{
int s;
int r;
s = socket_local_client(PROP_SERVICE_NAME,
ANDROID_SOCKET_NAMESPACE_RESERVED,
SOCK_STREAM);
if(s < 0) return -1;
while((r = send(s, msg, sizeof(prop_msg), 0)) < 0) {
if((errno == EINTR) || (errno == EAGAIN)) continue;
break;
}
if(r == sizeof(prop_msg)) {
r = 0;
} else {
r = -1;
}
close(s);
return r;
}
今日单词
- parser：解析程序，解析器
- property：性质，性能