其实init进程启动后，ServiceManager进程的启动，远比zygote要早，因为在启动zygote进程时需要用到ServiceManager进程的服务。ServiceManager是一个守护进程，它维护着系统服务和客户端的binder通信。
在Android系统中用到最多的通信机制就是Binder，Binder主要由Client、Server、ServiceManager和Binder驱动程序组成。其中Client、Service和ServiceManager运行在用户空间，而Binder驱动程序运行在内核空间。核心组件就是Binder驱动程序了，而ServiceManager提供辅助管理的功能，无论是Client还是Service进行通信前首先要和ServiceManager取得联系。而ServiceManager是一个守护进程，负责管理Server并向Client提供查询Server的功能。
service servicemanager /system/bin/servicemanager class core user system group system critical onrestart restart zygote onrestart restart media onrestart restart surfaceflinger onrestart restart drm
int main(int argc, char **argv)
{
struct binder_state *bs;
//宏：#define BINDER_SERVICE_MANAGER ((void*)0)。表示ServiceManager对应的句柄为0，表面自己是服务器管理者。其他的Server进程句柄值都是大于0的。
void *svcmgr = BINDER_SERVICE_MANAGER;
//打开Binder设备，映射128K的内存地址空间
bs = binder_open(128*1024);
//告诉Binder驱动程序自己是Binder上下文管理者
if (binder_become_context_manager(bs)) {
ALOGE("cannot become context manager (%s)\n", strerror(errno));
return -1;
}
//ServiceManager对应的句柄赋值
svcmgr_handle = svcmgr;
//进入一个无线循环，充当server角色，等待Client的请求
binder_loop(bs, svcmgr_handler);
return 0;
}
struct binder_state
{
int fd; // 文件描述符，打开的/dev/binder设备
void* mapped; // 把设备文件/dev/binder映射到进程空间的起始地址
unsigned mapsize; // 映射内存空间的大小
};这个结构体也是在Binder.c中定义的。binder_open(unsigned mapsize)函数代码如下：struct binder_state *binder_open(unsigned mapsize)
{
struct binder_state *bs;
bs = malloc(sizeof(*bs));
if (!bs) {
errno = ENOMEM;
return 0;
}
//打开/dev/binder驱动文件
bs->fd = open("/dev/binder", O_RDWR);
if (bs->fd < 0) {
fprintf(stderr,"binder: cannot open device (%s)\n",
strerror(errno));
goto fail_open;
}
//设置要映射的空间大小128*1024
bs->mapsize = mapsize;
//开始映射
bs->mapped = mmap(NULL, mapsize, PROT_READ, MAP_PRIVATE, bs->fd, 0);
if (bs->mapped == MAP_FAILED) {
fprintf(stderr,"binder: cannot map device (%s)\n",
strerror(errno));
goto fail_map;
}
/* TODO: check version */
return bs;
fail_map:
close(bs->fd);
fail_open:
free(bs);
return 0;
}
参数mapsize表示它希望把binder驱动文件的多少字节映射到本地空间。可以看到，Service Manager Service和普通进程所映射的binder大小并不相同。它把binder驱动文件的128K字节映射到内存空间，而普通进程则会映射binder文件里的BINDER_VM_SIZE（即1M减去8K）字节。具体的映射动作由mmap()一句完成，该函数将binder驱动文件的一部分映射到进程空间。mmap()的函数原型如下：
void* mmap ( void * addr , size_t len , int prot , int flags , int fd , off_t offset );
参数addr用于指出文件应被映射到进程空间的起始地址，一般指定为空指针，此时会由内核来决定起始地址。
参数len为映射的字节长度。参数offset是偏移量的起始位置。
此函数作用是让当前进程成为整个系统中唯一的上下文管理器，即 service管理器。其代码非常简单，仅仅是把BINDER_SET_CONTEXT_MGR发送到binder驱动而已。源码如下：
int binder_become_context_manager(struct binder_state *bs)
{
return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);
}
binder_loop(struct binder_state *bs, binder_handler func)函数源码位置在frameworks/base/cmds/servicemanager/Binder.c。此时已经正式进入循环，转正为一个server。注意这个函数的参数：bs是文件/dev/binder的描述符，func是函数svcmgr_handler。
void binder_loop(struct binder_state *bs, binder_handler func)
{
int res;
struct binder_write_read bwr;
unsigned readbuf[32];
bwr.write_size = 0;
bwr.write_consumed = 0;
bwr.write_buffer = 0;
readbuf[0] = BC_ENTER_LOOPER;
binder_write(bs, readbuf, sizeof(unsigned));
for (;;) {
bwr.read_size = sizeof(readbuf);
bwr.read_consumed = 0;
bwr.read_buffer = (unsigned) readbuf;
//通过设备描述符，将数据发给binder驱动
res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
if (res < 0) {
ALOGE("binder_loop: ioctl failed (%s)\n", strerror(errno));
break;
}
//解析驱动的数据，调用回调函数func
res = binder_parse(bs, 0, readbuf, bwr.read_consumed, func);
if (res == 0) {
ALOGE("binder_loop: unexpected reply?!\n");
break;
}
if (res < 0) {
ALOGE("binder_loop: io error %d %s\n", res, strerror(errno));
break;
}
}
}
这个函数源码位置在frameworks/base/cmds/servicemanager/Binder.c
这个函数主要是对binder文件解析并执行相应的指令。这个函数的大头其实就是那个一路传过来的svcmgr_handler函数，就是那个参数func。
int binder_parse(struct binder_state *bs, struct binder_io *bio,
uint32_t *ptr, uint32_t size, binder_handler func)
{
int r = 1;
uint32_t *end = ptr + (size / 4);
while (ptr < end) {
uint32_t cmd = *ptr++;
...
//注意这个BR_TRANSACTION
case BR_TRANSACTION: {
struct binder_txn *txn = (void *) ptr;
if ((end - ptr) * sizeof(uint32_t) < sizeof(struct binder_txn)) {
ALOGE("parse: txn too small!\n");
return -1;
}
binder_dump_txn(txn);
if (func) {
unsigned rdata[256/4];
struct binder_io msg;
struct binder_io reply;
int res;
bio_init(&reply, rdata, sizeof(rdata), 4);
bio_init_from_txn(&msg, txn);
res = func(bs, txn, &msg, &reply);
binder_send_reply(bs, &reply, txn->data, res);
}
ptr += sizeof(*txn) / sizeof(uint32_t);
break;
}
...
}
return r;
}
这个函数源码位置在frameworks/base/cmds/servicemanager/service_manager.c
int svcmgr_handler(struct binder_state *bs,
struct binder_txn *txn,
struct binder_io *msg,
struct binder_io *reply)
{
...
switch(txn->code) {
//客户端获取服务的请求
case SVC_MGR_GET_SERVICE:
case SVC_MGR_CHECK_SERVICE:
s = bio_get_string16(msg, &len);
ptr = do_find_service(bs, s, len, txn->sender_euid);
if (!ptr)
break;
bio_put_ref(reply, ptr);
return 0;
//服务端请求添加服务
case SVC_MGR_ADD_SERVICE:
s = bio_get_string16(msg, &len);
ptr = bio_get_ref(msg);
allow_isolated = bio_get_uint32(msg) ? 1 : 0;
if (do_add_service(bs, s, len, ptr, txn->sender_euid, allow_isolated))
return -1;
break;
...
}
bio_put_uint32(reply, 0);
return 0;
}
service_manager.c中声明了一个链表svclist
struct svcinfo *svclist = 0;
svclist记录着所有添加进系统的“service代理”信息，这些信息被组织成一条单向链表。svclist链表节点类型为svcinfo，如下：
struct svcinfo
{
struct svcinfo *next;
void *ptr;//记录的就是系统service对应的 binder句柄值
struct binder_death death;
int allow_isolated;
unsigned len;
uint16_t name[0];//系统服务的名称
};
当应用调用getService()获取系统服务的代理接口时，servicemanager就会搜索这张svclist链表，下面就要介绍。
这个函数源码位置在frameworks/base/cmds/servicemanager/service_manager.c
int do_add_service(struct binder_state *bs,
uint16_t *s, unsigned len,
void *ptr, unsigned uid, int allow_isolated)
{
struct svcinfo *si;
//ALOGI("add_service('%s',%p,%s) uid=%d\n", str8(s), ptr,
// allow_isolated ? "allow_isolated" : "!allow_isolated", uid);
if (!ptr || (len == 0) || (len > 127))
return -1;
//查看该服务是否有注册权限
if (!svc_can_register(uid, s)) {
ALOGE("add_service('%s',%p) uid=%d - PERMISSION DENIED\n",
str8(s), ptr, uid);
return -1;
}
//在服务列表中查找服务
si = find_svc(s, len);
//查看该服务是否已经注册
if (si) {
if (si->ptr) {
ALOGE("add_service('%s',%p) uid=%d - ALREADY REGISTERED, OVERRIDE\n",
str8(s), ptr, uid);
svcinfo_death(bs, si);
}
si->ptr = ptr;
} else {
//如果没有注册，则创建一个，并添加到svclist服务列表的表首。
si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));
if (!si) {
ALOGE("add_service('%s',%p) uid=%d - OUT OF MEMORY\n",
str8(s), ptr, uid);
return -1;
}
si->ptr = ptr;
si->len = len;
memcpy(si->name, s, (len + 1) * sizeof(uint16_t));
si->name[len] = '\0';
si->death.func = svcinfo_death;
si->death.ptr = si;
si->allow_isolated = allow_isolated;
si->next = svclist;
svclist = si;
}
//通知binder设备有一个service注册进来。
binder_acquire(bs, ptr);
binder_link_to_death(bs, ptr, &si->death);
return 0;
}
这个函数源码位置在frameworks/base/cmds/servicemanager/service_manager.c
void *do_find_service(struct binder_state *bs, uint16_t *s, unsigned len, unsigned uid)
{
struct svcinfo *si;
//上一个函数也调用了这个函数
si = find_svc(s, len);
// ALOGI("check_service('%s') ptr = %p\n", str8(s), si ? si->ptr : 0);
if (si && si->ptr) {
if (!si->allow_isolated) {
// If this service doesn't allow access from isolated processes,
// then check the uid to see if it is isolated.
unsigned appid = uid % AID_USER;
if (appid >= AID_ISOLATED_START && appid <= AID_ISOLATED_END) {
return 0;
}
}
return si->ptr;
} else {
return 0;
}
这个函数源码位置在frameworks/base/cmds/servicemanager/service_manager.c
struct svcinfo *find_svc(uint16_t *s16, unsigned len)
{
struct svcinfo *si;
//遍历svclist，查找服务
for (si = svclist; si; si = si->next) {
if ((len == si->len) &&
!memcmp(s16, si->name, len * sizeof(uint16_t))) {
return si;
}
}
return 0;
}
守护进程servicemanager循环从binder设备文件读取数据，然后解析并响应请求，包括服务端的添加服务请求和客户端的查询，获取服务的请求。
android启动之ServiceManager启动,布布扣,bubuko.com
原文地址：http://blog.csdn.net/w2865673691/article/details/26724701
