Bind机制由4个部分组成。bind驱动，Client，ServiceManager &Service
1.Bind其实是一个基于linux系统的驱动，目的是为了实现内存共享。
bind驱动的东西，由于偏向内核，并且bind机制的内容非常庞大，所以我们暂时略去这个部分。
2.ServiceManager
Service Manager顾名思义，是一个“管家”。更确切的说，是所有系统service 的manager。
首先从service_manager.c开始\frameworks\native\cmds\servicemanager\service_manager.c
static struct {
unsigned uid;
const char *name;
} allowed[] = {
{ AID_MEDIA, "media.audio_flinger" },
{ AID_MEDIA, "media.log" },
{ AID_MEDIA, "media.player" },
{ AID_MEDIA, "media.camera" },
{ AID_MEDIA, "media.audio_policy" },
{ AID_DRM, "drm.drmManager" },
{ AID_NFC, "nfc" },
{ AID_BLUETOOTH, "bluetooth" },
{ AID_RADIO, "radio.phone" },
{ AID_RADIO, "radio.sms" },
{ AID_RADIO, "radio.phonesubinfo" },
{ AID_RADIO, "radio.simphonebook" },
/* TODO: remove after phone services are updated: */
{ AID_RADIO, "phone" },
{ AID_RADIO, "sip" },
{ AID_RADIO, "isms" },
{ AID_RADIO, "iphonesubinfo" },
{ AID_RADIO, "simphonebook" },
{ AID_MEDIA, "common_time.clock" },
{ AID_MEDIA, "common_time.config" },
{ AID_KEYSTORE, "android.security.keystore" },
};以上就是系统服务的一个部分。这些都是注册在servicemanager来管理。
那service manager干那些事：
I.提供IBind对象，也就是各个service的引用，供每个进程使用，且对于每个进程来说，该Ibind对象是唯一的。
II.让各个系统service注册到servicemanager中。
这里binder驱动，不是我们通常操作系统结构里的驱动概念，可以理解为是client和ServiceManager交流的媒介。
binder驱动的本质是内存共享。
其实这是整个bind机制的前面部分，就是从client到servicemanager，这样client可以拿到Ibind对象，进而可以直接“操作servie”。
举个例子：
AlarmManager alarmManager = context.getSystemService(Context.ALARM_SERVICE);
alarmManager.setExact(AlarmManager.ELAPSED_REALTIME, elapsedRealtime,
pendingIntent);拿到alaram service bind对象，进而操作service提供的“服务”。
而且这个操作是同步的！
就好象在操作同一个进程的东西。
下面我们看看service Manager究竟是如何做到上面说的几点的。
2.1 Service Manager的启动：
既然SM是管理员，那么它应该是最勤快的，也就是必须最“早”启动。
是的，它的启动是定义在init.rc里面的：\system\core\rootdir\init.rc
# adbd on at boot in emulator
on property:ro.kernel.qemu=1
start adbd
service servicemanager /system/bin/servicemanager
class core
user system
group system
critical
onrestart restart healthd
onrestart restart zygote
onrestart restart media
onrestart restart surfaceflinger
onrestart restart drmService Manager启动后，在干什么？
还是在service_manager.c中：
int main(int argc, char **argv)
{
struct binder_state *bs;
void *svcmgr = BINDER_SERVICE_MANAGER;
bs = binder_open(128*1024);
if (binder_become_context_manager(bs)) {
ALOGE("cannot become context manager (%s)\n", strerror(errno));
return -1;
}
svcmgr_handle = svcmgr;
binder_loop(bs, svcmgr_handler);
return 0;
}binder_open打开bind驱动，并且分配128K大小。
binder_become_context_manager(bs)：
int binder_become_context_manager(struct binder_state *bs)
{
return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);
}把自己注册为Service 大管家。
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;
res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
if (res < 0) {
ALOGE("binder_loop: ioctl failed (%s)\n", strerror(errno));
break;
}
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;
}
}
}开始进入loop，和之前分析的andorid线程消息驱动机制非常相似。
读取消息队列，解析它们，知道出现异常。
接下来，看看bind_parse:
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++;
#if TRACE
fprintf(stderr,"%s:\n", cmd_name(cmd));
#endif
switch(cmd) {
case BR_NOOP:
break;
case BR_TRANSACTION_COMPLETE:
break;
case BR_INCREFS:
case BR_ACQUIRE:
case BR_RELEASE:
case BR_DECREFS:
#if TRACE
fprintf(stderr," %08x %08x\n", ptr[0], ptr[1]);
#endif
ptr += 2;
break;
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;
}
case BR_REPLY: {
struct binder_txn *txn = (void*) ptr;
if ((end - ptr) * sizeof(uint32_t) < sizeof(struct binder_txn)) {
ALOGE("parse: reply too small!\n");
return -1;
}
binder_dump_txn(txn);
if (bio) {
bio_init_from_txn(bio, txn);
bio = 0;
} else {
/* todo FREE BUFFER */
}
ptr += (sizeof(*txn) / sizeof(uint32_t));
r = 0;
break;
}
case BR_DEAD_BINDER: {
struct binder_death *death = (void*) *ptr++;
death->func(bs, death->ptr);
break;
}
case BR_FAILED_REPLY:
r = -1;
break;
case BR_DEAD_REPLY:
r = -1;
break;
default:
ALOGE("parse: OOPS %d\n", cmd);
return -1;
}
}
return r;
}关键是分析：BR_TRANSACTION，BR_REPLY。
BR_TRANSACTION中做了一些初始化，然后
res = func(bs, txn, &msg, &reply);
binder_send_reply(bs, &reply, txn->data, res);func函数就是在service_manager.c中传入的
int svcmgr_handler(struct binder_state *bs,
struct binder_txn *txn,
struct binder_io *msg,
struct binder_io *reply)所以bind_loop最终实现分析的函数是传入的函数！
至此整个service_manager的流程已经清楚。
事件驱动机制：
1.从bind驱动读取消息
2.处理消息
3.进入looper，永远不会主动退出，直到出现致命错误。
int svcmgr_handler(struct binder_state *bs,
struct binder_txn *txn,
struct binder_io *msg,
struct binder_io *reply)
{
struct svcinfo *si;
uint16_t *s;
unsigned len;
void *ptr;
uint32_t strict_policy;
int allow_isolated;
// ALOGI("target=%p code=%d pid=%d uid=%d\n",
// txn->target, txn->code, txn->sender_pid, txn->sender_euid);
if (txn->target != svcmgr_handle)
return -1;
// Equivalent to Parcel::enforceInterface(), reading the RPC
// header with the strict mode policy mask and the interface name.
// Note that we ignore the strict_policy and don't propagate it
// further (since we do no outbound RPCs anyway).
strict_policy = bio_get_uint32(msg);
s = bio_get_string16(msg, &len);
if ((len != (sizeof(svcmgr_id) / 2)) ||
memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {
fprintf(stderr,"invalid id %s\n", str8(s));
return -1;
}
switch(txn->code) {
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;
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;
case SVC_MGR_LIST_SERVICES: {
unsigned n = bio_get_uint32(msg);
si = svclist;
while ((n-- > 0) && si)
si = si->next;
if (si) {
bio_put_string16(reply, si->name);
return 0;
}
return -1;
}
default:
ALOGE("unknown code %d\n", txn->code);
return -1;
}
bio_put_uint32(reply, 0);
return 0;
}svcmgr_handler
switch语句，查询和获取service 或者注册。
查找svclist里面是否有相同name的服务。
svclist是链表的方式，与线程的消息队列一样！
struct svcinfo *find_svc(uint16_t *s16, unsigned len)
{
struct svcinfo *si;
for (si = svclist; si; si = si->next) {
if ((len == si->len) &&
!memcmp(s16, si->name, len * sizeof(uint16_t))) {
return si;
}
}
return 0;
}接下来我们看看void *do_find_service(struct binder_state *bs, uint16_t *s, unsigned len, unsigned uid)
return的到底是什么？
注册服务：SVC_MGR_ADD_SERVICE：
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;
if (!svc_can_register(uid, s)) {
ALOGE("add_service('%s',%p) uid=%d - PERMISSION DENIED\n",
str8(s), ptr, uid);
return -1;
}
si = find_svc(s, len);
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 {
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;
}
binder_acquire(bs, ptr);
binder_link_to_death(bs, ptr, &si->death);
return 0;
}do_add_service
int svc_can_register(unsigned uid, uint16_t *name)
判断是否在allowed表格里面。
先看看是否在列表里面？
si = find_svc(s, len);
如果不再的话，就注册一个新的si，到svclist。
至此service_manager就启动起来了。
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