在上一篇文章中，我们分析了Android系统进程间通信机制Binder中的Server在启动过程使用Service Manager的addService接口把自己添加到Service Manager守护过程中接受管理。在这一篇文章中，我们将深入到Binder驱动程序源代码去分析Client是如何通过Service Manager的getService接口中来获得Server远程接口的。Client只有获得了Server的远程接口之后，才能进一步调用Server提供的服务。
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这里，我们仍然是通过Android系统中自带的多媒体播放器为例子来说明Client是如何通过IServiceManager::getService接口来获得MediaPlayerService这个Server的远程接口的。假设计读者已经阅读过前面三篇文章浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路、浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路和Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析，即假设Service Manager和MediaPlayerService已经启动完毕，Service Manager现在等待Client的请求。
这里，我们要举例子说明的Client便是MediaPlayer了，它声明和实现在frameworks/base/include/media/mediaplayer.h和frameworks/base/media/libmedia/mediaplayer.cpp文件中。MediaPlayer继承于IMediaDeathNotifier类，这个类声明和实现在frameworks/base/include/media/IMediaDeathNotifier.h和frameworks/base/media/libmedia//IMediaDeathNotifier.cpp文件中，里面有一个静态成员函数getMeidaPlayerService，它通过IServiceManager::getService接口来获得MediaPlayerService的远程接口。
在介绍IMediaDeathNotifier::getMeidaPlayerService函数之前，我们先了解一下这个函数的目标。看来前面浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路这篇文章的读者知道，我们在获取Service Manager远程接口时，最终是获得了一个BpServiceManager对象的IServiceManager接口。类似地，我们要获得MediaPlayerService的远程接口，实际上就是要获得一个称为BpMediaPlayerService对象的IMediaPlayerService接口。现在，我们就先来看一下BpMediaPlayerService的类图：
从这个类图可以看到，BpMediaPlayerService继承于BpInterface<IMediaPlayerService>类，即BpMediaPlayerService继承了IMediaPlayerService类和BpRefBase类，这两个类又分别继续了RefBase类。BpRefBase类有一个成员变量mRemote，它的类型为IBinder，实际是一个BpBinder对象。BpBinder类使用了IPCThreadState类来与Binder驱动程序进行交互，而IPCThreadState类有一个成员变量mProcess，它的类型为ProcessState，IPCThreadState类借助ProcessState类来打开Binder设备文件/dev/binder，因此，它可以和Binder驱动程序进行交互。
BpMediaPlayerService的构造函数有一个参数impl，它的类型为const sp<IBinder>&，从上面的描述中，这个实际上就是一个BpBinder对象。这样，要创建一个BpMediaPlayerService对象，首先就要有一个BpBinder对象。再来看BpBinder类的构造函数，它有一个参数handle，类型为int32_t，这个参数的意义就是请求MediaPlayerService这个远程接口的进程对MediaPlayerService这个Binder实体的引用了。因此，获取MediaPlayerService这个远程接口的本质问题就变为从Service Manager中获得MediaPlayerService的一个句柄了。
现在，我们就来看一下IMediaDeathNotifier::getMeidaPlayerService的实现：
// establish binder interface to MediaPlayerService/*static*/const sp<IMediaPlayerService>&函数首先通过defaultServiceManager函数来获得Service Manager的远程接口，实际上就是获得BpServiceManager的IServiceManager接口，具体可以参考浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路一文。总的来说，这里的语句：
IMediaDeathNotifier::getMediaPlayerService()
{
LOGV("getMediaPlayerService");
Mutex::Autolock _l(sServiceLock);
if (sMediaPlayerService.get() == 0) {
sp<IServiceManager> sm = defaultServiceManager();
sp<IBinder> binder;
do {
binder = sm->getService(String16("media.player"));
if (binder != 0) {
break;
}
LOGW("Media player service not published, waiting...");
usleep(500000); // 0.5 s
} while(true);
if (sDeathNotifier == NULL) {
sDeathNotifier = new DeathNotifier();
}
binder->linkToDeath(sDeathNotifier);
sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
}
LOGE_IF(sMediaPlayerService == 0, "no media player service!?");
return sMediaPlayerService;
}
sp<IServiceManager> sm = defaultServiceManager();相当于是：
sp<IServiceManager> sm = new BpServiceManager(new BpBinder(0));这里的0表示Service Manager的远程接口的句柄值是0。
接下去的while循环是通过sm->getService接口来不断尝试获得名称为“media.player”的Service，即MediaPlayerService。为什么要通过这无穷循环来得MediaPlayerService呢？因为这时候MediaPlayerService可能还没有启动起来，所以这里如果发现取回来的binder接口为NULL，就睡眠0.5秒，然后再尝试获取，这是获取Service接口的标准做法。
我们来看一下BpServiceManager::getService的实现：
class BpServiceManager : public BpInterface<IServiceManager>{ ......virtual sp<IBinder> getService(const String16& name) const{unsigned n;for (n = 0; n < 5; n++){sp<IBinder> svc = checkService(name);if (svc != NULL) return svc;LOGI("Waiting for service %s...\n", String8(name).string());sleep(1);}return NULL;}virtual sp<IBinder> checkService( const String16& name) const{Parcel data, reply;data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());data.writeString16(name);remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);return reply.readStrongBinder();}......}; BpServiceManager::getService通过BpServiceManager::checkService执行操作。
在BpServiceManager::checkService中，首先是通过Parcel::writeInterfaceToken往data写入一个RPC头，这个我们在Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析一文已经介绍过了，就是写往data里面写入了一个整数和一个字符串“android.os.IServiceManager”， Service Manager来处理CHECK_SERVICE_TRANSACTION请求之前，会先验证一下这个RPC头，看看是否正确。接着再往data写入一个字符串name，这里就是“media.player”了。回忆一下Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析这篇文章，那里已经往Service Manager中注册了一个名字为“media.player”的MediaPlayerService。
这里的remote()返回的是一个BpBinder，具体可以参考浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路一文，于是，就进行到BpBinder::transact函数了：
status_t BpBinder::transact( uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){ // Once a binder has died, it will never come back to life. if (mAlive) { status_t status = IPCThreadState::self()->transact( mHandle, code, data, reply, flags); if (status == DEAD_OBJECT) mAlive = 0; return status; } return DEAD_OBJECT;}
这里的mHandle = 0，code = CHECK_SERVICE_TRANSACTION，flags = 0。
这里再进入到IPCThread::transact函数中：
status_t IPCThreadState::transact(int32_t handle, uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){ status_t err = data.errorCheck(); flags |= TF_ACCEPT_FDS; IF_LOG_TRANSACTIONS() { TextOutput::Bundle _b(alog); alog << "BC_TRANSACTION thr " << (void*)pthread_self() << " / hand " << handle << " / code " << TypeCode(code) << ": " << indent << data << dedent << endl; } if (err == NO_ERROR) { LOG_ONEWAY(">>>> SEND from pid %d uid %d %s", getpid(), getuid(), (flags & TF_ONE_WAY) == 0 ? "READ REPLY" : "ONE WAY"); err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL); } if (err != NO_ERROR) { if (reply) reply->setError(err); return (mLastError = err); } if ((flags & TF_ONE_WAY) == 0) { #if 0 if (code == 4) { // relayout LOGI(">>>>>> CALLING transaction 4"); } else { LOGI(">>>>>> CALLING transaction %d", code); } #endif if (reply) { err = waitForResponse(reply); } else { Parcel fakeReply; err = waitForResponse(&fakeReply); } #if 0 if (code == 4) { // relayout LOGI("<<<<<< RETURNING transaction 4"); } else { LOGI("<<<<<< RETURNING transaction %d", code); } #endif IF_LOG_TRANSACTIONS() { TextOutput::Bundle _b(alog); alog << "BR_REPLY thr " << (void*)pthread_self() << " / hand " << handle << ": "; if (reply) alog << indent << *reply << dedent << endl; else alog << "(none requested)" << endl; } } else { err = waitForResponse(NULL, NULL); } return err;} 首先是调用函数writeTransactionData写入将要传输的数据到IPCThreadState的成员变量mOut中去：
status_t IPCThreadState::writeTransactionData(int32_t cmd, uint32_t binderFlags, int32_t handle, uint32_t code, const Parcel& data, status_t* statusBuffer){ binder_transaction_data tr; tr.target.handle = handle; tr.code = code; tr.flags = binderFlags; const status_t err = data.errorCheck(); if (err == NO_ERROR) { tr.data_size = data.ipcDataSize(); tr.data.ptr.buffer = data.ipcData(); tr.offsets_size = data.ipcObjectsCount()*sizeof(size_t); tr.data.ptr.offsets = data.ipcObjects(); } else if (statusBuffer) { tr.flags |= TF_STATUS_CODE; *statusBuffer = err; tr.data_size = sizeof(status_t); tr.data.ptr.buffer = statusBuffer; tr.offsets_size = 0; tr.data.ptr.offsets = NULL; } else { return (mLastError = err); } mOut.writeInt32(cmd); mOut.write(&tr, sizeof(tr)); return NO_ERROR;} 结构体binder_transaction_data在上一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析已经介绍过，这里不再累述，这个结构体是用来描述要传输的参数的内容的。这里着重描述一下将要传输的参数tr里面的内容，handle = 0，code = CHECK_SERVICE_TRANSACTION，cmd = BC_TRANSACTION，data里面的数据分别为：
writeInt32(IPCThreadState::self()->getStrictModePolicy() | STRICT_MODE_PENALTY_GATHER);writeString16("android.os.IServiceManager");writeString16("media.player"); 这是在BpServiceManager::checkService函数里面写进去的，其中前两个是RPC头，Service Manager在收到这个请求时会验证这两个参数是否正确，这点前面也提到了。IPCThread->getStrictModePolicy默认返回0，STRICT_MODE_PENALTY_GATHER定义为：
// Note: must be kept in sync with android/os/StrictMode.java's PENALTY_GATHER#define STRICT_MODE_PENALTY_GATHER 0x100我们不关心这个参数的含义，这不会影响我们分析下面的源代码，有兴趣的读者可以研究一下。这里要注意的是，要传输的参数不包含有Binder对象，因此tr.offsets_size = 0。要传输的参数最后写入到IPCThreadState的成员变量mOut中，包括cmd和tr两个数据。
回到IPCThread::transact函数中，由于(flags & TF_ONE_WAY) == 0为true，即这是一个同步请求，并且reply != NULL，最终调用：
err = waitForResponse(reply);进入到waitForResponse函数中：
status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult){ int32_t cmd; int32_t err; while (1) { if ((err=talkWithDriver()) < NO_ERROR) break; err = mIn.errorCheck(); if (err < NO_ERROR) break; if (mIn.dataAvail() == 0) continue; cmd = mIn.readInt32(); IF_LOG_COMMANDS() { alog << "Processing waitForResponse Command: " << getReturnString(cmd) << endl; } switch (cmd) { case BR_TRANSACTION_COMPLETE: if (!reply && !acquireResult) goto finish; break; case BR_DEAD_REPLY: err = DEAD_OBJECT; goto finish; case BR_FAILED_REPLY: err = FAILED_TRANSACTION; goto finish; case BR_ACQUIRE_RESULT: { LOG_ASSERT(acquireResult != NULL, "Unexpected brACQUIRE_RESULT"); const int32_t result = mIn.readInt32(); if (!acquireResult) continue; *acquireResult = result ? NO_ERROR : INVALID_OPERATION; } goto finish; case BR_REPLY: { binder_transaction_data tr; err = mIn.read(&tr, sizeof(tr)); LOG_ASSERT(err == NO_ERROR, "Not enough command data for brREPLY"); if (err != NO_ERROR) goto finish; if (reply) { if ((tr.flags & TF_STATUS_CODE) == 0) { reply->ipcSetDataReference( reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer), tr.data_size, reinterpret_cast<const size_t*>(tr.data.ptr.offsets), tr.offsets_size/sizeof(size_t), freeBuffer, this); } else { err = *static_cast<const status_t*>(tr.data.ptr.buffer); freeBuffer(NULL, reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer), tr.data_size, reinterpret_cast<const size_t*>(tr.data.ptr.offsets), tr.offsets_size/sizeof(size_t), this); } } else { freeBuffer(NULL, reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer), tr.data_size, reinterpret_cast<const size_t*>(tr.data.ptr.offsets), tr.offsets_size/sizeof(size_t), this); continue; } } goto finish; default: err = executeCommand(cmd); if (err != NO_ERROR) goto finish; break; } }finish: if (err != NO_ERROR) { if (acquireResult) *acquireResult = err; if (reply) reply->setError(err); mLastError = err; } return err;} 这个函数通过IPCThreadState::talkWithDriver与驱动程序进行交互：
status_t IPCThreadState::talkWithDriver(bool doReceive){LOG_ASSERT(mProcess->mDriverFD >= 0, "Binder driver is not opened");binder_write_read bwr;// Is the read buffer empty?const bool needRead = mIn.dataPosition() >= mIn.dataSize();// We don't want to write anything if we are still reading// from data left in the input buffer and the caller// has requested to read the next data.const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0;bwr.write_size = outAvail;bwr.write_buffer = (long unsigned int)mOut.data();// This is what we'll read.if (doReceive && needRead) {bwr.read_size = mIn.dataCapacity();bwr.read_buffer = (long unsigned int)mIn.data();} else {bwr.read_size = 0;}......// Return immediately if there is nothing to do.if ((bwr.write_size == 0) && (bwr.read_size == 0)) return NO_ERROR;bwr.write_consumed = 0;bwr.read_consumed = 0;status_t err;do {......#if defined(HAVE_ANDROID_OS)if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0)err = NO_ERROR;elseerr = -errno;#elseerr = INVALID_OPERATION;#endif......} while (err == -EINTR);......if (err >= NO_ERROR) {if (bwr.write_consumed > 0) {if (bwr.write_consumed < (ssize_t)mOut.dataSize())mOut.remove(0, bwr.write_consumed);elsemOut.setDataSize(0);}if (bwr.read_consumed > 0) {mIn.setDataSize(bwr.read_consumed);mIn.setDataPosition(0);}......return NO_ERROR;}return err;} 这里的needRead为true，因此，bwr.read_size大于0；outAvail也大于0，因此，bwr.write_size也大于0。函数最后通过：
ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)进入到Binder驱动程序的binder_ioctl函数中。注意，这里的mProcess->mDriverFD是在我们前面调用defaultServiceManager函数获得Service Manager远程接口时，打开的设备文件/dev/binder的文件描述符，mProcess是IPCSThreadState的成员变量。
Binder驱动程序的binder_ioctl函数中，我们只关注BINDER_WRITE_READ命令相关的逻辑：
static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg){int ret;struct binder_proc *proc = filp->private_data;struct binder_thread *thread;unsigned int size = _IOC_SIZE(cmd);void __user *ubuf = (void __user *)arg;/*printk(KERN_INFO "binder_ioctl: %d:%d %x %lx\n", proc->pid, current->pid, cmd, arg);*/ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2);if (ret)return ret;mutex_lock(&binder_lock);thread = binder_get_thread(proc);if (thread == NULL) {ret = -ENOMEM;goto err;}switch (cmd) {case BINDER_WRITE_READ: {struct binder_write_read bwr;if (size != sizeof(struct binder_write_read)) {ret = -EINVAL;goto err;}if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {ret = -EFAULT;goto err;}if (binder_debug_mask & BINDER_DEBUG_READ_WRITE)printk(KERN_INFO "binder: %d:%d write %ld at %08lx, read %ld at %08lx\n",proc->pid, thread->pid, bwr.write_size, bwr.write_buffer, bwr.read_size, bwr.read_buffer);if (bwr.write_size > 0) {ret = binder_thread_write(proc, thread, (void __user *)bwr.write_buffer, bwr.write_size, &bwr.write_consumed);if (ret < 0) {bwr.read_consumed = 0;if (copy_to_user(ubuf, &bwr, sizeof(bwr)))ret = -EFAULT;goto err;}}if (bwr.read_size > 0) {ret = binder_thread_read(proc, thread, (void __user *)bwr.read_buffer, bwr.read_size, &bwr.read_consumed, filp->f_flags & O_NONBLOCK);if (!list_empty(&proc->todo))wake_up_interruptible(&proc->wait);if (ret < 0) {if (copy_to_user(ubuf, &bwr, sizeof(bwr)))ret = -EFAULT;goto err;}}if (binder_debug_mask & BINDER_DEBUG_READ_WRITE)printk(KERN_INFO "binder: %d:%d wrote %ld of %ld, read return %ld of %ld\n",proc->pid, thread->pid, bwr.write_consumed, bwr.write_size, bwr.read_consumed, bwr.read_size);if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {ret = -EFAULT;goto err;}break;}......default:ret = -EINVAL;goto err;}ret = 0;err:......return ret;} 这里的filp->private_data的值是在defaultServiceManager函数创建ProcessState对象时，在ProcessState构造函数通过open文件操作函数打开设备文件/dev/binder时设置好的，它表示的是调用open函数打开设备文件/dev/binder的进程上下文信息，这里将它取出来保存在proc本地变量中。
这里的thread本地变量表示当前线程上下文信息，通过binder_get_thread函数获得。在前面执行ProcessState构造函数时，也会通过ioctl文件操作函数进入到这个函数，那是第一次进入到binder_ioctl这里，因此，调用binder_get_thread时，表示当前进程上下文信息的proc变量还没有关于当前线程的上下文信息，因此，会为proc创建一个表示当前线程上下文信息的thread，会保存在proc->threads表示的红黑树结构中。这里调用binder_get_thread就可以直接从proc找到并返回了。
进入到BINDER_WRITE_READ相关的逻辑。先看看BINDER_WRITE_READ的定义：
#define BINDER_WRITE_READ _IOWR('b', 1, struct binder_write_read) 这里可以看出，BINDER_WRITE_READ命令的参数类型为struct binder_write_read：
struct binder_write_read {signed longwrite_size;/* bytes to write */signed longwrite_consumed;/* bytes consumed by driver */unsigned longwrite_buffer;signed longread_size;/* bytes to read */signed longread_consumed;/* bytes consumed by driver */unsigned longread_buffer;}; 这个结构体的含义可以参考浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路一文。这里首先是通过copy_from_user函数把用户传进来的参数的内容拷贝到本地变量bwr中。从上面的调用过程，我们知道，这里bwr.write_size是大于0的，因此进入到binder_thread_write函数中，我们只关注BC_TRANSACTION相关的逻辑：
intbinder_thread_write(struct binder_proc *proc, struct binder_thread *thread,void __user *buffer, int size, signed long *consumed){uint32_t cmd;void __user *ptr = buffer + *consumed;void __user *end = buffer + size;while (ptr < end && thread->return_error == BR_OK) {if (get_user(cmd, (uint32_t __user *)ptr))return -EFAULT;ptr += sizeof(uint32_t);if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {binder_stats.bc[_IOC_NR(cmd)]++;proc->stats.bc[_IOC_NR(cmd)]++;thread->stats.bc[_IOC_NR(cmd)]++;}switch (cmd) {......case BC_TRANSACTION:case BC_REPLY: {struct binder_transaction_data tr;if (copy_from_user(&tr, ptr, sizeof(tr)))return -EFAULT;ptr += sizeof(tr);binder_transaction(proc, thread, &tr, cmd == BC_REPLY);break; }......default:printk(KERN_ERR "binder: %d:%d unknown command %d\n", proc->pid, thread->pid, cmd);return -EINVAL;}*consumed = ptr - buffer;}return 0;} 这里再次把用户传出来的参数拷贝到本地变量tr中，tr的类型为struct binder_transaction_data，这个就是前面我们在IPCThreadState::writeTransactionData写入的内容了。接着进入到binder_transaction函数中，不相关的代码我们忽略掉：
static voidbinder_transaction(struct binder_proc *proc, struct binder_thread *thread,struct binder_transaction_data *tr, int reply){struct binder_transaction *t;struct binder_work *tcomplete;size_t *offp, *off_end;struct binder_proc *target_proc;struct binder_thread *target_thread = NULL;struct binder_node *target_node = NULL;struct list_head *target_list;wait_queue_head_t *target_wait;struct binder_transaction *in_reply_to = NULL;struct binder_transaction_log_entry *e;uint32_t return_error;.......if (reply) {......} else {if (tr->target.handle) {......} else {target_node = binder_context_mgr_node;if (target_node == NULL) {return_error = BR_DEAD_REPLY;goto err_no_context_mgr_node;}}......target_proc = target_node->proc;if (target_proc == NULL) {return_error = BR_DEAD_REPLY;goto err_dead_binder;}if (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) {......}}if (target_thread) {......} else {target_list = &target_proc->todo;target_wait = &target_proc->wait;}....../* TODO: reuse incoming transaction for reply */t = kzalloc(sizeof(*t), GFP_KERNEL);if (t == NULL) {return_error = BR_FAILED_REPLY;goto err_alloc_t_failed;}binder_stats.obj_created[BINDER_STAT_TRANSACTION]++;tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);if (tcomplete == NULL) {return_error = BR_FAILED_REPLY;goto err_alloc_tcomplete_failed;}binder_stats.obj_created[BINDER_STAT_TRANSACTION_COMPLETE]++;t->debug_id = ++binder_last_id;......if (!reply && !(tr->flags & TF_ONE_WAY))t->from = thread;elset->from = NULL;t->sender_euid = proc->tsk->cred->euid;t->to_proc = target_proc;t->to_thread = target_thread;t->code = tr->code;t->flags = tr->flags;t->priority = task_nice(current);t->buffer = binder_alloc_buf(target_proc, tr->data_size,tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));if (t->buffer == NULL) {return_error = BR_FAILED_REPLY;goto err_binder_alloc_buf_failed;}t->buffer->allow_user_free = 0;t->buffer->debug_id = t->debug_id;t->buffer->transaction = t;t->buffer->target_node = target_node;if (target_node)binder_inc_node(target_node, 1, 0, NULL);offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {......return_error = BR_FAILED_REPLY;goto err_copy_data_failed;}......if (reply) {......} else if (!(t->flags & TF_ONE_WAY)) {BUG_ON(t->buffer->async_transaction != 0);t->need_reply = 1;t->from_parent = thread->transaction_stack;thread->transaction_stack = t;} else {......}t->work.type = BINDER_WORK_TRANSACTION;list_add_tail(&t->work.entry, target_list);tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;list_add_tail(&tcomplete->entry, &thread->todo);if (target_wait)wake_up_interruptible(target_wait);return; ......} 注意，这里的参数reply = 0，表示这是一个BC_TRANSACTION命令。前面我们提到，传给驱动程序的handle值为0，即这里的tr->target.handle = 0，表示请求的目标Binder对象是Service Manager，因此有：
target_node = binder_context_mgr_node;target_proc = target_node->proc;target_list = &target_proc->todo;target_wait = &target_proc->wait;
其中binder_context_mgr_node是在Service Manager通知Binder驱动程序它是守护过程时创建的。
接着创建一个待完成事项tcomplete，它的类型为struct binder_work，这是等一会要保存在当前线程的todo队列去的，表示当前线程有一个待完成的事务。紧跟着创建一个待处理事务t，它的类型为struct binder_transaction，这是等一会要存在到Service Manager的todo队列去的，表示Service Manager当前有一个事务需要处理。同时，这个待处理事务t也要存放在当前线程的待完成事务transaction_stack列表中去：
t->from_parent = thread->transaction_stack;thread->transaction_stack = t;这样表明当前线程还有事务要处理。
继续往下看，就是分别把tcomplete和t放在当前线程thread和Service Manager进程的todo队列去了：
t->work.type = BINDER_WORK_TRANSACTION;list_add_tail(&t->work.entry, target_list);tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;list_add_tail(&tcomplete->entry, &thread->todo);最后，Service Manager有事情可做了，就要唤醒它了：
wake_up_interruptible(target_wait);前面我们提到，此时Service Manager正在等待Client的请求，也就是Service Manager此时正在进入到Binder驱动程序的binder_thread_read函数中，并且休眠在target->wait上，具体参考浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路一文。
这里，我们暂时忽略Service Manager被唤醒之后的情景，继续看当前线程的执行。
函数binder_transaction执行完成之后，就一路返回到binder_ioctl函数里去了。函数binder_ioctl从binder_thread_write函数调用处返回后，发现bwr.read_size大于0，于是就进入到binder_thread_read函数去了：
static intbinder_thread_read(struct binder_proc *proc, struct binder_thread *thread, void __user *buffer, int size, signed long *consumed, int non_block){void __user *ptr = buffer + *consumed;void __user *end = buffer + size;int ret = 0;int wait_for_proc_work;if (*consumed == 0) {if (put_user(BR_NOOP, (uint32_t __user *)ptr))return -EFAULT;ptr += sizeof(uint32_t);}retry:wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo);......if (wait_for_proc_work) {......} else {if (non_block) {if (!binder_has_thread_work(thread))ret = -EAGAIN;} elseret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));}......while (1) {uint32_t cmd;struct binder_transaction_data tr;struct binder_work *w;struct binder_transaction *t = NULL;if (!list_empty(&thread->todo))w = list_first_entry(&thread->todo, struct binder_work, entry);else if (!list_empty(&proc->todo) && wait_for_proc_work)w = list_first_entry(&proc->todo, struct binder_work, entry);else {if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */goto retry;break;}if (end - ptr < sizeof(tr) + 4)break;switch (w->type) {......case BINDER_WORK_TRANSACTION_COMPLETE: {cmd = BR_TRANSACTION_COMPLETE;if (put_user(cmd, (uint32_t __user *)ptr))return -EFAULT;ptr += sizeof(uint32_t);binder_stat_br(proc, thread, cmd);if (binder_debug_mask & BINDER_DEBUG_TRANSACTION_COMPLETE)printk(KERN_INFO "binder: %d:%d BR_TRANSACTION_COMPLETE\n",proc->pid, thread->pid);list_del(&w->entry);kfree(w);binder_stats.obj_deleted[BINDER_STAT_TRANSACTION_COMPLETE]++; } break;......}if (!t)continue;......}done:......return 0;} 函数首先是写入一个操作码BR_NOOP到用户传进来的缓冲区中去。
回忆一下上面的binder_transaction函数，这里的thread->transaction_stack != NULL，并且thread->todo也不为空，所以线程不会进入休眠状态。
进入while循环中，首先是从thread->todo队列中取回待处理事项w，w的类型为BINDER_WORK_TRANSACTION_COMPLETE，这也是在binder_transaction函数里面设置的。对BINDER_WORK_TRANSACTION_COMPLETE的处理也很简单，只是把一个操作码BR_TRANSACTION_COMPLETE写回到用户传进来的缓冲区中去。这时候，用户传进来的缓冲区就包含两个操作码了，分别是BR_NOOP和BINDER_WORK_TRANSACTION_COMPLETE。
binder_thread_read执行完之后，返回到binder_ioctl函数中，将操作结果写回到用户空间中去：
if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {ret = -EFAULT;goto err;} 最后就返回到IPCThreadState::talkWithDriver函数中了。
IPCThreadState::talkWithDriver函数从下面语句：
ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)返回后，首先是清空之前写入Binder驱动程序的内容：
if (bwr.write_consumed > 0) { if (bwr.write_consumed < (ssize_t)mOut.dataSize()) mOut.remove(0, bwr.write_consumed); else mOut.setDataSize(0);} 接着是设置从Binder驱动程序读取的内容：
if (bwr.read_consumed > 0) { mIn.setDataSize(bwr.read_consumed); mIn.setDataPosition(0);} 然后就返回到IPCThreadState::waitForResponse去了。IPCThreadState::waitForResponse函数的处理也很简单，就是处理刚才从Binder驱动程序读入内容了。从前面的分析中，我们知道，从Binder驱动程序读入的内容就是两个整数了，分别是BR_NOOP和BR_TRANSACTION_COMPLETE。对BR_NOOP的处理很简单，正如它的名字所示，什么也不做；而对BR_TRANSACTION_COMPLETE的处理，就分情况了，如果这个请求是异步的，那个整个BC_TRANSACTION操作就完成了，如果这个请求是同步的，即要等待回复的，也就是reply不为空，那么还要继续通过IPCThreadState::talkWithDriver进入到Binder驱动程序中去等待BC_TRANSACTION操作的处理结果。
这里属于后一种情况，于是再次通过IPCThreadState::talkWithDriver进入到Binder驱动程序的binder_ioctl函数中。不过这一次在binder_ioctl函数中，bwr.write_size等于0，而bwr.read_size大于0，于是再次进入到binder_thread_read函数中。这时候thread->transaction_stack仍然不为NULL，不过thread->todo队列已经为空了，因为前面我们已经处理过thread->todo队列的内容了，于是就通过下面语句：
ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));进入休眠状态了，等待Service Manager的唤醒。
现在，我们终于可以回到Service Manager被唤醒之后的过程了。前面我们说过，Service Manager此时正在binder_thread_read函数中休眠中：
static intbinder_thread_read(struct binder_proc *proc, struct binder_thread *thread, void __user *buffer, int size, signed long *consumed, int non_block){void __user *ptr = buffer + *consumed;void __user *end = buffer + size;int ret = 0;int wait_for_proc_work;if (*consumed == 0) {if (put_user(BR_NOOP, (uint32_t __user *)ptr))return -EFAULT;ptr += sizeof(uint32_t);}retry:wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo);......if (wait_for_proc_work) {......if (non_block) {if (!binder_has_proc_work(proc, thread))ret = -EAGAIN;} elseret = wait_event_interruptible_exclusive(proc->wait, binder_has_proc_work(proc, thread));} else {......}......while (1) {uint32_t cmd;struct binder_transaction_data tr;struct binder_work *w;struct binder_transaction *t = NULL;if (!list_empty(&thread->todo))w = list_first_entry(&thread->todo, struct binder_work, entry);else if (!list_empty(&proc->todo) && wait_for_proc_work)w = list_first_entry(&proc->todo, struct binder_work, entry);else {if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */goto retry;break;}if (end - ptr < sizeof(tr) + 4)break;switch (w->type) {case BINDER_WORK_TRANSACTION: {t = container_of(w, struct binder_transaction, work); } break;......}if (!t)continue;BUG_ON(t->buffer == NULL);if (t->buffer->target_node) {struct binder_node *target_node = t->buffer->target_node;tr.target.ptr = target_node->ptr;tr.cookie = target_node->cookie;t->saved_priority = task_nice(current);if (t->priority < target_node->min_priority &&!(t->flags & TF_ONE_WAY))binder_set_nice(t->priority);else if (!(t->flags & TF_ONE_WAY) ||t->saved_priority > target_node->min_priority)binder_set_nice(target_node->min_priority);cmd = BR_TRANSACTION;} else {......}tr.code = t->code;tr.flags = t->flags;tr.sender_euid = t->sender_euid;if (t->from) {struct task_struct *sender = t->from->proc->tsk;tr.sender_pid = task_tgid_nr_ns(sender, current->nsproxy->pid_ns);} else {......}tr.data_size = t->buffer->data_size;tr.offsets_size = t->buffer->offsets_size;tr.data.ptr.buffer = (void *)t->buffer->data + proc->user_buffer_offset;tr.data.ptr.offsets = tr.data.ptr.buffer + ALIGN(t->buffer->data_size, sizeof(void *));if (put_user(cmd, (uint32_t __user *)ptr))return -EFAULT;ptr += sizeof(uint32_t);if (copy_to_user(ptr, &tr, sizeof(tr)))return -EFAULT;ptr += sizeof(tr);......list_del(&t->work.entry);t->buffer->allow_user_free = 1;if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {t->to_parent = thread->transaction_stack;t->to_thread = thread;thread->transaction_stack = t;} else {......}break;}done:*consumed = ptr - buffer;......return 0;} 这里就是从语句中唤醒了：
ret = wait_event_interruptible_exclusive(proc->wait, binder_has_proc_work(proc, thread));Service Manager唤醒过来看，继续往下执行，进入到while循环中。首先是从proc->todo中取回待处理事项w。这个事项w的类型是BINDER_WORK_TRANSACTION，这是上面调用binder_transaction的时候设置的，于是通过w得到待处理事务t：
t = container_of(w, struct binder_transaction, work);接下来的内容，就把cmd和t->buffer的内容拷贝到用户传进来的缓冲区去了，这里就是Service Manager从用户空间传进来的缓冲区了：
if (put_user(cmd, (uint32_t __user *)ptr))return -EFAULT;ptr += sizeof(uint32_t);if (copy_to_user(ptr, &tr, sizeof(tr)))return -EFAULT;ptr += sizeof(tr);注意，这里先是把t->buffer的内容拷贝到本地变量tr中，再拷贝到用户空间缓冲区去。关于t->buffer内容的拷贝，请参考Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析一文，它的一个关键地方是Binder驱动程序和Service Manager守护进程共享了同一个物理内存的内容，拷贝的只是这个物理内存在用户空间的虚拟地址回去：
tr.data.ptr.buffer = (void *)t->buffer->data + proc->user_buffer_offset;tr.data.ptr.offsets = tr.data.ptr.buffer + ALIGN(t->buffer->data_size, sizeof(void *));对于Binder驱动程序这次操作来说，这个事项就算是处理完了，就要从todo队列中删除了：
list_del(&t->work.entry);紧接着，还不放删除这个事务，因为它还要等待Service Manager处理完成后，再进一步处理，因此，放在thread->transaction_stack队列中：
t->to_parent = thread->transaction_stack;t->to_thread = thread;thread->transaction_stack = t;还要注意的一个地方是，上面写入的cmd = BR_TRANSACTION，告诉Service Manager守护进程，它要做什么事情，后面我们会看到相应的分析。
这样，binder_thread_read函数就处理完了，回到binder_ioctl函数中，同样是操作结果写回到用户空间的缓冲区中去：
if (copy_to_user(ubuf, &bwr, sizeof(bwr))) { ret = -EFAULT; goto err;} 最后，就返回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函数去了：
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) { LOGE("binder_loop: ioctl failed (%s)\n", strerror(errno)); break; } res = binder_parse(bs, 0, readbuf, bwr.read_consumed, func); if (res == 0) { LOGE("binder_loop: unexpected reply?!\n"); break; } if (res < 0) { LOGE("binder_loop: io error %d %s\n", res, strerror(errno)); break; } }} 这里就是从下面的语句：
res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);返回来了。接着就进入binder_parse函数处理从Binder驱动程序里面读取出来的数据：
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++;switch(cmd) {......case BR_TRANSACTION: {struct binder_txn *txn = (void *) ptr;......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; }......default:LOGE("parse: OOPS %d\n", cmd);return -1;}}return r;} 前面我们说过，Binder驱动程序写入到用户空间的缓冲区中的cmd为BR_TRANSACTION，因此，这里我们只关注BR_TRANSACTION相关的逻辑。
这里用到的两个数据结构struct binder_txn和struct binder_io可以参考前面一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析，这里就不复述了。
接着往下看，函数调bio_init来初始化reply变量：
void bio_init(struct binder_io *bio, void *data, uint32_t maxdata, uint32_t maxoffs){ uint32_t n = maxoffs * sizeof(uint32_t); if (n > maxdata) { bio->flags = BIO_F_OVERFLOW; bio->data_avail = 0; bio->offs_avail = 0; return; } bio->data = bio->data0 = data + n; bio->offs = bio->offs0 = data; bio->data_avail = maxdata - n; bio->offs_avail = maxoffs; bio->flags = 0;} 接着又调用bio_init_from_txn来初始化msg变量：
void bio_init_from_txn(struct binder_io *bio, struct binder_txn *txn){ bio->data = bio->data0 = txn->data; bio->offs = bio->offs0 = txn->offs; bio->data_avail = txn->data_size; bio->offs_avail = txn->offs_size / 4; bio->flags = BIO_F_SHARED;} 最后，真正进行处理的函数是从参数中传进来的函数指针func，这里就是定义在frameworks/base/cmds/servicemanager/service_manager.c文件中的svcmgr_handler函数：
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;// LOGI("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); if (!ptr) break; bio_put_ref(reply, ptr); return 0; ...... } default: LOGE("unknown code %d\n", txn->code); return -1; } bio_put_uint32(reply, 0); return 0;} 这里， Service Manager要处理的code是SVC_MGR_CHECK_SERVICE，这是在前面的BpServiceManager::checkService函数里面设置的。
回忆一下，在BpServiceManager::checkService时，传给Binder驱动程序的参数为：
writeInt32(IPCThreadState::self()->getStrictModePolicy() | STRICT_MODE_PENALTY_GATHER); writeString16("android.os.IServiceManager"); writeString16("media.player"); 这里的语句：
strict_policy = bio_get_uint32(msg); s = bio_get_string16(msg, &len); s = bio_get_string16(msg, &len);其中，会验证一下传进来的第二个参数，即"android.os.IServiceManager"是否正确，这个是验证RPC头，注释已经说得很清楚了。
最后，就是调用do_find_service函数查找是存在名称为"media.player"的服务了。回忆一下前面一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析，MediaPlayerService已经把一个名称为"media.player"的服务注册到Service Manager中，所以这里一定能找到。我们看看do_find_service这个函数：
void *do_find_service(struct binder_state *bs, uint16_t *s, unsigned len){ struct svcinfo *si; si = find_svc(s, len);// LOGI("check_service('%s') ptr = %p\n", str8(s), si ? si->ptr : 0); if (si && si->ptr) { return si->ptr; } else { return 0; }} 这里又调用了find_svc函数：
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;} 就是在svclist列表中查找对应名称的svcinfo了。
然后返回到do_find_service函数中。回忆一下前面一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析，这里的si->ptr就是指MediaPlayerService这个Binder实体在Service Manager进程中的句柄值了。
回到svcmgr_handler函数中，调用bio_put_ref函数将这个Binder引用写回到reply参数。我们看看bio_put_ref的实现：
void bio_put_ref(struct binder_io *bio, void *ptr){ struct binder_object *obj; if (ptr) obj = bio_alloc_obj(bio); else obj = bio_alloc(bio, sizeof(*obj)); if (!obj) return; obj->flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS; obj->type = BINDER_TYPE_HANDLE; obj->pointer = ptr; obj->cookie = 0;} 这里很简单，就是把一个类型为BINDER_TYPE_HANDLE的binder_object写入到reply缓冲区中去。这里的binder_object就是相当于是flat_binder_obj了，具体可以参考Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析一文。
再回到svcmgr_handler函数中，最后，还写入一个0值到reply缓冲区中，表示操作结果码：
bio_put_uint32(reply, 0);最后返回到binder_parse函数中，调用binder_send_reply函数将操作结果反馈给Binder驱动程序：
void binder_send_reply(struct binder_state *bs, struct binder_io *reply, void *buffer_to_free, int status){ struct { uint32_t cmd_free; void *buffer; uint32_t cmd_reply; struct binder_txn txn; } __attribute__((packed)) data; data.cmd_free = BC_FREE_BUFFER; data.buffer = buffer_to_free; data.cmd_reply = BC_REPLY; data.txn.target = 0; data.txn.cookie = 0; data.txn.code = 0; if (status) { data.txn.flags = TF_STATUS_CODE; data.txn.data_size = sizeof(int); data.txn.offs_size = 0; data.txn.data = &status; data.txn.offs = 0; } else { data.txn.flags = 0; data.txn.data_size = reply->data - reply->data0; data.txn.offs_size = ((char*) reply->offs) - ((char*) reply->offs0); data.txn.data = reply->data0; data.txn.offs = reply->offs0; } binder_write(bs, &data, sizeof(data));} 注意，这里的status参数为0。从这里可以看出，binder_send_reply告诉Binder驱动程序执行BC_FREE_BUFFER和BC_REPLY命令，前者释放之前在binder_transaction分配的空间，地址为buffer_to_free，buffer_to_free这个地址是Binder驱动程序把自己在内核空间用的地址转换成用户空间地址再传给Service Manager的，所以Binder驱动程序拿到这个地址后，知道怎么样释放这个空间；后者告诉Binder驱动程序，它的SVC_MGR_CHECK_SERVICE操作已经完成了,要查询的服务的句柄值也是保存在data.txn.data，操作结果码是0，也是保存在data.txn.data中。再来看binder_write函数：
int binder_write(struct binder_state *bs, void *data, unsigned len){ struct binder_write_read bwr; int res; bwr.write_size = len; bwr.write_consumed = 0; bwr.write_buffer = (unsigned) data; bwr.read_size = 0; bwr.read_consumed = 0; bwr.read_buffer = 0; res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr); if (res < 0) { fprintf(stderr,"binder_write: ioctl failed (%s)\n", strerror(errno)); } return res;} 这里可以看出，只有写操作，没有读操作，即read_size为0。这里又是一个ioctl的BINDER_WRITE_READ操作。直入到驱动程序的binder_ioctl函数后，执行BINDER_WRITE_READ命令，这里就不累述了。
最后，从binder_ioctl执行到binder_thread_write函数，首先是执行BC_FREE_BUFFER命令，这个命令的执行在前面一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析已经介绍过了，这里就不再累述了。
我们重点关注BC_REPLY命令的执行：
int binder_thread_write(struct binder_proc *proc, struct binder_thread *thread, void __user *buffer, int size, signed long *consumed) { uint32_t cmd; void __user *ptr = buffer + *consumed; void __user *end = buffer + size; while (ptr < end && thread->return_error == BR_OK) { if (get_user(cmd, (uint32_t __user *)ptr)) return -EFAULT; ptr += sizeof(uint32_t); if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) { binder_stats.bc[_IOC_NR(cmd)]++; proc->stats.bc[_IOC_NR(cmd)]++; thread->stats.bc[_IOC_NR(cmd)]++; } switch (cmd) { ...... case BC_TRANSACTION: case BC_REPLY: { struct binder_transaction_data tr; if (copy_from_user(&tr, ptr, sizeof(tr))) return -EFAULT; ptr += sizeof(tr); binder_transaction(proc, thread, &tr, cmd == BC_REPLY); break; } ...... *consumed = ptr - buffer; } return 0; } 又再次进入到binder_transaction函数：
static voidbinder_transaction(struct binder_proc *proc, struct binder_thread *thread,struct binder_transaction_data *tr, int reply){struct binder_transaction *t;struct binder_work *tcomplete;size_t *offp, *off_end;struct binder_proc *target_proc;struct binder_thread *target_thread = NULL;struct binder_node *target_node = NULL;struct list_head *target_list;wait_queue_head_t *target_wait;struct binder_transaction *in_reply_to = NULL;struct binder_transaction_log_entry *e;uint32_t return_error;......if (reply) {in_reply_to = thread->transaction_stack;if (in_reply_to == NULL) {......return_error = BR_FAILED_REPLY;goto err_empty_call_stack;}......thread->transaction_stack = in_reply_to->to_parent;target_thread = in_reply_to->from;......target_proc = target_thread->proc;} else {......}if (target_thread) {e->to_thread = target_thread->pid;target_list = &target_thread->todo;target_wait = &target_thread->wait;} else {......}/* TODO: reuse incoming transaction for reply */t = kzalloc(sizeof(*t), GFP_KERNEL);if (t == NULL) {return_error = BR_FAILED_REPLY;goto err_alloc_t_failed;}binder_stats.obj_created[BINDER_STAT_TRANSACTION]++;tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);if (tcomplete == NULL) {return_error = BR_FAILED_REPLY;goto err_alloc_tcomplete_failed;}......if (!reply && !(tr->flags & TF_ONE_WAY))t->from = thread;elset->from = NULL;t->sender_euid = proc->tsk->cred->euid;t->to_proc = target_proc;t->to_thread = target_thread;t->code = tr->code;t->flags = tr->flags;t->priority = task_nice(current);t->buffer = binder_alloc_buf(target_proc, tr->data_size,tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));if (t->buffer == NULL) {return_error = BR_FAILED_REPLY;goto err_binder_alloc_buf_failed;}t->buffer->allow_user_free = 0;t->buffer->debug_id = t->debug_id;t->buffer->transaction = t;t->buffer->target_node = target_node;if (target_node)binder_inc_node(target_node, 1, 0, NULL);offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {binder_user_error("binder: %d:%d got transaction with invalid ""data ptr\n", proc->pid, thread->pid);return_error = BR_FAILED_REPLY;goto err_copy_data_failed;}if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {binder_user_error("binder: %d:%d got transaction with invalid ""offsets ptr\n", proc->pid, thread->pid);return_error = BR_FAILED_REPLY;goto err_copy_data_failed;}......off_end = (void *)offp + tr->offsets_size;for (; offp < off_end; offp++) {struct flat_binder_object *fp;......fp = (struct flat_binder_object *)(t->buffer->data + *offp);switch (fp->type) {......case BINDER_TYPE_HANDLE:case BINDER_TYPE_WEAK_HANDLE: {struct binder_ref *ref = binder_get_ref(proc, fp->handle);if (ref == NULL) {......return_error = BR_FAILED_REPLY;goto err_binder_get_ref_failed;}if (ref->node->proc == target_proc) {......} else {struct binder_ref *new_ref;new_ref = binder_get_ref_for_node(target_proc, ref->node);if (new_ref == NULL) {return_error = BR_FAILED_REPLY;goto err_binder_get_ref_for_node_failed;}fp->handle = new_ref->desc;binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);......}} break;......}}if (reply) {BUG_ON(t->buffer->async_transaction != 0);binder_pop_transaction(target_thread, in_reply_to);} else if (!(t->flags & TF_ONE_WAY)) {......} else {......}t->work.type = BINDER_WORK_TRANSACTION;list_add_tail(&t->work.entry, target_list);tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;list_add_tail(&tcomplete->entry, &thread->todo);if (target_wait)wake_up_interruptible(target_wait);return; ......} 这次进入binder_transaction函数的情形和上面介绍的binder_transaction函数的情形基本一致，只是这里的proc、thread和target_proc、target_thread调换了角色，这里的proc和thread指的是Service Manager进程，而target_proc和target_thread指的是刚才请求SVC_MGR_CHECK_SERVICE的进程。
那么，这次是如何找到target_proc和target_thread呢。首先，我们注意到，这里的reply等于1，其次，上面我们提到，Binder驱动程序在唤醒Service Manager，告诉它有一个事务t要处理时，事务t虽然从Service Manager的todo队列中删除了，但是仍然保留在transaction_stack中。因此，这里可以从thread->transaction_stack找回这个等待回复的事务t，然后通过它找回target_proc和target_thread：
in_reply_to = thread->transaction_stack;target_thread = in_reply_to->from;target_list = &target_thread->todo;target_wait = &target_thread->wait;再接着往下看，由于Service Manager返回来了一个Binder引用，所以这里要处理一下，就是中间的for循环了。这是一个BINDER_TYPE_HANDLE类型的Binder引用，这是前面设置的。先把t->buffer->data的内容转换为一个struct flat_binder_object对象fp，这里的fp->handle值就是这个Service在Service Manager进程里面的引用值了。接通过调用binder_get_ref函数得到Binder引用对象struct binder_ref类型的对象ref：
struct binder_ref *ref = binder_get_ref(proc, fp->handle);这里一定能找到，因为前面MediaPlayerService执行IServiceManager::addService的时候把自己添加到Service Manager的时候，会在Service Manager进程中创建这个Binder引用，然后把这个Binder引用的句柄值返回给Service Manager用户空间。
这里面的ref->node->proc不等于target_proc，因为这个Binder实体是属于创建MediaPlayerService的进程的，而不是请求这个服务的远程接口的进程的，因此，这里调用binder_get_ref_for_node函数为这个Binder实体在target_proc创建一个引用：
struct binder_ref *new_ref;new_ref = binder_get_ref_for_node(target_proc, ref->node);然后增加引用计数：
binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);这样，返回数据中的Binder对象就处理完成了。注意，这里会把fp->handle的值改为在target_proc中的引用值：
fp->handle = new_ref->desc;这里就相当于是把t->buffer->data里面的Binder对象的句柄值改写了。因为这是在另外一个不同的进程里面的Binder引用，所以句柄值当然要用新的了。这个值最终是要拷贝回target_proc进程的用户空间去的。
再往下看：
if (reply) { BUG_ON(t->buffer->async_transaction != 0); binder_pop_transaction(target_thread, in_reply_to);} else if (!(t->flags & TF_ONE_WAY)) { ......} else { ......} 这里reply等于1，执行binder_pop_transaction函数把当前事务in_reply_to从target_thread->transaction_stack队列中删掉，这是上次调用binder_transaction函数的时候设置的，现在不需要了，所以把它删掉。
再往后的逻辑就跟前面执行binder_transaction函数时候一样了，这里不再介绍。最后的结果就是唤醒请求SVC_MGR_CHECK_SERVICE操作的线程：
if (target_wait) wake_up_interruptible(target_wait);这样，Service Manger回复调用SVC_MGR_CHECK_SERVICE请求就算完成了，重新回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函数等待下一个Client请求的到来。事实上，Service Manger回到binder_loop函数再次执行ioctl函数时候，又会再次进入到binder_thread_read函数。这时个会发现thread->todo不为空，这是因为刚才我们调用了：
list_add_tail(&tcomplete->entry, &thread->todo);把一个工作项tcompelete放在了在thread->todo中，这个tcompelete的type为BINDER_WORK_TRANSACTION_COMPLETE，因此，Binder驱动程序会执行下面操作：
switch (w->type) { case BINDER_WORK_TRANSACTION_COMPLETE: { cmd = BR_TRANSACTION_COMPLETE; if (put_user(cmd, (uint32_t __user *)ptr)) return -EFAULT; ptr += sizeof(uint32_t); list_del(&w->entry); kfree(w); } break; ...... } binder_loop函数执行完这个ioctl调用后，才会在下一次调用ioctl进入到Binder驱动程序进入休眠状态，等待下一次Client的请求。上面讲到调用请求SVC_MGR_CHECK_SERVICE操作的线程被唤醒了，于是，重新执行binder_thread_read函数：
static int binder_thread_read(struct binder_proc *proc, struct binder_thread *thread, void __user *buffer, int size, signed long *consumed, int non_block) { void __user *ptr = buffer + *consumed; void __user *end = buffer + size; int ret = 0; int wait_for_proc_work; if (*consumed == 0) { if (put_user(BR_NOOP, (uint32_t __user *)ptr)) return -EFAULT; ptr += sizeof(uint32_t); } retry: wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo); ...... if (wait_for_proc_work) { ...... } else { if (non_block) { if (!binder_has_thread_work(thread)) ret = -EAGAIN; } else ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread)); } ...... while (1) { uint32_t cmd; struct binder_transaction_data tr; struct binder_work *w; struct binder_transaction *t = NULL; if (!list_empty(&thread->todo)) w = list_first_entry(&thread->todo, struct binder_work, entry); else if (!list_empty(&proc->todo) && wait_for_proc_work) w = list_first_entry(&proc->todo, struct binder_work, entry); else { if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */ goto retry; break; } ...... switch (w->type) { case BINDER_WORK_TRANSACTION: { t = container_of(w, struct binder_transaction, work); } break; ...... } if (!t) continue; BUG_ON(t->buffer == NULL); if (t->buffer->target_node) { ...... } else { tr.target.ptr = NULL; tr.cookie = NULL; cmd = BR_REPLY; } tr.code = t->code; tr.flags = t->flags; tr.sender_euid = t->sender_euid; if (t->from) { ...... } else { tr.sender_pid = 0; } tr.data_size = t->buffer->data_size; tr.offsets_size = t->buffer->offsets_size; tr.data.ptr.buffer = (void *)t->buffer->data + proc->user_buffer_offset; tr.data.ptr.offsets = tr.data.ptr.buffer + ALIGN(t->buffer->data_size, sizeof(void *)); if (put_user(cmd, (uint32_t __user *)ptr)) return -EFAULT; ptr += sizeof(uint32_t); if (copy_to_user(ptr, &tr, sizeof(tr))) return -EFAULT; ptr += sizeof(tr); ...... list_del(&t->work.entry); t->buffer->allow_user_free = 1; if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) { ...... } else { t->buffer->transaction = NULL; kfree(t); binder_stats.obj_deleted[BINDER_STAT_TRANSACTION]++; } break; } done: ...... return 0; } 就是从下面这个调用：
ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));被唤醒过来了。在while循环中，从thread->todo得到w，w->type为BINDER_WORK_TRANSACTION，于是，得到t。从上面可以知道，Service Manager返回来了一个Binder引用和一个结果码0回来，写在t->buffer->data里面，现在把t->buffer->data加上proc->user_buffer_offset，得到用户空间地址，保存在tr.data.ptr.buffer里面，这样用户空间就可以访问这个数据了。由于cmd不等于BR_TRANSACTION，这时就可以把t删除掉了，因为以后都不需要用了。
执行完这个函数后，就返回到binder_ioctl函数，执行下面语句，把数据返回给用户空间：
if (copy_to_user(ubuf, &bwr, sizeof(bwr))) { ret = -EFAULT; goto err; } 接着返回到用户空间IPCThreadState::talkWithDriver函数，最后返回到IPCThreadState::waitForResponse函数，最终执行到下面语句：
status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult) { int32_t cmd; int32_t err; while (1) { if ((err=talkWithDriver()) < NO_ERROR) break; ...... cmd = mIn.readInt32(); ...... switch (cmd) { ...... case BR_REPLY: { binder_transaction_data tr; err = mIn.read(&tr, sizeof(tr)); LOG_ASSERT(err == NO_ERROR, "Not enough command data for brREPLY"); if (err != NO_ERROR) goto finish; if (reply) { if ((tr.flags & TF_STATUS_CODE) == 0) { reply->ipcSetDataReference( reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer), tr.data_size, reinterpret_cast<const size_t*>(tr.data.ptr.offsets), tr.offsets_size/sizeof(size_t), freeBuffer, this); } else { ...... } } else { ...... } } goto finish; ...... } } finish: ...... return err; } 注意，这里的tr.flags等于0，这个是在上面的binder_send_reply函数里设置的。接着就把结果保存在reply了：
reply->ipcSetDataReference( reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer), tr.data_size, reinterpret_cast<const size_t*>(tr.data.ptr.offsets), tr.offsets_size/sizeof(size_t), freeBuffer, this);我们简单看一下Parcel::ipcSetDataReference函数的实现：
void Parcel::ipcSetDataReference(const uint8_t* data, size_t dataSize, const size_t* objects, size_t objectsCount, release_func relFunc, void* relCookie){ freeDataNoInit(); mError = NO_ERROR; mData = const_cast<uint8_t*>(data); mDataSize = mDataCapacity = dataSize; //LOGI("setDataReference Setting data size of %p to %lu (pid=%d)\n", this, mDataSize, getpid()); mDataPos = 0; LOGV("setDataReference Setting data pos of %p to %d\n", this, mDataPos); mObjects = const_cast<size_t*>(objects); mObjectsSize = mObjectsCapacity = objectsCount; mNextObjectHint = 0; mOwner = relFunc; mOwnerCookie = relCookie; scanForFds();} 上面提到，返回来的数据中有一个Binder引用，因此，这里的mObjectSize等于1，这个Binder引用对应的位置记录在mObjects成员变量中。
从这里层层返回，最后回到BpServiceManager::checkService函数中：
virtual sp<IBinder> BpServiceManager::checkService( const String16& name) const{ Parcel data, reply; data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor()); data.writeString16(name); remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply); return reply.readStrongBinder();} 这里就是从：
remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);返回来了。我们接着看一下reply.readStrongBinder函数的实现：
sp<IBinder> Parcel::readStrongBinder() const{ sp<IBinder> val; unflatten_binder(ProcessState::self(), *this, &val); return val;} 这里调用了unflatten_binder函数来构造一个Binder对象：
status_t unflatten_binder(const sp<ProcessState>& proc, const Parcel& in, sp<IBinder>* out){ const flat_binder_object* flat = in.readObject(false); if (flat) { switch (flat->type) { case BINDER_TYPE_BINDER: *out = static_cast<IBinder*>(flat->cookie); return finish_unflatten_binder(NULL, *flat, in); case BINDER_TYPE_HANDLE: *out = proc->getStrongProxyForHandle(flat->handle); return finish_unflatten_binder( static_cast<BpBinder*>(out->get()), *flat, in); } } return BAD_TYPE;} 这里的flat->type是BINDER_TYPE_HANDLE，因此调用ProcessState::getStrongProxyForHandle函数：
sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle){ sp<IBinder> result; AutoMutex _l(mLock); handle_entry* e = lookupHandleLocked(handle); if (e != NULL) { // We need to create a new BpBinder if there isn't currently one, OR we // are unable to acquire a weak reference on this current one. See comment // in getWeakProxyForHandle() for more info about this. IBinder* b = e->binder; if (b == NULL || !e->refs->attemptIncWeak(this)) { b = new BpBinder(handle); e->binder = b; if (b) e->refs = b->getWeakRefs(); result = b; } else { // This little bit of nastyness is to allow us to add a primary // reference to the remote proxy when this team doesn't have one // but another team is sending the handle to us. result.force_set(b); e->refs->decWeak(this); } } return result;} 这里我们可以看到，ProcessState会把使用过的Binder远程接口（BpBinder）缓存起来，这样下次从Service Manager那里请求得到相同的句柄（Handle）时就可以直接返回这个Binder远程接口了，不用再创建一个出来。这里是第一次使用，因此，e->binder为空，于是创建了一个BpBinder对象：
b = new BpBinder(handle); e->binder = b;if (b) e->refs = b->getWeakRefs();result = b;最后，函数返回到IMediaDeathNotifier::getMediaPlayerService这里，从这个语句返回：
binder = sm->getService(String16("media.player")); 这里，就相当于是：
binder = new BpBinder(handle);最后，函数调用：
sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);到了这里，我们可以参考一下前面一篇文章浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager，就会知道，这里的interface_cast实际上最终调用了IMediaPlayerService::asInterface函数：
android::sp<IMediaPlayerService> IMediaPlayerService::asInterface(const android::sp<android::IBinder>& obj){android::sp<IServiceManager> intr;if (obj != NULL) { intr = static_cast<IMediaPlayerService*>( obj->queryLocalInterface(IMediaPlayerService::descriptor).get());if (intr == NULL) {intr = new BpMediaPlayerService(obj);}}return intr; } 这里的obj就是BpBinder，而BpBinder::queryLocalInterface返回NULL，因此就创建了一个BpMediaPlayerService对象：intr = new BpMediaPlayerService(new BpBinder(handle));因此，我们最终就得到了一个BpMediaPlayerService对象，达到我们最初的目标。
有了这个BpMediaPlayerService这个远程接口之后，MediaPlayer就可以调用MediaPlayerService的服务了。
至此，Android系统进程间通信（IPC）机制Binder中的Client如何通过Service Manager的getService函数获得Server远程接口的过程就分析完了，Binder机制的学习就暂告一段落了。
不过，细心的读者可能会发现，我们这里介绍的Binder机制都是基于C/C++语言实现的，但是我们在编写应用程序都是基于Java语言的，那么，我们如何使用Java语言来使用系统的Binder机制来进行进程间通信呢？这就是下一篇文章要介绍的内容了，敬请关注。
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