Android系统进程间通信（IPC）机制Binder中的Client获得Server远程接口过程源代码分析
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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>&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;} 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：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 阅读全文
