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

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。

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;
  else
   err = -errno;
#else
  err = 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);
   else
    mOut.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 long write_size; /* bytes to write */
 signed long write_consumed; /* bytes consumed by driver */
 unsigned long write_buffer;
 signed long read_size; /* bytes to read */
 signed long read_consumed; /* bytes consumed by driver */
 unsigned long read_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：

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;
      }
  ......
  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 void
binder_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;
 else
  t->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 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;
  } 

  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 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) {
  ......
  if (non_block) {
   if (!binder_has_proc_work(proc, thread))
    ret = -EAGAIN;
  } else
   ret = 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 void
binder_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;
 else
  t->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。

以上就是对Android IPC Binder Client获得Server 远程接口过程的源码分析，后续继续补充相关文章，谢谢大家对本站的支持！