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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：

		
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