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)