status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags) {
    // 数据错误检查
    status_t err = data.errorCheck();
    flags |= TF_ACCEPT_FDS;
    if (err == NO_ERROR) {
        // 传输数据
        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 (reply) {
            // 等待响应
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }
        ......
    } else {
        err = waitForResponse(NULL, NULL);
    }
    return err;
}

传送数据请查看writeTransactionData。