diff --git a/include/dmlc/io.h b/include/dmlc/io.h
index 48cea081a..e6e01c6d3 100644
--- a/include/dmlc/io.h
+++ b/include/dmlc/io.h
@@ -32,16 +32,17 @@ class Stream {  // NOLINT(*)
   /*!
    * \brief reads data from a stream
    * \param ptr pointer to a memory buffer
-   * \param size block size
-   * \return the size of data read
+   * \param size The maximum number of bytes to read
+   * \return The number of bytes read from the stream
    */
-  virtual size_t Read(void *ptr, size_t size) = 0;
+  virtual size_t Read(void* ptr, size_t size) = 0;
   /*!
    * \brief writes data to a stream
    * \param ptr pointer to a memory buffer
-   * \param size block size
+   * \param size The maximum number of bytes to write
+   * \return The number of bytes written
    */
-  virtual void Write(const void *ptr, size_t size) = 0;
+  virtual size_t Write(const void* ptr, size_t size) = 0;
   /*! \brief virtual destructor */
   virtual ~Stream(void) {}
   /*!
diff --git a/include/dmlc/memory_io.h b/include/dmlc/memory_io.h
index 4e807585c..4e0d1a1f4 100644
--- a/include/dmlc/memory_io.h
+++ b/include/dmlc/memory_io.h
@@ -30,23 +30,24 @@ struct MemoryFixedSizeStream : public SeekStream {
         buffer_size_(buffer_size) {
     curr_ptr_ = 0;
   }
-  virtual size_t Read(void *ptr, size_t size) {
+  virtual size_t Read(void *ptr, size_t size) override {
     CHECK(curr_ptr_ + size <= buffer_size_);
     size_t nread = std::min(buffer_size_ - curr_ptr_, size);
     if (nread != 0) std::memcpy(ptr, p_buffer_ + curr_ptr_, nread);
     curr_ptr_ += nread;
     return nread;
   }
-  virtual void Write(const void *ptr, size_t size) {
-    if (size == 0) return;
+  virtual size_t Write(const void *ptr, size_t size) override {
+    if (size == 0) return 0;
     CHECK(curr_ptr_ + size <=  buffer_size_);
     std::memcpy(p_buffer_ + curr_ptr_, ptr, size);
     curr_ptr_ += size;
+    return size;
   }
-  virtual void Seek(size_t pos) {
+  virtual void Seek(size_t pos) override {
     curr_ptr_ = static_cast<size_t>(pos);
   }
-  virtual size_t Tell(void) {
+  virtual size_t Tell(void) override {
     return curr_ptr_;
   }
 
@@ -73,25 +74,26 @@ struct MemoryStringStream : public dmlc::SeekStream {
       : p_buffer_(p_buffer) {
     curr_ptr_ = 0;
   }
-  virtual size_t Read(void *ptr, size_t size) {
+  virtual size_t Read(void *ptr, size_t size) override {
     CHECK(curr_ptr_ <= p_buffer_->length());
     size_t nread = std::min(p_buffer_->length() - curr_ptr_, size);
     if (nread != 0) std::memcpy(ptr, &(*p_buffer_)[0] + curr_ptr_, nread);
     curr_ptr_ += nread;
     return nread;
   }
-  virtual void Write(const void *ptr, size_t size) {
-    if (size == 0) return;
+  virtual size_t Write(const void *ptr, size_t size) override {
+    if (size == 0) return 0;
     if (curr_ptr_ + size > p_buffer_->length()) {
       p_buffer_->resize(curr_ptr_+size);
     }
     std::memcpy(&(*p_buffer_)[0] + curr_ptr_, ptr, size);
     curr_ptr_ += size;
+    return size;
   }
-  virtual void Seek(size_t pos) {
+  virtual void Seek(size_t pos) override {
     curr_ptr_ = static_cast<size_t>(pos);
   }
-  virtual size_t Tell(void) {
+  virtual size_t Tell(void) override {
     return curr_ptr_;
   }
 
diff --git a/src/io/hdfs_filesys.cc b/src/io/hdfs_filesys.cc
index 09503c43c..3a45c898d 100644
--- a/src/io/hdfs_filesys.cc
+++ b/src/io/hdfs_filesys.cc
@@ -28,7 +28,7 @@ class HDFSStream : public SeekStream {
     }
   }
 
-  virtual size_t Read(void *ptr, size_t size) {
+  virtual size_t Read(void *ptr, size_t size) override {
     char *buf = static_cast<char*>(ptr);
     size_t nleft = size;
     size_t nmax = static_cast<size_t>(std::numeric_limits<tSize>::max());
@@ -48,7 +48,7 @@ class HDFSStream : public SeekStream {
     return size - nleft;
   }
 
-  virtual void Write(const void *ptr, size_t size) {
+  virtual size_t Write(const void *ptr, size_t size) override {
     const char *buf = reinterpret_cast<const char*>(ptr);
     size_t nleft = size;
     // When using builtin-java classes to write, the maximum write size
@@ -70,14 +70,15 @@ class HDFSStream : public SeekStream {
         LOG(FATAL) << "HDFSStream.hdfsWrite Error:" << strerror(errsv);
       }
     }
+    return size - nleft;
   }
-  virtual void Seek(size_t pos) {
+  virtual void Seek(size_t pos) override {
     if (hdfsSeek(fs_, fp_, pos) != 0) {
       int errsv = errno;
       LOG(FATAL) << "HDFSStream.hdfsSeek Error:" << strerror(errsv);
     }
   }
-  virtual size_t Tell(void) {
+  virtual size_t Tell(void) override {
     tOffset offset = hdfsTell(fs_, fp_);
     if (offset == -1) {
       int errsv = errno;
diff --git a/src/io/local_filesys.cc b/src/io/local_filesys.cc
index ce90f2863..e51cba8ea 100644
--- a/src/io/local_filesys.cc
+++ b/src/io/local_filesys.cc
@@ -31,21 +31,22 @@ class FileStream : public SeekStream {
   virtual ~FileStream(void) {
     this->Close();
   }
-  virtual size_t Read(void *ptr, size_t size) {
+  virtual size_t Read(void *ptr, size_t size) override {
     return std::fread(ptr, 1, size, fp_);
   }
-  virtual void Write(const void *ptr, size_t size) {
+  virtual size_t Write(const void *ptr, size_t size) override {
     CHECK(std::fwrite(ptr, 1, size, fp_) == size)
         << "FileStream.Write incomplete";
+    return 0;
   }
-  virtual void Seek(size_t pos) {
+  virtual void Seek(size_t pos) override {
 #ifndef _MSC_VER
     CHECK(!std::fseek(fp_, static_cast<long>(pos), SEEK_SET));  // NOLINT(*)
 #else  // _MSC_VER
     CHECK(!_fseeki64(fp_, pos, SEEK_SET));
 #endif  // _MSC_VER
   }
-  virtual size_t Tell(void) {
+  virtual size_t Tell(void) override {
 #ifndef _MSC_VER
     return std::ftell(fp_);
 #else  // _MSC_VER
diff --git a/src/io/s3_filesys.cc b/src/io/s3_filesys.cc
index 778613092..f597b2f98 100644
--- a/src/io/s3_filesys.cc
+++ b/src/io/s3_filesys.cc
@@ -424,23 +424,24 @@ class CURLReadStreamBase : public SeekStream {
   virtual ~CURLReadStreamBase() {
     this->Cleanup();
   }
-  virtual size_t Tell(void) {
+  virtual size_t Tell(void) override {
     return curr_bytes_;
   }
   virtual bool AtEnd(void) const {
     return at_end_;
   }
-  virtual void Write(const void *ptr, size_t size) {
+  virtual size_t Write(const void *ptr, size_t size) override {
     LOG(FATAL) << "CURL.ReadStream cannot be used for write";
+    return 0;
   }
   // lazy seek function
-  virtual void Seek(size_t pos) {
+  virtual void Seek(size_t pos) override {
     if (curr_bytes_ != pos) {
       this->Cleanup();
       curr_bytes_ = pos;
     }
   }
-  virtual size_t Read(void *ptr, size_t size);
+  virtual size_t Read(void *ptr, size_t size) override ;
 
  protected:
   CURLReadStreamBase()
@@ -790,11 +791,11 @@ class WriteStream : public Stream {
     ecurl_ = curl_easy_init();
     this->Init();
   }
-  virtual size_t Read(void *ptr, size_t size) {
+  virtual size_t Read(void *ptr, size_t size) override {
     LOG(FATAL) << "S3.WriteStream cannot be used for read";
     return 0;
   }
-  virtual void Write(const void *ptr, size_t size);
+  virtual size_t Write(const void *ptr, size_t size) override;
   // destructor
   virtual ~WriteStream() {
     this->Close();
@@ -863,13 +864,14 @@ class WriteStream : public Stream {
   void Finish(void);
 };
 
-void WriteStream::Write(const void *ptr, size_t size) {
+size_t WriteStream::Write(const void *ptr, size_t size) {
   size_t rlen = buffer_.length();
   buffer_.resize(rlen + size);
   std::memcpy(BeginPtr(buffer_) + rlen, ptr, size);
   if (buffer_.length() >= max_buffer_size_) {
     this->Upload();
   }
+  return size;
 }
 
 void WriteStream::Run(const std::string &method,
diff --git a/src/io/single_file_split.h b/src/io/single_file_split.h
index 9d2f9805e..f25c9fdb8 100644
--- a/src/io/single_file_split.h
+++ b/src/io/single_file_split.h
@@ -70,8 +70,9 @@ class SingleFileSplit : public InputSplit {
     CHECK(part_index == 0 && num_parts == 1);
     this->BeforeFirst();
   }
-  virtual void Write(const void * /*ptr*/, size_t /*size*/) {
+  virtual size_t Write(const void * /*ptr*/, size_t /*size*/) {
     LOG(FATAL) << "InputSplit do not support write";
+    return 0;
   }
   virtual bool NextRecord(Blob *out_rec) {
     if (chunk_begin_ == chunk_end_) {