diff --git a/file_writer.cc b/file_writer.cc
new file mode 100644
index 0000000..07bdb46
--- /dev/null
+++ b/file_writer.cc
@@ -0,0 +1,47 @@
+// Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "update_engine/file_writer.h"
+#include <errno.h>
+
+namespace chromeos_update_engine {
+
+int DirectFileWriter::Open(const char* path, int flags, mode_t mode) {
+  CHECK_EQ(fd_, -1);
+  fd_ = open(path, flags, mode);
+  if (fd_ < 0)
+    return -errno;
+  return 0;
+}
+
+int DirectFileWriter::Write(const void* bytes, size_t count) {
+  CHECK_GE(fd_, 0);
+  const char* char_bytes = reinterpret_cast<const char*>(bytes);
+
+  size_t bytes_written = 0;
+  while (bytes_written < count) {
+    ssize_t rc = write(fd_, char_bytes + bytes_written,
+                       count - bytes_written);
+    if (rc < 0)
+      return -errno;
+    bytes_written += rc;
+  }
+  CHECK_EQ(bytes_written, count);
+  return bytes_written;
+}
+
+int DirectFileWriter::Close() {
+  CHECK_GE(fd_, 0);
+  int rc = close(fd_);
+
+  // This can be any negative number that's not -1. This way, this FileWriter
+  // won't be used again for another file.
+  fd_ = -2;
+
+  if (rc < 0)
+    return -errno;
+  return rc;
+}
+
+}  // namespace chromeos_update_engine