diff --git a/runtime/base/scoped_flock.cc b/runtime/base/scoped_flock.cc
index 351de3d..bf091d0 100644
--- a/runtime/base/scoped_flock.cc
+++ b/runtime/base/scoped_flock.cc
@@ -58,6 +58,22 @@
   }
 }
 
+bool ScopedFlock::Init(File* file, std::string* error_msg) {
+  file_.reset(new File(dup(file->Fd())));
+  if (file_->Fd() == -1) {
+    file_.reset();
+    *error_msg = StringPrintf("Failed to duplicate open file '%s': %s",
+                              file->GetPath().c_str(), strerror(errno));
+    return false;
+  }
+  if (0 != TEMP_FAILURE_RETRY(flock(file_->Fd(), LOCK_EX))) {
+    file_.reset();
+    *error_msg = StringPrintf("Failed to lock file '%s': %s", file->GetPath().c_str(), strerror(errno));
+    return false;
+  }
+  return true;
+}
+
 File* ScopedFlock::GetFile() {
   CHECK(file_.get() != NULL);
   return file_.get();
diff --git a/runtime/base/scoped_flock.h b/runtime/base/scoped_flock.h
index f8ed805..08612e3 100644
--- a/runtime/base/scoped_flock.h
+++ b/runtime/base/scoped_flock.h
@@ -37,6 +37,10 @@
   // changed (usually due to a new file being created at the same path)
   // between attempts to lock it.
   bool Init(const char* filename, std::string* error_msg);
+  // Attempt to acquire an exclusive file lock (see flock(2)) on 'file'.
+  // Returns true if the lock could be acquired or false if an error
+  // occured.
+  bool Init(File* file, std::string* error_msg);
 
   // Returns the (locked) file associated with this instance.
   File* GetFile();
@@ -45,6 +49,7 @@
   bool HasFile();
 
   ~ScopedFlock();
+
  private:
   std::unique_ptr<File> file_;
   DISALLOW_COPY_AND_ASSIGN(ScopedFlock);