Index: lib/Fuzzer/CMakeLists.txt
===================================================================
--- lib/Fuzzer/CMakeLists.txt
+++ lib/Fuzzer/CMakeLists.txt
@@ -11,8 +11,9 @@
   add_library(LLVMFuzzerNoMainObjects OBJECT
     FuzzerCrossOver.cpp
     FuzzerDriver.cpp
-    FuzzerExtFunctionsDlsym.cpp
-    FuzzerExtFunctionsWeak.cpp
+    FuzzerExtFunctionsApple.cpp
+    FuzzerExtFunctionsLinux.cpp
+    FuzzerExtFunctionsWindows.cpp
     FuzzerIO.cpp
     FuzzerIOPosix.cpp
     FuzzerIOWindows.cpp
Index: lib/Fuzzer/FuzzerCorpus.h
===================================================================
--- lib/Fuzzer/FuzzerCorpus.h
+++ lib/Fuzzer/FuzzerCorpus.h
@@ -14,6 +14,7 @@
 
 #include <random>
 #include <unordered_set>
+#include <numeric>
 
 #include "FuzzerDefs.h"
 #include "FuzzerIO.h"
Index: lib/Fuzzer/FuzzerExtFunctionsApple.cpp
===================================================================
--- lib/Fuzzer/FuzzerExtFunctionsApple.cpp
+++ lib/Fuzzer/FuzzerExtFunctionsApple.cpp
@@ -1,4 +1,4 @@
-//===- FuzzerExtFunctionsDlsym.cpp - Interface to external functions ------===//
+//===- FuzzerExtFunctionsApple.cpp - Interface to external functions ------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -13,8 +13,8 @@
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
 #if LIBFUZZER_APPLE
-
 #include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
 #include <dlfcn.h>
 
 using namespace fuzzer;
Index: lib/Fuzzer/FuzzerExtFunctionsLinux.cpp
===================================================================
--- lib/Fuzzer/FuzzerExtFunctionsLinux.cpp
+++ lib/Fuzzer/FuzzerExtFunctionsLinux.cpp
@@ -1,4 +1,4 @@
-//===- FuzzerExtFunctionsWeak.cpp - Interface to external functions -------===//
+//===- FuzzerExtFunctionsLinux.cpp - Interface to external functions ------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -14,9 +14,8 @@
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
 #if LIBFUZZER_LINUX
-
-#include "FuzzerIO.h"
 #include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
 
 extern "C" {
 // Declare these symbols as weak to allow them to be optionally defined.
Index: lib/Fuzzer/FuzzerExtFunctionsWindows.cpp
===================================================================
--- /dev/null
+++ lib/Fuzzer/FuzzerExtFunctionsWindows.cpp
@@ -0,0 +1,53 @@
+//===- FuzzerExtFunctionsWindows.cpp - Interface to external functions ----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Implementation for Windows.
+//===----------------------------------------------------------------------===//
+#include "FuzzerDefs.h"
+#if LIBFUZZER_WINDOWS
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+
+using namespace fuzzer;
+
+extern "C" {
+// Declare these symbols as weak to allow them to be optionally defined.
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  RETURN_TYPE NAME ## Def FUNC_SIG {                                           \
+    Printf("ERROR: Function \"%s\" not defined.\n", #NAME);                    \
+    exit(1);                                                                   \
+  }                                                                            \
+  RETURN_TYPE NAME FUNC_SIG __attribute__ ((weak, alias (#NAME "Def")));
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+template <typename T>
+static T* GetFnPtr(T *Fun, T *FunDef, const char *FnName, bool WarnIfMissing) {
+  if (Fun == FunDef) {
+    if (WarnIfMissing)
+      Printf("WARNING: Failed to find function \"%s\".\n", FnName);
+    return nullptr;
+  }
+  return Fun;
+}
+
+namespace fuzzer {
+
+ExternalFunctions::ExternalFunctions() {
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  this->NAME = GetFnPtr<decltype(::NAME)>(::NAME, ::NAME ## Def, #NAME, WARN);
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+} // namespace fuzzer
+#endif // LIBFUZZER_WINDOWS