Index: lib/Fuzzer/CMakeLists.txt
===================================================================
--- lib/Fuzzer/CMakeLists.txt
+++ lib/Fuzzer/CMakeLists.txt
@@ -12,6 +12,7 @@
     FuzzerCrossOver.cpp
     FuzzerTraceState.cpp
     FuzzerDriver.cpp
+    FuzzerExtFunctions.cpp
     FuzzerIO.cpp
     FuzzerLoop.cpp
     FuzzerMutate.cpp
Index: lib/Fuzzer/FuzzerExtFunctions.h
===================================================================
--- /dev/null
+++ lib/Fuzzer/FuzzerExtFunctions.h
@@ -0,0 +1,31 @@
+//===- FuzzerExtFunctions.h - Interface to external functions ---*- C++ -* ===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Defines an interface to (possibly optional) functions.
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_FUZZER_EXT_FUNCTIONS_H
+#define LLVM_FUZZER_EXT_FUNCTIONS_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+namespace fuzzer {
+
+struct ExternalFunctions {
+  // Initialize function pointers.
+  void Init();
+
+  // Optional user functions
+  int (*LLVMFuzzerInitialize)(int *argc, char ***argv) = nullptr;
+  size_t (*LLVMFuzzerCustomMutator)(uint8_t *Data, size_t Size, size_t MaxSize,
+                                    unsigned int Seed) = nullptr;
+
+  // TODO: Sanitizer functions
+};
+}
+#endif
Index: lib/Fuzzer/FuzzerExtFunctions.cpp
===================================================================
--- /dev/null
+++ lib/Fuzzer/FuzzerExtFunctions.cpp
@@ -0,0 +1,44 @@
+//===- FuzzerExtFunctions.cpp - Interface to external functions --*- C++ -* ==//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Implementation for operating systems that support dlsym()
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerInternal.h"
+#include <dlfcn.h>
+
+using namespace fuzzer;
+
+template <typename T>
+static T GetFnPtr(const char *FnName, bool WarnIfMissing) {
+  dlerror(); // Clear any previous errors
+  void *Fn = dlsym(RTLD_DEFAULT, FnName);
+  if (Fn == nullptr) {
+    if (WarnIfMissing) {
+      const char *ErrorMsg = dlerror();
+      Printf("WARNING: Failed to find function \"%s\"", FnName);
+      if (ErrorMsg)
+        Printf("Reason %s.", ErrorMsg);
+      Printf("\n");
+    }
+  }
+  return reinterpret_cast<T>(Fn);
+}
+
+namespace fuzzer {
+
+void ExternalFunctions::Init() {
+#define INIT_FUNC(NAME, WARN)                                                  \
+  this->NAME = GetFnPtr<decltype(ExternalFunctions::NAME)>(#NAME, WARN)
+
+  INIT_FUNC(LLVMFuzzerInitialize, false);
+  INIT_FUNC(LLVMFuzzerCustomMutator, false);
+#undef INIT_FUNC
+}
+}
Index: lib/Fuzzer/FuzzerInternal.h
===================================================================
--- lib/Fuzzer/FuzzerInternal.h
+++ lib/Fuzzer/FuzzerInternal.h
@@ -25,6 +25,7 @@
 #include <unordered_set>
 #include <vector>
 
+#include "FuzzerExtFunctions.h"
 #include "FuzzerInterface.h"
 #include "FuzzerTracePC.h"
 
@@ -468,6 +469,9 @@
 
   // Need to know our own thread.
   static thread_local bool IsMyThread;
+
+  // Interface to functions that might be available.
+  ExternalFunctions EF;
 };
 
 }; // namespace fuzzer
Index: lib/Fuzzer/FuzzerLoop.cpp
===================================================================
--- lib/Fuzzer/FuzzerLoop.cpp
+++ lib/Fuzzer/FuzzerLoop.cpp
@@ -47,9 +47,6 @@
 __attribute__((weak)) uintptr_t
 __sanitizer_get_coverage_pc_buffer(uintptr_t **data);
 
-__attribute__((weak)) size_t LLVMFuzzerCustomMutator(uint8_t *Data, size_t Size,
-                                                     size_t MaxSize,
-                                                     unsigned int Seed);
 __attribute__((weak)) void __sanitizer_malloc_hook(void *ptr, size_t size);
 __attribute__((weak)) void __sanitizer_free_hook(void *ptr);
 __attribute__((weak)) void __lsan_enable();
@@ -150,7 +147,8 @@
 };
 
 Fuzzer::Fuzzer(UserCallback CB, MutationDispatcher &MD, FuzzingOptions Options)
-    : CB(CB), MD(MD), Options(Options) {
+    : CB(CB), MD(MD), Options(Options), EF(ExternalFunctions()) {
+  EF.Init();
   SetDeathCallback();
   InitializeTraceState();
   assert(!F);
@@ -692,9 +690,9 @@
 
   for (int i = 0; i < Options.MutateDepth; i++) {
     size_t NewSize = 0;
-    if (LLVMFuzzerCustomMutator)
-      NewSize = LLVMFuzzerCustomMutator(CurrentUnitData, Size,
-                                        Options.MaxLen, MD.GetRand().Rand());
+    if (EF.LLVMFuzzerCustomMutator)
+      NewSize = EF.LLVMFuzzerCustomMutator(CurrentUnitData, Size,
+                                           Options.MaxLen, MD.GetRand().Rand());
     else
       NewSize = MD.Mutate(CurrentUnitData, Size, Options.MaxLen);
     assert(NewSize > 0 && "Mutator returned empty unit");
Index: lib/Fuzzer/FuzzerMain.cpp
===================================================================
--- lib/Fuzzer/FuzzerMain.cpp
+++ lib/Fuzzer/FuzzerMain.cpp
@@ -9,18 +9,20 @@
 // main() and flags.
 //===----------------------------------------------------------------------===//
 
+#include "FuzzerExtFunctions.h"
 #include "FuzzerInterface.h"
 #include "FuzzerInternal.h"
 
 extern "C" {
 // This function should be defined by the user.
 int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
-// This function may optionally be defined by the user.
-__attribute__((weak)) int LLVMFuzzerInitialize(int *argc, char ***argv);
 }  // extern "C"
 
 int main(int argc, char **argv) {
-  if (LLVMFuzzerInitialize)
-    LLVMFuzzerInitialize(&argc, &argv);
+  // FIXME: We don't propagate these into libfuzzer's internals
+  fuzzer::ExternalFunctions EF;
+  EF.Init();
+  if (EF.LLVMFuzzerInitialize)
+    EF.LLVMFuzzerInitialize(&argc, &argv);
   return fuzzer::FuzzerDriver(argc, argv, LLVMFuzzerTestOneInput);
 }