Index: llvm/trunk/lib/Fuzzer/FuzzerInternal.h
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerInternal.h
+++ llvm/trunk/lib/Fuzzer/FuzzerInternal.h
@@ -27,6 +27,17 @@
 #include "FuzzerInterface.h"
 #include "FuzzerTracePC.h"
 
+// Platform detection.
+#ifdef __linux__
+#define LIBFUZZER_LINUX 1
+#define LIBFUZZER_APPLE 0
+#elif __APPLE__
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_APPLE 1
+#else
+#error "Support for your platform has not been implemented"
+#endif
+
 namespace fuzzer {
 
 typedef int (*UserCallback)(const uint8_t *Data, size_t Size);
Index: llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
+++ llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
@@ -437,9 +437,19 @@
 
 static thread_local MallocFreeTracer AllocTracer;
 
+// FIXME: The hooks only count on Linux because
+// on Mac OSX calls to malloc are intercepted before
+// thread local storage is initialised leading to
+// crashes when accessing ``AllocTracer``.
 extern "C" {
-void __sanitizer_malloc_hook(void *ptr, size_t size) { AllocTracer.Mallocs++; }
-void __sanitizer_free_hook(void *ptr) { AllocTracer.Frees++; }
+void __sanitizer_malloc_hook(void *ptr, size_t size) {
+  if (!LIBFUZZER_APPLE)
+    AllocTracer.Mallocs++;
+}
+void __sanitizer_free_hook(void *ptr) {
+  if (!LIBFUZZER_APPLE)
+    AllocTracer.Frees++;
+}
 }  // extern "C"
 
 void Fuzzer::ExecuteCallback(const uint8_t *Data, size_t Size) {