Index: lib/Fuzzer/FuzzerUtil.cpp
===================================================================
--- lib/Fuzzer/FuzzerUtil.cpp
+++ lib/Fuzzer/FuzzerUtil.cpp
@@ -113,7 +113,13 @@
 void SetSigTermHandler() { SetSigaction(SIGTERM, InterruptHandler); }
 
 int NumberOfCpuCores() {
+#ifdef __linux__
   FILE *F = popen("nproc", "r");
+#elif __APPLE__
+  FILE *F = popen("sysctl -n hw.ncpu", "r");
+#else
+#error "NumberOfCpuCores() is not implemented for your platform"
+#endif
   int N = 0;
   if (fscanf(F, "%d", &N) != 1)
     N = 1;