Index: lib/Fuzzer/FuzzerUtil.cpp
===================================================================
--- lib/Fuzzer/FuzzerUtil.cpp
+++ lib/Fuzzer/FuzzerUtil.cpp
@@ -113,11 +113,31 @@
 void SetSigTermHandler() { SetSigaction(SIGTERM, InterruptHandler); }
 
 int NumberOfCpuCores() {
-  FILE *F = popen("nproc", "r");
+  const char *cmdLine = nullptr;
+  if (LIBFUZZER_LINUX) {
+    cmdLine = "nproc";
+  } else if (LIBFUZZER_APPLE) {
+    cmdLine = "sysctl -n hw.ncpu";
+  } else {
+    assert(0 && "NumberOfCpuCores() is not implemented for your platform");
+  }
+
+  FILE *F = popen(cmdLine, "r");
   int N = 0;
-  if (fscanf(F, "%d", &N) != 1)
+  if (!F || fscanf(F, "%d", &N) != 1) {
+    Printf("WARNING: Failed to parse output of command \"%s\" in %s(). "
+           "Assuming CPU count of 1.\n",
+           cmdLine, __func__);
+    N = 1;
+  }
+
+  int PcloseRet = pclose(F);
+  if (PcloseRet != 0) {
+    Printf("WARNING: Executing command \"%s\" failed in %s(). ",
+           "Assuming CPU count of 1.\n", cmdLine, __func__);
     N = 1;
-  fclose(F);
+  }
+  assert(N > 0 && "Number of cpu cores must be > 0");
   return N;
 }