diff --git a/lib/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index 1eceaaa615e90f77d11a3c74f602d3ae3d1acfc5..dd2a89a18b7213b778f20bed4488e4231154be43 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -365,7 +365,7 @@ protected:
     DefineStd(Builder, "unix", Opts);
     Builder.defineMacro("__ELF__");
     if (Opts.POSIXThreads)
-      Builder.defineMacro("_POSIX_THREADS");
+      Builder.defineMacro("_REENTRANT");
   }
 public:
   OpenBSDTargetInfo(const std::string &triple)