diff --git a/src/clock.hpp b/src/clock.hpp
index 22fb8ae6..b9ca50b3 100644
--- a/src/clock.hpp
+++ b/src/clock.hpp
@@ -38,9 +38,13 @@
 #include <time.h>
 #include <sys/time.h>
 int clock_gettime (int clock_id, timespec *ts);
+#ifndef CLOCK_REALTIME
 #define CLOCK_REALTIME 0
+#endif
+#ifndef HAVE_CLOCK_GETTIME
 #define HAVE_CLOCK_GETTIME
 #endif
+#endif
 
 namespace zmq
 {