diff --git a/src/libCom/Makefile b/src/libCom/Makefile
index 84614dfdc..f1142d5cb 100644
--- a/src/libCom/Makefile
+++ b/src/libCom/Makefile
@@ -3,6 +3,8 @@ include $(TOP)/configure/CONFIG
 
 LIBCOM = $(TOP)/src/libCom
 
+USR_CXXFLAGS_hpux += -D_REENTRANT
+
 #epicsVersion is created by this Makefile
 INC += epicsVersion.h