diff --git a/src/libCom/calc/calcPerform.c b/src/libCom/calc/calcPerform.c
index d77a2a2db..f8cb182ba 100644
--- a/src/libCom/calc/calcPerform.c
+++ b/src/libCom/calc/calcPerform.c
@@ -47,6 +47,7 @@
 #include	"epicsMath.h"
 #include	"postfix.h"
 #include	"postfixPvt.h"
+#include	"osiUnistd.h"
 
 static double	local_random(void);