diff --git a/src/libCom/osi/epicsStdio.h b/src/libCom/osi/epicsStdio.h
index 497388f2b..e2d343ef1 100644
--- a/src/libCom/osi/epicsStdio.h
+++ b/src/libCom/osi/epicsStdio.h
@@ -17,6 +17,7 @@
 extern "C" {
 #endif
 
+#include <stdarg.h>
 #include "shareLib.h"
 
 epicsShareFunc int epicsShareAPI epicsSnprintf(