diff --git a/xlhtml/support.h b/xlhtml/support.h
index c4cdef5..a02b26a 100644
--- a/xlhtml/support.h
+++ b/xlhtml/support.h
@@ -1,12 +1,17 @@
+#ifndef __SUPPORT_H_INCLUDED
+#define __SUPPORT_H_INCLUDED
+
 #include "../cole/cole.h"
 
-void display_usage(void);
-void NumToDate(long num, int *year, int *month, int *day);
-U16 getShort(U8 *ptr);
-void getDouble(U8 *ptr, F64 *d);
-int null_string(U8 *str);
-void print_version(void);
-void do_cr(void);
-void FracToTime(U8 *cnum, int *hr, int *minut, int *sec, int *msec);
-S32 getLong(U8 *ptr);
-void RKtoDouble(S32 n, F64 *d);
+extern void display_usage(void);
+extern void NumToDate(long num, int *year, int *month, int *day);
+extern U16 getShort(U8 *ptr);
+extern void getDouble(U8 *ptr, F64 *d);
+extern int null_string(U8 *str);
+extern void print_version(void);
+extern void do_cr(void);
+extern void FracToTime(U8 *cnum, int *hr, int *minut, int *sec, int *msec);
+extern S32 getLong(U8 *ptr);
+extern void RKtoDouble(S32 n, F64 *d);
+
+#endif