diff --git a/src/include/ctype.h b/src/include/ctype.h
index ed4d8846..9f5127bf 100644
--- a/src/include/ctype.h
+++ b/src/include/ctype.h
@@ -11,6 +11,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #define isdigit(c)	((c) >= '0' && (c) <= '9')
 #define islower(c)	((c) >= 'a' && (c) <= 'z')
 #define isupper(c)	((c) >= 'A' && (c) <= 'Z')
+#define isxdigit(c)	(isdigit(c) || ((c) >= 'A' && (c) <= 'F') || ((c) >= 'a' && (c) <= 'f'))
 
 static inline unsigned char tolower(unsigned char c)
 {