diff --git a/kernel/include/kstring.h b/kernel/include/kstring.h
index 6093263..399f653 100644
--- a/kernel/include/kstring.h
+++ b/kernel/include/kstring.h
@@ -1,6 +1,11 @@
+#ifndef __K_STRING
+#define __K_STRING
+
 #include <kstdint.h>
 
 int strcmp(const char *s1, const char *s2);
 char *strcpy(char *dst, const char *src);
 char *strcat(char *dst, const char *src);
 int strlen(const char *s);
+
+#endif