diff --git a/c-programming/math/opt_int_div.h b/c-programming/math/opt_int_div.h
index 7e1e5e1..fc8d1e1 100644
--- a/c-programming/math/opt_int_div.h
+++ b/c-programming/math/opt_int_div.h
@@ -9,6 +9,9 @@
  * License: Unlicense
  */
 
+#ifndef _OPT_INT_DIV_H
+#define _OPT_INT_DIV_H
+
 #include <math.h>
 #include <stdint.h>
 
@@ -46,3 +49,5 @@
         :                                                                     \
             ((a) / (b)) ))                                                    \
     ) /* end */
+
+#endif /* _OPT_INT_DIV_H */
\ No newline at end of file