diff --git a/tests/tests2/24_math_library.c b/tests/tests2/24_math_library.c
index 4cc3d9e..514a25f 100644
--- a/tests/tests2/24_math_library.c
+++ b/tests/tests2/24_math_library.c
@@ -1,3 +1,5 @@
+#define _ISOC99_SOURCE 1
+
 #include <stdio.h>
 #include <math.h>