diff --git a/src/functional/search_hsearch.c b/src/functional/search_hsearch.c
index dc1538e..be9fb64 100644
--- a/src/functional/search_hsearch.c
+++ b/src/functional/search_hsearch.c
@@ -1,3 +1,6 @@
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
+#endif
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
diff --git a/src/functional/search_insque.c b/src/functional/search_insque.c
index 0da76a8..0f4c370 100644
--- a/src/functional/search_insque.c
+++ b/src/functional/search_insque.c
@@ -1,3 +1,6 @@
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
+#endif
 #include <stdlib.h>
 #include <search.h>
 #include "test.h"
diff --git a/src/functional/search_lsearch.c b/src/functional/search_lsearch.c
index 8aaffa6..711a7a0 100644
--- a/src/functional/search_lsearch.c
+++ b/src/functional/search_lsearch.c
@@ -1,3 +1,6 @@
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
+#endif
 #include <string.h>
 #include <search.h>
 #include "test.h"
diff --git a/src/functional/search_tsearch.c b/src/functional/search_tsearch.c
index 80fa5de..b79095f 100644
--- a/src/functional/search_tsearch.c
+++ b/src/functional/search_tsearch.c
@@ -1,3 +1,6 @@
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
+#endif
 #include <stdlib.h>
 #include <string.h>
 #include <search.h>