diff --git a/c/string.c b/c/string.c
index 9d83e3e..b29ea5f 100644
--- a/c/string.c
+++ b/c/string.c
@@ -2,6 +2,8 @@
   string functions
 */
 
+#define _XOPEN_SOURCE
+
 #include <sys/types.h>
 #include <sys/time.h>
 
diff --git a/c/time_unix.c b/c/time_unix.c
index d48da3f..4cca98b 100644
--- a/c/time_unix.c
+++ b/c/time_unix.c
@@ -1,3 +1,5 @@
+#define _XOPEN_SOURCE
+
 #include <sys/poll.h>
 #include <sys/time.h>
 
diff --git a/c/utf8.c b/c/utf8.c
index 2547e75..7c77b46 100644
--- a/c/utf8.c
+++ b/c/utf8.c
@@ -13,6 +13,8 @@
   A UTF-8 validation routine is included.
 */
 
+#define _XOPEN_SOURCE
+
 #include <assert.h>
 #include <stdarg.h>
 #include <stddef.h>