diff --git a/examples/posix_sockets/udp.c b/examples/posix_sockets/udp.c
index ecf30bf6e8..b49edf9fc6 100644
--- a/examples/posix_sockets/udp.c
+++ b/examples/posix_sockets/udp.c
@@ -18,6 +18,11 @@
  * @}
  */
 
+/* needed for posix usleep */
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
+
 #include <inttypes.h>
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/tests/posix_time/main.c b/tests/posix_time/main.c
index 5c94cbefb5..e79fbced2b 100644
--- a/tests/posix_time/main.c
+++ b/tests/posix_time/main.c
@@ -19,6 +19,11 @@
  * @}
  */
 
+/* needed for posix usleep */
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
+
 #include <stdio.h>
 #include <unistd.h>