diff --git a/src/tests/system/libroot/posix/wcs_test.cpp b/src/tests/system/libroot/posix/wcs_test.cpp
index bfb55d5262..75ecd3bed5 100644
--- a/src/tests/system/libroot/posix/wcs_test.cpp
+++ b/src/tests/system/libroot/posix/wcs_test.cpp
@@ -3,7 +3,7 @@
  * Distributed under the terms of the MIT License.
  */
 
-#define __USE_GNU
+#define _GNU_SOURCE
 	// for wmempcpy() and wcschrnul()
 
 #include <errno.h>