diff --git a/src/thread/pthread/SDL_sysmutex.c b/src/thread/pthread/SDL_sysmutex.c
index 4284fa9bf..cc2d75632 100644
--- a/src/thread/pthread/SDL_sysmutex.c
+++ b/src/thread/pthread/SDL_sysmutex.c
@@ -21,6 +21,7 @@
 */
 #include "SDL_config.h"
 
+#define _GNU_SOURCE
 #include <pthread.h>
 
 #include "SDL_thread.h"