diff --git a/thpool.c b/thpool.c
index 004b4bc..0d60a65 100644
--- a/thpool.c
+++ b/thpool.c
@@ -14,6 +14,9 @@
 #ifndef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200809L
 #endif
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 500
+#endif
 #endif
 #include <unistd.h>
 #include <signal.h>