diff --git a/sources/util.h b/sources/util.h
index 5847e4ae..e7dd6e48 100644
--- a/sources/util.h
+++ b/sources/util.h
@@ -7,6 +7,12 @@
  * Scalable PostgreSQL connection pooler.
  */
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
+#include <stdio.h>
+
 static inline int od_vsnprintf(char *buf, int size, char *fmt, va_list args)
 {
 	int rc;