diff --git a/src/os_unix.c b/src/os_unix.c
index 94e1a53..4d902d9 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -6194,7 +6194,9 @@
 
 	struct timeval  tv;
 	struct timeval	*tvp;
-	fd_set		rfds, wfds, efds;
+	// These are static because they can take 8 Kbyte each and cause the
+	// signal stack to run out with -O3.
+	static fd_set	rfds, wfds, efds;
 	int		maxfd;
 	long		towait = msec;