diff --git a/lib/libc/stdlib/exit.c b/lib/libc/stdlib/exit.c
index a8f6e24164d..5df19dc1bd4 100644
--- a/lib/libc/stdlib/exit.c
+++ b/lib/libc/stdlib/exit.c
@@ -41,6 +41,15 @@ static char sccsid[] = "@(#)exit.c	8.1 (Berkeley) 6/4/93";
 
 void (*__cleanup)();
 
+/*
+ * This variable is zero until a process has created a thread.
+ * It is used to avoid calling locking functions in libc when they
+ * are not required. By default, libc is intended to be(come)
+ * thread-safe, but without a (significant) penalty to non-threaded
+ * processes.
+ */
+int	__isthreaded	= 0;
+
 /*
  * Exit, flushing stdio buffers if necessary.
  */