1 files changed, 12 insertions, 1 deletions
diff --git a/signal.c b/signal.c
index ba19e6d0af..f419d45845 100644
--- a/signal.c
+++ b/signal.c
@@ -23,7 +23,7 @@
 # include "nacl/signal.h"
 #endif
 
-#ifdef NEED_RUBY_ATOMIC_EXCHANGE
+#ifdef NEED_RUBY_ATOMIC_OPS
 rb_atomic_t
 ruby_atomic_exchange(rb_atomic_t *ptr, rb_atomic_t val)
 {
@@ -31,6 +31,17 @@ ruby_atomic_exchange(rb_atomic_t *ptr, rb_atomic_t val)
     *ptr = val;
     return old;
 }
+
+rb_atomic_t
+ruby_atomic_compare_and_swap(rb_atomic_t *ptr, rb_atomic_t cmp,
+			     rb_atomic_t newval)
+{
+    rb_atomic_t old = *ptr;
+    if (old == cmp) {
+      *ptr = newval;
+    }
+    return old;
+}
 #endif
 
 #if defined(__BEOS__) || defined(__HAIKU__)