1 files changed, 3 insertions, 0 deletions
diff --git a/vm_core.h b/vm_core.h
index ea59e417ef..7852ca4625 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -608,6 +608,9 @@ typedef struct rb_vm_struct {
     VALUE thgroup_default;
     int living_thread_num;
 
+    /* set in single-threaded processes only: */
+    volatile int ubf_async_safe;
+
     unsigned int running: 1;
     unsigned int thread_abort_on_exception: 1;
     unsigned int thread_report_on_exception: 1;