add bitmap .

src/sync/sync.h

@@ -0,0 +1,20 @@
+#ifndef XGBOOST_SYNC_SYNC_H_
+#define XGBOOST_SYNC_SYNC_H_
+/*!
+ * \file sync.h
+ * \brief interface to do synchronization
+ * \author Tianqi Chen
+ */
+namespace xgboost {
+namespace sync {
+/*! 
+ * \brief synchronization context interface of xgboost,
+ *        will be provided as a singleton
+ */
+class IContext {
+  
+};
+
+}  // namespace sync
+}  // namespace xgboost
+#endif