+ /** @brief Return the transaction's ID.
+ *
+ * This returns the identifier associated with this transaction. For a
+ * read-only transaction, this corresponds to the snapshot being read;
+ * concurrent readers will frequently have the same transaction ID.
+ *
+ * @param[in] txn A transaction handle returned by #mdb_txn_begin()
+ * @return A transaction ID, valid if input is an active transaction.
+ */
+size_t mdb_txn_id(MDB_txn *txn);
+