How to use trigger in VoltDB
VoltDB has very limited support for triggers in DDL, the way some traditional SQL databases do. You can tell it to run a SQL statement when a table reaches a certain row count, which is often used to truncate older data.

Within stored procedures, there is tremendous support for very flexible logic. You can perform arbitrary database operations when certain conditions are met.

VoltDB does not have an official way to call a C function or executable with data. Perhaps you could expand on what you're trying to accomplish? If this is part of a change-data-capture scenario, you might want to take a look at VoltDB's Export technology, which is a push-based system for moving data to a downstream system in a reliable way.

https://voltdb.com/products/fast-data-ingest-and-export
https://docs.voltdb.com/UsingVoltDB/ChapExport.php