A user-defined function (UDF) is a way to extend MariaDB with a new function that works like a native (built-in) MariaDB function such as ABS() or CONCAT().

Statements making use of user-defined functions are not safe for replication.

For an example, see sql/udf_example.cc in the source tree. For a collection of existing UDFs see http://www.mysqludf.org/.

There are alternative ways to add a new function: writing a native function (which requires modifying and compiling the server source code), or writing a stored function.