The virtual table API is one of the more advanced SQLite APIs. In specific, it does very little hand-holding and will often fool those that make assumptions. The functions you need to write are often required to do a very specific set of operations. If you fail to do any one of those, or forget to initialize a data structure field, the result might very well be a bus error or segmentation fault.
That street goes two ways, however. While the SQLite core expects you to do your job, it does a very good job of always doing its job in a very predictable and documented way. Most of this code operates fairly deeply in the SQLite core, and SQLite does a solid job of protecting your code against odd user behavior. For example, none of the example code checks for NULL parameter values, as you can be sure SQLite will never allow a NULL database pointer (or some equally critical parameter) to be passed into your function.
Implementing a virtual table module is a bit like developing an aggregate function, only a lot more complex. You must write a series of functions that, taken together, define the behavior of the module. This block of functions is then registered under a module name.
int sqlite3_create_module( sqlite3 *db, const char *name, const sqlite3_module *module, void *udp )
Creates and registers a virtual table module with a database connection. The second parameter is the name of the module. The third parameter is a block of function pointers that implements the virtual table. ...