SQLite extensions are libraries with dynamic linkage. You can find some examples here (This is a fossil repository, click on “login/fill captcha” to enable hyperlinks). See for example md5.c.
load_extension must be enabled in SQLite (pragma IIRC)
it requires as first argument the path of the library
The second argument is the name of the entry point function (in md5.c it is sqlite3_extension_init). Its prototype must be int(sqlite3*, char **, const sqlite3_api_routines *).
In SQL you can try SELECT load_extension('md5.so', 'sqlite3_extension_init'); or simply SELECT load_extension('md5.so');
You can try to compile md5.c, and from the sqlite shell use .load md5.so
