Runtime systems may or may not have an automatically generated main
program. This is controlled by the --main option to spld.
If --main=user is given, a function user_main() must be
supplied:
int user_main(int argc, char *argv[])
user_main() is responsible for initializing the Prolog engine,
loading code, and issuing any Prolog queries. And alternative is to use
--main=none and write your own main() function.