The book was written 20 years ago, before the C99 spec was released. main() was acceptable but discouraged in C89, which is why -Wall (or more precisely -Wimpli

The book was written 20 years ago, before the C99 spec was released. main() was acceptable but discouraged in C89, which is why -Wall (or more precisely -Wimplicit-int) displays a warning. It's disallowed by the C99 spec and will always give you a warning.