Preprocessor Directives

A preprocessor directive is a directive, rather than a keyword; it tells the preprocessor to perform a search-and-replace.

A preprocessor directive begins with # followed by spaces or tabs, then the directive name, the term, and the value; it ends as soon as it encounters the first newline.

Any \ followed by a newline, i.e. \n, is removed. These make directives logically only one line long.