19.4.2 The Extra Argument

All functions other than rule hooks take one additional argument: yyscanner.

Notice that the calls to yy_push_state and yy_pop_state both have an argument, yyscanner , that is not present in a non-reentrant scanner. Here are the declarations of yy_push_state and yy_pop_state in the reentrant scanner:

    static void yy_push_state  ( int new_state , yyscan_t yyscanner ) ;
    static void yy_pop_state  ( yyscan_t yyscanner  ) ;

Notice that the argument yyscanner appears in the declaration of both functions. In fact, all flex functions in a reentrant scanner have this additional argument, except for rule hooks which get it supplied implicitly.

It is always the last argument in the argument list, it is always of type yyscan_t (which is typedef’d to void *) and it is always named yyscanner. As you may have guessed, yyscanner is a pointer to an opaque data structure encapsulating the current state of the scanner. For a list of function declarations, see Functions Available in Reentrant Scanners. Rule hooks, such as yybegin(), yyecho(), yyreject(), and yystart(), do not take this additional argument.

Rule hooks don’t need to take a scanner context argument because, under the hood, the context is supplied by the call location.

The type name yscan_t follows C conventions. It may differ in other target languages.