19.4.1 Declaring a Scanner As Reentrant

%option reentrant (–reentrant) must be specified.

Notice that %option reentrant is specified in the above example (see Reentrant Example. Had this option not been specified, flex would have happily generated a non-reentrant scanner without complaining. You may explicitly specify %option noreentrant, if you do not want a reentrant scanner, although it is not necessary. The default is to generate a non-reentrant scanner.