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Flex supports several directives, only a few of them being presented below, see (FIXME: cite Flex documentation.), for more information. Most of them have command line option equivalent, but in typical uses it is better to embed them within the file.
yy_flex_debug, which, when set to a non zero value, triggers tracing messages on the standard error output.
You are encouraged to use this option, in particular when developing
your scanner, and to have some option to set
particular, never write
printf-like tracing code in your scanner:
that's an absolute waste of time.
yywrap. These two functions are beyond the scope of this book and won't be detailed. Nevertheless we present these options so that (i) we no longer need the Flex library (which provides a default
yywrap), and (ii) our scanners compile without triggering
warning: `yyunput' defined but not used.
yyprefix with prefix. It also changes the default output file from