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A.6 Deprecated interfaces, , Retargeting Flex

Long-time users of Flex and its predecessor, Lex, may notice that the examples in this manual look a little different than they used to. They have changed because of the support for targeting languages other than C/C++.

All the old interfaces are still in place for legacy C code to use. But some are now deprecated and should not be used in new code. Doing so will hinder forward portability someday.

These changes are necessary because the documented Flex interface can no longer rely on controlling scanner generation by defining preprocessor macros. Most languages other than C/C++ don’t have text macros, and none of those that do emulate C #define closely enough to preserve the behavior dependent on it. Thus interface calls in multi-language Flex need to be functions, at least syntactically (though many are still implemented as macros for C/C++).

A list of deprecated interfaces and their replacements follows. Again, all are still available in the default C/C++ back end, but not in any other.

• BEGIN: Replaced by yybegin()
• ECHO: Replaced by yyecho()
• REJECT: Replaced by yyreject()
• #define YY_DECL: Replaced by the yydecl option.
• #define YYLMAX: Replaced by the yylmax option.
• #define YY_INPUT: Replaced by the noyyread option.
• YYSTART: Replaced by yystart().
• YY_AT_BOL(): Replaced by yyatbol().
• yy_set_bol(): Replaced by yysetbol()
• input(): Replaced by yyinput(). This function was already yyinput() in the C++ back end.
• : unput(): Replaced by yyunput().
• #define YY_EXTRA_TYPE: Replaced by the extra-type option.
• #define YY_USER_INIT: Replaced by the user-init option.
• #define YY_USER_ACTION replaced by pre-action option.
• #define YY_BREAK replaced by post-action option.
• YYSTATE: is accepted as an alias for yystart() (since that is what’s used by AT&T lex).
• YY_FLUSH_BUFFER: replaced by yy_flush_current_buffer().
• YY_CURRENT_BUFFER: replaced by yy_current_buffer().
• YY_BUFFER_STATE: replaced by yybuffer.
• YY_FATAL_ERROR: replaced by yypanic(). To supply your own handler, simply use %option noyypanic so the default handler is not generated, then write your own handler with the same prototype.
• YY_NEW_FILE: In previous versions of flex, when assigning yyin to a new input file, after doing the assignment you had to call the special action YY_NEW_FILE. This is no longer necessary.
• FLEX_SCANNER: Not meaningful outside of the C back end, and not defined.
• FLEX_DEBUG: Outside the default C back end, this is a constant of type bool rather rthan a preprocessor symbol.
• FLEX_BETA: Its behavior can’t be replicated without the C preprocessor. Test for YY_FLEX_SUBMINOR_VERSION instead.
• YY_BUF_SIZE: replaced by %option bufsize.
• : yyterminate(): Backends that don’t use the C preprocessor cannot support redefining this hook with #define. Instead, set it with the yyterminate option.

Where entry point names have been downcased and had underscores removed, it’s because some languages in the expected target set for Flex - notably Go and Python - have validator/linter tools that would complain about uppercase or underscores or both. It would obviously be best if the parts of the public API that have to change to conform to local conventions are small and not commonly used.

This is also why the functions yy_current_buffer(), yy_flush_current_buffer(), and yy_set_interactive() do not have rewrite rules (see See option-rewrite); simply removing the underscores would have produced names that are uncomfortably long and hard on the eyeballs. We leave it to the API binding for each individual target language to choose whether they should be left as is, camel-cased, or otherwise adapted to local conventions.