(flex.info)Bison Bridge

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A.2 C Scanners with Bison Parsers
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This section describes the 'flex' features useful when integrating
'flex' with 'GNU bison'(1).  Skip this section if you are not using
'bison' with your scanner.  Here we discuss only the 'flex' half of the
'flex' and 'bison' pair.  We do not discuss 'bison' in any detail.  For
more information about generating 'bison' parsers, see Note:
(bison)Top.

   A compatible 'bison' scanner is generated by declaring '%option
bison-bridge' or by supplying '--bison-bridge' when invoking 'flex' from
the command line.  This instructs 'flex' that the macro 'yylval' may be
used.  The data type for 'yylval', 'YYSTYPE', is typically defined in a
header file, included in section 1 of the 'flex' input file.  For a list
of functions and macros available, Note: bison-functions.

   The declaration of yylex becomes,

           int yylex ( YYSTYPE * lvalp, yyscan_t scanner );

   If '%option bison-locations' is specified, then the declaration
becomes,

           int yylex ( YYSTYPE * lvalp, YYLTYPE * llocp, yyscan_t scanner );

   Note that the macros 'yylval' and 'yylloc' evaluate to pointers.
Support for 'yylloc' is optional in 'bison', so it is optional in 'flex'
as well.  The following is an example of a 'flex' scanner that is
compatible with 'bison'.

         /* Scanner for "C" assignment statements... sort of. */
         %{
         #include "y.tab.h"  /* Generated by bison. */
         %}
     
         %option bison-bridge bison-locations
         %
     
         [[:digit:]]+  { yylval->num = atoi(yytext);   return NUMBER;}
         [[:alnum:]]+  { yylval->str = strdup(yytext); return STRING;}
         "="|";"       { return yytext[0];}
         .  {}
         %

   As you can see, there really is no magic here.  We just use 'yylval'
as we would any other variable.  The data type of 'yylval' is generated
by 'bison', and included in the file 'y.tab.h'.  Here is the
corresponding 'bison' parser:

         /* Parser to convert "C" assignments to lisp. */
         %{
         /* Pass the argument to yyparse through to yylex. */
         #define YYPARSE_PARAM scanner
         #define YYLEX_PARAM   scanner
         %}
         %locations
         %pure_parser
         %union {
             int num;
             char* str;
         }
         %token <str> STRING
         %token <num> NUMBER
         %%
         assignment:
             STRING '=' NUMBER ';' {
                 printf( "(setf %s %d)", $1, $3 );
            }
         ;

   ---------- Footnotes ----------

   (1) The features described here are purely optional, and are by no
means the only way to use flex with bison.  We merely provide some glue
to ease development of your parser-scanner pair.