CODEWITHSUNDEEP
c++cparsingyacc
jmgunn87
jmgunn87

Reputation: 11

Bison Grammar: yylval is embedded in yyparse

No wonder i cant link to it from my flex file. I have checked this and taken out the declaration "YYSTYPE yylval;" from the beginning of yyparse and it works as intended. Surely this is not the correct way to use bison and flex? Can somebody show me another way? Thank you.

Upvotes: 1

Views: 1653

Answers (3)

Viktor Engelmann
Viktor Engelmann

Reputation: 11

Instead of using

#define YY_DECL int yylex(YYSTYPE *val)

you can also use

%option bison-bridge

But if you want to write a flex+bison parser in C++, then this method does not work.

For C++ parsers, check this example out.

Upvotes: 1

neuront
neuront

Reputation: 9612

I have checked this and taken out the declaration "YYSTYPE yylval;"

I wonder if there is something wrong with your "taken out", but you could try

bison -d your-yacc-file.y

then bison will generate a header file for you with all those declarations.

Upvotes: 0

Chris Dodd
Chris Dodd

Reputation: 126175

It is normal that yylval is declared and defined in the y.tab.c file output by bison. Its also declared (as extern) in the y.tab.h file, so if you include that in your lexer, you can access yylval as a global var. This is the normal way in which flex/bison works and there should be no need to edit the files to take out things -- it should 'just work'

This use of a global var causes problems if you want to have more than one parser in a program, or want to use multiple parsers in different threads (or otherwise simultaneously). Bison provides a way to avoid this with %define api.pure, which gets rid of yylval as a global -- instead the parser will call yylex with the address of a YYSTYPE (a pointer) and the lexer should put the token value there instead of in yylval. If you're using flex, you'll want #define YY_DECL int yylex(YYSTYPE *val) in the top of your flex file to change the declaration it uses for yylex.

Upvotes: 1

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