yylval undefined with flex and bison

Question

I have searched almost every material online. But I am still confused why lexer cannot identify yylval.

Here is the case: I have defined a bunch of ADT in node.h and realize them in node.c, my purpose is to generate a AST after these structures are properly stored. But I am stucked with bison file.

First, I change %union to union YYSTYPE {...}; and typedef union YYSTYPE YYSTYPE;, I don't why I need to do this, some other files posted online seems to work well with %uinion.

Then, I am stucked with yylval things. I have done bison -d things, and checked it already in parser.c(I have specified the bison output), so I think extern YYSTYPE yylval; should work. But it doesn't. So I wonder if there is another way to solve yylval undeclared problem.

I only use the two types of YYSTYPE struct, int and char *, can I separate the union YYSYTPE and struct for the AST? This means, the nonterminals will have no associated types. Do you guys have any other ideas??

Answer 1

In case it's of help to others, I found (OpenBSD lex v2.5.4 + yacc) that including

extern YYSTYPE yylval;

in my lex source was insufficient to prevent a 'yylval undefined' problem, even though the y.tab.c file contains:

#ifndef YYSTYPE
typedef int YYSTYPE;
#endif

I fixed this by putting an explicit definition in the lex source:

#define YYSTYPE int
extern YYSTYPE yyltype

However, I am unclear whether defining in this way will propagate to the locale of the yacc source file...

Answer 2

You should use %union. You do not need to typedef the union.

You use bison -d to get a header file with any declarations (such as types for terminals, nonterminals, and tokens). Example:

bison -d parser.y Would yield two files, parser.tab.c and parser.tab.h. You can include the latter file into your lexer file, so it'll know about your %token definitions. This file also includes the definition of yylval and its type.

You should not separate the union because you'll most likely expand it and need it for communicating between the lexer and parser.