How can I know the value of a variable in bison while debugging with GDB?

Question

I'm trying to write a small compiler which can parse some commands I type.

The command I try to parse is:

create class something = create class do_something ; 

My code would be something like this:

grammar : my_grammar
        {
            list<Class1 *> *obj = new list<Class1 *>;
            obj->push_back($1);
        }

my_grammar : my_definition SEMICOLON
           {
               report("something detected!");
               $$ = $1;
            }

my_definition : CREATE CLASS class_name EQU class_expression
               {
                   $5->setClassName(*$3);
                   $$ = $5;
                }

class_expression : CREATE CLASS operand_name
                 {
                     $$ = new OperandClass();
                     $$->setOperationType("createClass");
                     $$->setOperandName(*$3);
                  }

However, when I try to call the parser somewhere else, I can't get the Class I have defined before.

I guess there must be something wrong with the parser and have done some debugging with GDB. But I just can't step in the function push_back(), neither can I print the information of obj correctly.

So, I am wondering if there is a way that I can get the value of $$ or $1 while using GDB. Simply type in p $$ would print something else.

Answer 1

If you're using bison with the C templates, bison has a variable yyval and an array yyvsp, both of type YYSTYPE, which we define using the %union option in the bison parser.y file. The $$ of a rule is given by the union member of yyval, and the symbols in the production are members in the array yyvsp. For example, for the union:

%union {
  int t1;
  float t2;
}

%type <t1> nt1;
%type <t2> nt2;
%%
input : nt1 | nt2;
nt1 : CONSTANT { $$ = $1; } ;
nt2 : FCONSTANT { $$ = $1 };
%%
int main() { yyparse(); }

while using gdb:

1. You can refer to the left hand of a production, for example nt1 by it's type, i.e $$ for nt1 can be referred to by yyval.t1
2. $i on the R.H.S can be referred to by: yyvsp[i - t].type where i is the index of the symbol to be referenced, and t is the total number of symbols(terminal and non-terminals) in production.

So, for example, $1 in both rules can be referred to by yyvsp[1-1].t1 and yyvsp.[0].t2, respectively.