What does "$$ = 1" mean in Bison/Yacc?

3
votes

Can anybody tell me what {$$ = 0} or {$$ = 1} or {$$ = $1 +1} mean in Bison/Yacc rule actions? I know what $$ = $1 means in Bison.

1
bisonyacc

1 Answers

7
votes

Bison is used for declaring rules of your grammar. Whatever is in the braces after the rule, it is the action to be taken when the rule applies to a certain group of tokens.

On the other hand, $$ stands for the semantic value of the grouping created under the respective rule.

Below I build a brief example based on your questions. The first expression states that the int_var derived from a NUM should be assigned the value that was assigned to NUM.

The second expression states that if: int_var is followed by a '=' which is followed by a 'FALSE' string, then the int_var token should be set to 0. Then, when the int_token is followed by '=' and 'TRUE' then it is set to 1.

The 4th rule states that if an int_var is followed by a '++' string, then the value assigned to the token should be the value of the int_var found plus 1.

In the same manner, this can be applied to rules for different arithmetic expressions as the addition:

int_var:  NUM                 { $$ = $1       }
        | int_var '=' 'FALSE' { $$ = 0;       }
        | int_var '=' 'TRUE'  { $$ = 1;       }
        | int_var '++'        { $$ = $1 + 1;  }
        | int_var '+' int_var { $$ = $1 + $3; }
        ...
;

Hope this solves your question and good luck taming Bison.