compiler design code to parse goto and blocks

Question

Can someone please help me understand how to code lex and yacc file for a C program to parse goto and blocks(labels).

Here is some example I found on the net:

<statement> ::= 
<variable> <inc> <semi>
| <variable> <assign> <null> <semi> 
| <goto> <label> <semi>

But how to uniquely identify label. Can someone give me an example of this or any link where it is mentioned.

Answer 1

You don't need to "uniquely identify" as part of the parser, a label is identified by the syntax of the language. It is entirely legal to allow labels to overlap with other symbol / variable names. Just add a rule for a legal label, reuse the lexer TOKEN for IDENTIFIER, then store labels in a symbol table per function so you can detect duplicate labels.

Typically in C-like languages, a label is part of a statement rule. It doesn't usually make sense anywhere else. The way I do it:

labeled_statement:
    IDENTIFIER ':' statement
    {
        $$ = $3;
        $$->label = new Label($1);
    }
    ;

Another way:

 label:
     IDENTIFIER ':'
     { $$ = new Label($1); }
     ;

labeled_statement:
    label statement
    {
        $$ = $2;
        $$->label = $1;
    }
    ;