Structure of a YACC source program

A YACC source program is structurally similar to a LEX one.

declarations
%%
rules
%%
routines

• The declaration section may be empty. Moreover, if the routines section is omitted, the second %% mark may be omitted also.
• Blanks, tabs, and newlines are ignored except that they may not appear in names.

THE DECLARATIONS SECTION may contain the following items.

• Declarations of tokens. Yacc requires token names to be declared as such using the keyword %token.
• Declaration of the start symbol using the keyword %start
• C declarations: included files, global variables, types.
• C code between %{ and %}.

RULES SECTION.

A rule has the form:

nonterminal : sentential form
            | sentential form
            .................
            | sentential form
            ;

Actions may be associated with rules and are executed when the associated sentential form is matched.

LEX-YACC INTERACTION

yyparse() calls yylex() when it needs a new token.

LEX	YACC
return(TOKEN)	%token TOKEN
	TOKEN is used in production

The external variable yylval

• is used in a LEX source program to return values of lexemes,
• yylval is assumed to be integer if you take no other action.
• Changes related to yylval must be made
  • in the definitions section of YACC specification
    • by adding new types in the following way

      %union {
          (type fieldname)
          (type fieldname)
          ...............
      }
      

    • and defining which token and non-terminals will use these types

      %token <fieldname> token 
      %type <fieldname> non-terminal
      

  • in LEX specification by using the fieldnames in the assignment as follows

    yylval.fieldname = ...........
    

If you need a record type, then add it in the union. Example:

%union {
   struct s {
      double fvalue;
      int ivalue;
   } t;
}

• in the LEX specification use the record name and record field in assignments:

  yylval.t.ivalue = ......
  

• in the YACC rules specification use the record field only in the assignment:

  $1.ivalue = ......
  

  assuming that $1 has the appropriate type, whatever it denotes.