Introduction to YACC

A parser generator is a program that takes as input a specification of a syntax, and produces as output a procedure for recognizing that language. Historically, they are also called compiler-compilers.

YACC (yet another compiler-compiler) is an LALR(1) (LookAhead, Left-to-right, Rightmost derivation producer with 1 lookahead token) parser generator. YACC was originally designed for being complemented by Lex.

Input File:
YACC input file is divided in three parts.



/* definitions */
 ....

%% 
/* rules */ 
....
%% 

/* auxiliary routines */
.... 

Input File: Definition Part:

  • The definition part includes information about the tokens used in the syntax definition:
    %token NUMBER 
    %token ID 
  • Yacc automatically assigns numbers for tokens, but it can be overridden by
    %token NUMBER 621 
  • Yacc also recognizes single characters as tokens. Therefore, assigned token numbers should no overlap ASCII codes.
  • The definition part can include C code external to the definition of the parser and variable declarations, within %{ and %} in the first column.
  • It can also include the specification of the starting symbol in the grammar:
    %start nonterminal 

Input File: Rule Part:

  • The rules part contains grammar definition in a modified BNF form.
  • Actions is C code in { } and can be embedded inside (Translation schemes).

Input File: Auxiliary Routines Part:

  • The auxiliary routines part is only C code.
  • It includes function definitions for every function needed in rules part.
  • It can also contain the main() function definition if the parser is going to be run as a program.
  • The main() function must call the function yyparse().

Input File:

  • If yylex() is not defined in the auxiliary routines sections, then it should be included:
    #include "lex.yy.c"  
  • YACC input file generally finishes with:
     .y 

Output Files:

  • The output of YACC is a file named y.tab.c
  • If it contains the main() definition, it must be compiled to be executable.
  • Otherwise, the code can be an external function definition for the function int yyparse()
  • If called with the –d option in the command line, Yacc produces as output a header file y.tab.h with all its specific definition (particularly important are token definitions to be included, for example, in a Lex input file).
  • If called with the –v option, Yacc produces as output a file y.output containing a textual description of the LALR(1) parsing table used by the parser. This is useful for tracking down how the parser solves conflicts.

Example:
Yacc File (.y)

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%{ 
   #include <ctype.h> 
   #include <stdio.h> 
   #define YYSTYPE double /* double type for yacc stack */ 
%} 
  
%%
 Lines :  Lines S '\n' { printf("OK \n"); }
       |  S '\n’
       |  error '\n' {yyerror("Error: reenter last line:");
                        yyerrok; };
 S     :  '(' S ')’
       '[' S ']’
       |   /* empty */    ;
%% 
  
#include "lex.yy.c" 
   
void yyerror(char * s)
/* yacc error handler */
{   
 fprintf (stderr, "%s\n", s);
   
int main(void
 {
 return yyparse();
 }  

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Lex File (.l)

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%{ 
%}
  
%% 
[ \t]     { /* skip blanks and tabs */
\n|.      { return yytext[0]; }
%% 

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For Compiling YACC Program:

  1. Write lex program in a file file.l and yacc in a file file.y
  2. Open Terminal and Navigate to the Directory where you have saved the files.
  3. type lex file.l
  4. type yacc file.y
  5. type cc lex.yy.c y.tab.h -ll
  6. type ./a.out


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