YACC in Compiler Design

Workings of YACC

YACC in compiler design is set to work in C programming language along with its parser generator.

• An input with a .y extension is given.
• The file is invoked, and 2 files, y.tab.h and y.tab.c, are created. These files contain long codes implementing the LARl (1) Parser for grammar.
• This file then provides yyparse.y, which tries to parse a valid sentence successfully.

For the output files, 

• If called with the –d option in the command line, YACC produces y.tab.h with all its specific definitions.
• If called with the –v option, YACC produces y.output, having a textual description of the LALR(1) parsing table.

Example of YACC 

The YACC program code for a simple calculator is given below:

%{
#include <ctype.h>
#include <stdio.h>
int yylex();
void yyerror();
int tmp=0;
%}


%token num
%left '+' '-'
%left '*' '/'
%left '(' ')'


%%


line :exp  {printf("=%d\n",$$); return 0;};


exp  :exp '+' exp {$$ =$1+$3;}
     | exp '-' exp {$$ =$1-$3;}
     | exp '*' exp {$$ =$1*$3;}
     | exp '/' exp {$$ =$1/$3;}
     | '(' exp ')' {$$=$2;}
     | num {$$=$1;};
     
%%


void yyerror(){
printf("Incorrect\n");
tmp=1;
}
int main(){
printf("Enter an expression( +,-,*,/ or parenthesis):\n");
yyparse();
}

The lex file of the program is given below:

%option noinput nounput noyywrap
%{
#include <stdlib.h>
#include <stdio.h>
#include "y.tab.h"
extern int yylval;
%}


%%


[\t]      ;
[\n]      return 0;


[0-9]+    { yylval = atoi(yytext);
            return num;
          }
.         return yytext[0];
%%

How to Execute 

• Save the program code to a file with a .y extension, such as calculator.y.
• Install Yacc/Bison on your system, if it is not already installed.
• Open a terminal or command prompt and navigate to the directory where the .y file is saved.
• Run the following command to generate a C source file from the Yacc/Bison grammar file

yacc -d calculator.y

• This will create two output files: y.tab.c (the C source file), and y.tab.h (the header file).
• Compile the generated C source file and link it with the Yacc/Bison runtime library using a C compiler such as GCC:

gcc -o calculator y.tab.c -ly

• Run the compiled program by typing its name and pressing Enter:

bash
./calculator

Explanation

Firstly, the header file is defined with all its function definitions. Then, the tokens for the grammar are initialised. Finally, the grammar to use the calculator is defined.

Frequently Asked Questions

What is the use of yacc in compiler design?

Yacc is used to generate a parser that converts grammar rules into syntax trees, helping in syntax analysis during compilation.

Where is Yacc used?

Yacc is mainly used in designing compilers and interpreters to automate syntax analysis based on context-free grammars.

What is yyerror in compiler design?

yyerror is a user-defined function in Yacc that handles syntax errors during parsing, typically displaying error messages for debugging.

Conclusion

We learned all about YACC in Compiler Design. It serves as a reliable tool for simplifying the creation of parsers. It transforms grammar rules into parsers efficiently, thanks to its LALR parsing algorithm. With YACC, developers can bridge the gap between human-readable code and machine instructions, making compiler design more accessible and manageable.

Recommended Readings:

• Lexical Analysis in Compiler Design
• Characteristics of Compiler