introduction to yacc (bison)

7/30/2019 Introduction to Yacc (Bison)

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Introduction toa parser generator

National

1 Compiler Lab.| Mr. B.

acc (Bison)

Mr. Bhaskar Mondal

nstitute of Technology, Jamshedpur

ondal | NIT Jamshedpur


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An Over View

yacc stands for "yet another co

Updated version is Bison

parsers what lex is to scanner

input of a grammar specificatio

generates an LALR(1) parser to

grammar.


piler compiler

recognize sentences in that

Mondal | NIT Jamshedpur


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How It Works

It use with C code and gener

Works conjunction with a le

provide a grammar rules in a

invokeyacc on the .yfile any.tab.c in C


tes a parser written in C.

-generated scanner

file, with .yextension

it creates they.tab.h and



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How It Works (cont)

The file provides an extern fun

attempt to successfully par

You compile that C file normall

By default, the parser reads frojust like a lex-generated scanne


tionyyparse.y that will

e a valid sentence.

with any C Compiler.

stdin and writes to stdoudoes.



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How It Works Steps

%yacc myFile.ycreatesy.tab.c o

% gcc -c y.tab.c compiles parser c

% gcc o parse y.tab.o lex.yy.o l

% parse invokes parser, reads fro


C code for parser

de

-lylink parser, scanner, libr

stdin



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yacc File Format

%{

Declarations

%}

Definitions

%%

Productions

%%

User subroutines

6 Compiler Lab.| Mr. B. Mondal | NIT Jamshedpur


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yacc File Format (cont

The optional Declarations and

used for ordinary C code The optional Definitions sectio

features: defining token codes,

establishing operator precedence

setting up the global variables usscanner and parser.

The Productions section is for specifcan associate an action with eaproduction), which allows you to do

reduce using that production.


)

ser subroutines sections ar

Contains various parser

and associativity,

d to communicate between the

the grammar rules. As in lex, yh pattern (this time ahatever processing is needed a



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Example:

Write ayacc input fcalculator that recogbinary postfix expre


le for a simplenizes and evaluatesions using a stack



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Declaration Part

%{

#include #include

static int Pop();

static int Top();

static void Push(int val);

%}

9 Compiler Lab.| Mr. B. Mondal | NIT Jamshedpur


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Definition and Production%token T_Int

%%S : S E '\n' { printf("= %d\n", Top())|;E : E E '+' { Push(Pop() + Pop()); }

| E E '-' { int op2 = Pop(); Push(Pop| E E '*' { Push(Pop() * Pop()); }| E E '/' { int op2 = Pop(); Push(Pop| T_Int { Push(yylval); };

%%


art

; }

) - op2); }

) / op2); }



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User Sub Routine

static int stack[100], count = 0;

static int Pop() {assert(count > 0);

return stack[--count];

}

}static void Push(int val) {

assert(count 0);

return stack[count-1];

}

int main() {return yyparse();

}



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Note about the Code:

Any token types returned from

using %token Used by scanner to tell the pars

The global variableyylval is usinformation about the lexeme i

A colon(:) is used in place of th a vertical bar(|) separates the v

A semicolon terminates the rul

Yacc dont minds about line bo

enough spaces.


he scanner must be defined

r about each token scanned

ed to store additional attribuself.

arrow() in the rules.rious productions.

.

ndaries like lex. You can use



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Note about the Code: (Con

Within the braces C code are th

a production. No Code no actio The Header symbol of first rule

symbol for the grammar.

yyparse is the function genera

stdin. yyparse returns 0 if the parse w

If it encounters an error it callsdefault prints the generic "pars


)

re for the action associated

at reduction.in the file is assumed the sta

ed byyacc. It reads input f

as successful and 1 otherwis

the routineyyerror,whicherror" message and quits.



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The lex file used:

%{

#include "y.tab.h"%}

%%

[0-9]+ { yylval = atoi(yytext); ret

[-+*/\n] { return yytext[0];}

. { /* ignore everything else */ }


rn T_Int;}



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What does it do

yylex will return the ASCII repr

calculator operators, recognize integers , and

ignore all other characters.

assembles a series of digits intonumeric value and stores inyypassing attributes from the sca

The token type T_Int is return


sentation of the

an integer, it converts to aval (the global reserved for

ner to the parser).

d.



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Tie this all together: a calc

runyacc/bison on the grammar

y.tab.c andy.tab.h files run lex/flex on the scanner speci

file.

Compile the two .c files and link

calc: lex.yy.o y.tab.o

gcc -o calc lex.yy.o y.tlex.yy.c: calc.l y.tab.c

flex calc.l

y.tab.c: calc.y

bison -vdty calc.y


lator to born!

pecification to generate the

ication to generate the lex.yy

hem together.

b.o -ly -ll


Wh h h f d LALR( )


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What happens when you feedgrammar?

yacc reports any conflicts whe

resolving the conflicts and buil In A shift/reduce conflict,yacc

In a reduce/reduce conflict, itfirst in the file.

Must control what happens byand associativity for your opera


yacc a non LALR(1)

trying to fill in the table

ing a table anyway.will choose the shift.

ill reduce using the rule dec

explicitly declaring precedenors.


I bi h i l d ddi i


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In put an ambiguous grammamultiplication, and exponenti

%token T_Int

%%E : E '+' E

| E '*' E

| E '^' E

| T_Int

;

conflicts: 9 shift/reduce

y.Out...State 6 cState 7 cState 8 c

...


that includes addition,tion

ut

ntains 3 shift/reduce conflictsntains 3 shift/reduce conflictsntains 3 shift/reduce conflicts



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Defining precedence levels

one per line, from lowest to hig

associativity (either left, right, Several terminals can be on the

precedence.

%token T_Int

%left '+'

%left '*'

%right ' '

%%

E : E '

| E '*'| E ' '| T_I

;19 Compiler Lab.| Mr. B.

est, and indicate the

r nonassoc).same line to assign them eq

+' E

EE



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Defining precedence levels

So if a 8 + 2 is on the stack a

higher precedence than the 8

If 8*2 is on the stack and + i

If 8+2 is on the stack and + ibreaks the tie, a left-to-rightrule and then go on, a right-tthe reduction.


Cont)

d * is coming up, the * ha

+2, so it shifts.

coming up, it reduces.

coming up, the associativssociativity would reduce-left would shift and post



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References:

J. Levine, T. Mason, and D. Bro

CA: OReilly & Associates, 1992 A. Pyster, Compiler Design and

Van Nostrand Reinhold, 1988.


wn, Lex and Yacc. Sebastopo

.Construction. New York, NY


introduction to yacc (bison)

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