Parsing Tools

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Parsing Tools

Introduction to Bison and Flex

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Scanning/parsing tools

lex - original UNIX lexics generator (Lesk, 1975)

create a C function that will parse input according to a set of regular expressions

yacc - "yet another compiler compiler" UNIX parser (Johnson, 1975)

generate a C program for a parser from BNF rules

bison and flex ("fast lex") - more powerful, free versions of yacc and lex, from GNU Software Fnd'n.

Jflex - generates Java code for a scanner

CUP - generates Java code for a parser

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Bison Overview

Purpose: automatically write a parser program for a grammar written in BNF.

Usage: you write a bison source file containing rules that look like BNF.

Bison creates a C program that parses according to the rules

term : term '*' factor { $$ = $1 * $3; } | term '/' factor { $$ = $1 / $3; } | factor { $$ = $1; } ;factor : ID { $$ = valueof($1); } | NUMBER { $$ = $1; } ;

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Bison Overview (2)

> bison myparser.y

myparser.tab.c

parser source code

myparser.y

BNF rules and actions for your grammar.

yylex.c

tokenizer function in C

> gcc -o myprog myparser.tab.c yylex.c

myprog

executable program

The programmer puts BNF rules and token rules for the parser he wants in a bison source file myparser.y

run bison to create a C program (*.tab.c) containing a parser function.

The programmer must also supply a tokenizer named yylex( )

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Bison Overview (3)

input file to be parsed.

yyparse( )

parser created by bison

In operation:

your main program calls yyparse( ).

yyparse( ) calls yylex when it wants a token.

yylex returns the type of the token.

yylex puts the value of the token in a global variable named yylval

yylex( )

tokenizer returns the type of the next token

yylval

parse tree or other result

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Bison source file

/* declarations go here */

%%/* grammar rules go here */

%%/* additional C code goes here */

The file has 3 sections, separated by "%%" lines.

Note: format for "yacc" is the same as for bison.

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Bison source file with C declarations

%{ /* C declarations and #define statements go here */#include <stdio.h>#define YYSTYPE double

%}/* bison declarations go here */

%%/* grammar rules go here */

%%/* additional C code goes here */

You usually include C code in the declarations section.

Declare that yylval will be a "double" type.

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Bison Example

Create a parser for this grammar:

expression => expression + term | expression - term| term

term => term * factor| term / factor| factor

factor => ( expression )| NUMBER

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Bison/Yacc file for example (1)

%{/* C declarations and #DEFINE statements go here */ #include <stdio.h> #define YYSTYPE double%}/* Bison/Yacc declarations go here */%token NUMBER /* define token type NUMBER */%left '+' '-' /* + and - are left associative */%left '*' '/' /* * and / are left associative */

%%/* grammar rules go here */%%/* additional C code goes here */

Structure of Bison or Yacc input:

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Bison/Yacc example (2)

%% /* Bison grammar rules */input : /* empty production to allow an empty input */ | input line ;line : expr '\n' { printf("Result is %f\n", $1); }expr : expr '+' term { $$ = $1 + $3; } | expr '-' term { $$ = $1 - $3; } | term { $$ = $1; } ;term : term '*' factor { $$ = $1 * $3; } | term '/' factor { $$ = $1 / $3; } | factor { $$ = $1; } ;factor : '(' expr ')' { $$ = $2; } | NUMBER { $$ = $1; } ;

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$1, $2, ... represent the actual values of tokens or non-terminals (rules) that match the production.

$$ is the result.

expr : expr '+' term { $$ = $1 + $3; } | expr '-' term { $$ = $1 - $3; } | term { $$ = $1; } ;

Example:if the input matches expr + term then set the result ($$) equal to the sum of expr plus term ($1 + $3).

pattern to match actionrule

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Q: why can we write "$$ = $1 + $3" ?

A: because we declared "#define YYSTYPE double", so all tokens and results are double.

expr : expr '+' term { $$ = $1 + $3; } | expr '-' term { $$ = $1 - $3; } | term { $$ = $1; } ;

pattern to match actionrule

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Scanner function: yylex( ) You must supply a scanner named yylex. yylex returns the token TYPE (not token value).

int yylex( void ) {int c = getchar(); /* read from stdin */if (c < 0) return 0; /* end of input */if ( c == '+' || c == '-' ) return c; /* for character tokens, TYPE = character itself */if ( isdigit(c) ) {

yylval = c - '0'; /* yylval is a global var */while( isdigit( c=getchar() ) )

yylval = 10*yylval + (c - '0');if (c >= 0) ungetc(c,stdin);return NUMBER; /* token type is NUMBER */

}...}

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Where is the token value? The value of the token is stored in a global variable named yylval.

int yylex( void ) {int c = getchar(); /* read from stdin */if (c < 0) return 0; /* end of input */if ( c == '+' || c == '-' ) return c; /* read number and store as yylval */if ( isdigit(c) ) {

yylval = c - '0'; /* get each digit */while( isdigit( c=getchar() ) )

yylval = 10*yylval + (c - '0');/* push next character back into input stream */if (c >= 0) ungetc(c,stdin);return NUMBER; /* token type is NUMBER */

}...}

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Useful C functions for yylex

int c = getchar( ); read next char from stdinreturns -1 at end of input

ungetc( c, stdin) put character back in input

#include <ctype.h>isdigit(c) true if c contains a digitisalpha(c) true if c contains a letterisalnum(c) isdigit(c) || isalnum(c)islower(c) true if c is lowercaseisupper(c) guess?isspace(c) space tab newline formfeed

scanf("%d", &num) read an integer from inputscanf("%lf", &dnum) read a double from input

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Scanner function for double values Suppose we want the data type of all tokens to be double.

In Bison input: #define YYSTYPE double.

This changes yylval to be a double!

%{ /* C declarations and #DEFINE statements go here */ #include <stdio.h> #define YYSTYPE double%}%%/* bison definitions go here */

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Scanner function for double Now yylex must know that yylval is "extern double".

Here is example of using scanf to parse numbers.

int yylex( void ) {int c = getchar(); /* read from stdin */if (c < 0) return 0; /* end of the input*/while ( c == ' ' || c == '\t' ) c = getchar( );if ( isdigit(c) || c == '.' ) {ungetc(c, stdin); /* put c back into input */scanf ("%lf", &yylval); /* get value using scanf */return NUMBER; /* return the token type */}return c; /* anything else... return char itself */

}

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Other C functions: yyerror

Bison requires an error routine named yyerror.

yyerror is called by the parser when there is an error.

You can include yyerror( ) in your Bison source file.

/* display error message */

int yyerror( char *errmsg ) {

printf("%s\n", errmsg);

}

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Other C functions: main

you need a write a main() function that starts the parser.

For a simple parser, main() calls yyparse().

/* main method to run the program */

int main( ) {

printf("Type some input. Enter ? for help.\n");

yyparse( );

}

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Running Bison

Compile the example file simple.y

CMD> bison simple.y

Output is "simple.tab.c" it is C code for the parser.

To compile the parser by itself using gcc:

CMD> gcc -o simple.exe simple.tab.c

To run the program at the command prompt:

CMD> simple.exe

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Simple example: definitions

/* The Bison declarations section */

%{

/* C declarations and #DEFINE statements go here */

#include <math.h>

#define YYSTYPE double

%}

%token NUMBER /* define token type for numbers */

%token '+' '-' /* + and - are left associative */

File: simple.y

No left/right associativity specified

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Simple example: grammar rules

%% /* Simple grammar rules */input : /* allow empty input */ | input line ;line : expr '\n' { printf("answer: %d\n", $1); }expr : expr '+' term { $$ = $1 + $3; } | expr '-' term { $$ = $1 - $3; } | term { $$ = $1; } ;term : NUMBER { $$ = $1; } ;

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yyerror and main

%% /* extra C code *//* display error message */int yyerror( char *errmsg ) { printf("%s\n", errmsg); }

/* main */int main() {

printf("type an expression:\n");yyparse( );

}

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Simple example: does it work?

cmd> bison simple.y

cmd> gcc -o calc.exe simple.tab.c

cmd> calc

Test the grammar rules:

3 + 5

3 - 4 - 5 + 6

How about this: -3

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Simple example: exploring BNF

%% /* Simple grammar rules */input : /* empty input */ | input line ;line : expr '\n' { printf("answer: %d\n", $1); }expr : expr '+' expr { $$ = $1 + $3; } | expr '-' expr { $$ = $1 - $3; } | term { $$ = $1; } ;term : NUMBER { $$ = $1; } | '-' NUMBER { $$ = -$2; } ;

%token NUMBER%right '+' '-'

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Exercise

Expand the grammar to include these operations:

4 * 5 multiplication

2 / 3 division

10 + 3 * 4 – 1 / 2 correct order of operations

2 * ( 3 + 4 ) grouping

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Complete example: definitions

/* The Bison declarations section */

%{

/* C declarations and #DEFINE statements go here */

#include <math.h>


%}

%token NUMBER /* define token type for numbers */

%left '+' '-' /* + and - are left associative */

%left '*' '/' /* * and / are left associative */

File: simple.y

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Complete example: grammar rules

%% /* Bison grammar rules */input : /* allow empty input */ | input line ;line : expr '\n' { printf("Result is %f\n", $1); }expr : expr '+' term { $$ = $1 + $3; } | expr '-' term { $$ = $1 - $3; } | term { $$ = $1; } ;term : term '*' factor { $$ = $1 * $3; } | term '/' factor { $$ = $1 / $3; } | factor { $$ = $1; } ;factor : '(' expr ')' { $$ = $2; } | NUMBER { $$ = $1; } | '-' NUMBER { $$ = -$2; } ;

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Common Errors in Bison

1. Forgetting to quote literals: term '+' term

2. Not skipping space where space is allowed

3. Ambiguity in grammar rules

4. Forgetting ';' at the end of rule

%token NUMBER%token + -%% /* grammar rules */

expr : term + term { $$ = $1 + $3; }| term - term { $$ = $1 - $3; }| term { $$ = $1; }

term : NUMBER { $$ = $1; }| - NUMBER { $$ = -$2; }

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Shift / Reduce and operator order

Bison uses token look-ahead and a token stack. It shifts tokens onto the stack until it can choose a rule to reduce (replace) tokens with a non-terminal. Example:

expr ::= expr + expr

| expr - expr

| expr * expr

| expr / expr

| term

term ::= NUMBER | ( expr )

Suppose the input read is 10 - shift these onto stack because no matching rule yet. Suppose the next token is 2. What should Bison do?

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Shift / Reduce and operator order

INPUT ACTION STACK

10 shift 10

- shift 10 -

2 shift 10 - 2

* shift 10 - 2 *

STACK: 10 - CURRENT TOKEN: 2 This grammar is ambiguous. Bison could match "expr - expr" or it could shift 2 onto the stack and look at the next token... it maybe * such as: 10 - 2 * 3

This is called a "shift / reduce conflict". Unless you specify a disambiguating rule (next slide),

Bison chooses "shift" over "reduce". Meaning: if it's not clear how to resolve conflicts, wait.

INPUT ACTION STACK

3 reduce 10 - 6

reduce 4done

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The declarations section

The declarations section can contain Bison directives and C directives.

We already saw the use of %left, %right, etc.

%{

/* semantic value of tokens, as C datatype */


#include <math.h>

/* why do we have to declare these? */

int yylex (void);

void yyerror (char const *);

%}

%token NUMBER

%left '+' '-'

%left '*' '/'

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Specifying operator order

In the declarations section you can write:

%noassoc NEG /* unary minus sign: - 3 */

%left '+' '-'

%left '*' '/' '%'

%right '^'

'+' and '-' are left associative and have the same precedence.

'*', '/', and '%' left associative and same precedence; they have higher precedence (later declarations are higher).

'^' is right associative and higher precedence than + - * / % 'NEG' has no associativity: "- - 3" is an error

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Scanner function main points

Tokens have a TYPE and a VALUE. The scanner (yylex) returns the TYPE of the next token For one-character tokens like '+', '=', "(' the character

itself can be used as the type. Define symbolic names for token types in Bison using %token NAME

Return the VALUE of a token using the global variable yylval.

By default, yylval is type "int". Change it using: #define YYSTYPE double

Rather than parse numbers yourself, use scanf.

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Scanner function main points (1)

%{#include <ctype.h>#include <math.h>#define YYSTYPE double

%}%token NUMBER%token SQRT%left '+' '-'%left '*' '/'%%line : expr '\n' { printf("%g\n", $$); }

;expr : expr * expr { $$ = $1 + $3; }

| SQRT '(' expr ')' { $$ = sqrt( $2 ); }... /* more rules */

Token for sqrt function

Token values are double

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Scanner function main points (2)

%%int yylex(void) {

int c = getchar( );while ( c == ' ' || c == '\t' ) c = getchar( );if ( isdigit(c) ) { ungetc(c,stdin);

scanf("%f", &yylval);return NUMBER;

}if ( isalpha(c) ) {

char *word = getword(c); /* get next word */if ( strcmp(word,"sqrt") ) return SQRT;

...}

Token is a number

Token is sqrt function

For a more general way of handling identifiers, see the Bison User's Guide, "mfcalc" example.

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Handling Multiple Data Types For a more general grammar, the scanner should be able to return

different data types as the value (yylval). In definitions section, define "%union" as union of all data types

that yylval (and hence $1, $2, ...) can have.

%union {double number;char* string;}

%token <number> NUMBER%token <string> IDENT%type <number> expr%type <number> term%left '+' '-'%left '*' '/'

Define all the data types that token values can have

For each token type, define the data type of its value.

For tokens that represent their own value, don't need to define

data type.

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Handling Multiple Data Types (2)

%%int yylex(void) {

int c = getchar( );while ( c == ' ' || c == '\t' ) c = getchar( );if ( isdigit(c) ) { ungetc(c,stdin);

double x;scanf("%f", &x);yylval.number = x;return NUMBER;

}if ( isalpha(c) ) {

char *word = getword(c); /* get next word */yylval.string = word;return IDENT;

...}

token value is a double

token value is a string

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Using a Separate File for yylex

You can put the scanner (yylex) in a separate file. BUT, yylex needs values that are defined by Bison,

such as NUMBER, IDENT, yylval. Solution: add the "%defines" option to your rules Bison will create a header file named

"simple.tab.h".

%defines

%{

#include <math.h>


%}

token NUMBER

...

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Using a Makefile

make is an automatic build facility. Just type "make"

Reads a Makefile of "rules" for how to make things

# Makefile for simple calculatorcalc.exe: simple.tab.c yylex.o

gcc -o calc.exe simple.tab.c yylex.o -lm

# how to make simple.tab.c from simple.ysimple.tab.c: simple.y

bison simple.y

# yylex.o (obj. file) depends on simple.tab.hyylex.o: yylex.c simple.tab.h

gcc –c yylex.c

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Running Make

C:/calculator> make

bison -d simple.ygcc -c simple.tab.cgcc -o simple.exe simple.tab.o yylex.c -lm

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Advantages of Make Makefile record all the dependencies in a project

make only builds the parts that are missing or out of date

can manage complex projects with multiple Makefiles in subdirectories

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Introduction to flex

Flex is a program that automatically creates a scanner in C, using rules for tokens as regular expressions.

Format of the input file is like Bison.

%{/* C definitions for scanner */

%}flex definitions %% rules %% user code (extra C code)

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Flex Example

Read console input and describes each token read.

%{ inline int yywrap(void) { return 1; };%}/* flex definitions */DIGIT [0-9]LETTER [A-Za-z]%%{LETTER}+ printf("Word: %s\n", yytext); return 1;-?{DIGIT}+ printf("Number: %s\n", yytext); return 2;[[:punct:]] printf("Punct: %s\n", yytext); return 3;\n printf("End of line\n"); return 0;%% /* main method calls yylex to tokenize input */int main( ) { printf("Type something: ");

while( yylex() > 0 ) { }; }

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Flex Example (2)

Run flex to create yylex.c (on Linux: lex.yy.c).

cmd> flex mylex.fl

Compile lex.yy.c and create myscanner.exe

cmd> gcc -o myscanner.exe yylex.c

Run the program:

cmd> myscanner

Type something: hello, it's 9:00 o'clock. word: hellopunct: , word: itpunct: '

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Flex Explained: definitions

%{ /* C definitions and includes */#include "myparser.tab.h"

/* this fixes an unknown symbol "yywrap" */#ifdef yywrap#undef yywrap#endifinline int yywrap(void) { return 0; };

%}/* flex definitions */DIGIT [0-9]LETTER [A-Za-z]%%

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Flex Explained: Parsing Rules (1) pattern is a regular expression or a literal value.

action is zero or more C statements to execute.

The current matched input is (char *) yytext

The length of matched input is yyleng

{LETTER}+ printf("Found a word: %s", yytext);-?[0-9]+ printf("Found an int: %s", yytext);

pattern action

yytext is a string containing the current token.

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Flex Explained: Parsing Rules (2) Keep matching input until a return statement.

Flex chooses the rule that produces longest match.

If there is a tie, choose the first rule that matches.

Use yylval (global variable) for value of token.

%{ /* Bison header file defines INT, IDENT...*/#include "myparser.tab.h"

%}%% [0-9]+ yylval.itype = atoi(yytext); return INT;{LETTER}+ yylval.ctype = yytext; return IDENT;"+"|"-" yylval.ctype = yytext; return OP;"=" return ASSIGN;

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Flex yywrap

Flex can read input from multiple input files. when flex reaches the end of a file, it calls yywrap() if yywrap() returns 0, it means that there is another

input file to read. if yywrap() returns 1, it means that there are no more

input files.

%{ inline int yywrap(void) { return 1; };%}

/* same thing */%option noyywrap

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Using flex with bison1. Run "bison -d file.y" to create file.tab.h

2. #include "file.tab.h" in your flex source.

3. Set yylval to the token value.

4. Run flex.

5. Compile and link lex.yy.c and file.tab.c

%{#include <math.h>#include "file.tab.h"

%}LETTER [A-Za-z] /* same as [[:alpha:]] */%% -?[0-9]+ yylval.itype = atoi(yytext); return INT;{LETTER}+ yylval.ctype = yytext; return IDENT;"+"|"-" yylval.ctype = yytext; return OP;"=" return ASSIGN;

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Using Flex and Bison

> bison –d mygrammar.y

mygrammar.tab.c

parser source code

mygrammer.y

BNF rules for your grammar.

lex.yy,c

tokenizer function in C

> gcc -o myprog mygrammar.tab.c lex.yy.c

myprog

executable program

mylex.fl

Regular expressions that match and return tokens

> flex mylex.fl

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Learning Bison, Flex, CUP

On Linux or most UNIX systems type:

info bison

Using "info": SPACEBAR = next pageALT-V = previous page, CTRL-C = quit

Documentation on the Web:

GNU Free Software Foundation

http://www.gnu.org/software/bison/bison.html

... or many tutorials online (search Google)

Similar documentation for flex. CUP manual: http://www2.cs.tum.edu/projects/cup

includes many easy examples!

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Getting the Software bison and flex are included with Linux distributions

MS Windows version of GNU programs

http://gnuwin32.sourceforge.net/

http://gnuwin32.sourceforge.net/packages.html

The packages include excellent user's guides with examples.

CUP:

http://www2.cs.tum.edu/projects/cup/

software (jar file) and excellent manual; links to resources

Jflex (lex that creates java code):

http://jflex.sourceforge.net/

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GNU Tools and C/C++ compiler

Cygwin: www.cygwin.com

MinGW: www.mingw.org

free GNU toolkit.

Dev-C++

C++ integrated development environment

two downloads: with gcc or without gcc

Documents

Parsing Tools