GNU Bison

GNU Bison

Developer(s)	The GNU Project
Repository	git.savannah.gnu.org/cgit/bison.git
Operating system	Cross-platform
Type	Parser generator
License	GPL (free software)
Website	www.gnu.org/software/bison

GNU bison, commonly known as Bison, is a parser generator that is part of the GNU Project. Bison reads a specification of a context-free language, warns about any parsing ambiguities, and generates a parser (either in C, C++, or Java) which reads sequences of tokens and decides whether the sequence conforms to the syntax specified by the grammar. Bison by default generates LALR parsers but can also create GLR parsers.

In POSIX mode, Bison is compatible with yacc, but also supports several improvements over this earlier program. flex, an automatic lexical analyser, is often used with Bison, to tokenise input data and provide Bison with tokens.

Bison is licensed as free software and is available in source code form. Earlier releases of Bison used to stipulate that parts of its output were protected under the GPL, due to the inclusion of the yyparse() function from the original source code in the output. An exception has since been made to allow other licenses to apply to the use of the output.

A complete reentrant parser example

The following example shows how to use Bison and flex to write a simple calculator program (only addition and multiplication) and a program for creating an abstract syntax tree. The next two files provide definition and implementation of the syntax tree functions.

/*
 * Expression.h
 * Definition of the structure used to build the syntax tree.
 */
#ifndef __EXPRESSION_H__
#define __EXPRESSION_H__

/**
 * @brief The operation type
 */
typedef enum tagEOperationType
{
    eVALUE,
    eMULTIPLY,
    ePLUS
}EOperationType;

/**
 * @brief The expression structure
 */
typedef struct tagSExpression
{
    EOperationType type;///< type of operation

    int value;///< valid only when type is eVALUE
    struct tagSExpression* left; ///< left side of the tree
    struct tagSExpression* right;///< right side of the tree
}SExpression;

/**
 * @brief It creates an identifier
 * @param value The number value
 * @return The expression or NULL in case of no memory
 */
SExpression* createNumber(int value);

/**
 * @brief It creates an operation
 * @param type The operation type
 * @param left The left operand
 * @param right The right operand
 * @return The expression or NULL in case of no memory
 */
SExpression* createOperation(
    EOperationType type,
    SExpression *left,
    SExpression *right);

/**
 * @brief Deletes a expression
 * @param b The expression
 */
void deleteExpression(SExpression *b);

#endif // __EXPRESSION_H__

/*
 * Expression.c
 * Implementation of functions used to build the syntax tree.
 */

#include "Expression.h"

#include <stdlib.h>

/**
 * @brief Allocates space for expression
 * @return The expression or NULL if not enough memory
 */
static SExpression* allocateExpression()
{
    SExpression* b = (SExpression *)malloc(sizeof *b);

    if (b == NULL)
        return NULL;

    b->type = eVALUE;
    b->value = 0;

    b->left = NULL;
    b->right = NULL;

    return b;
}

SExpression* createNumber(int value)
{
    SExpression* b = allocateExpression();

    if (b == NULL)
        return NULL;

    b->type = eVALUE;
    b->value = value;

    return b;
}

SExpression *createOperation(
    EOperationType type,
    SExpression *left,
    SExpression *right)
{
    SExpression* b = allocateExpression();

    if (b == NULL)
        return NULL;

    b->type = type;
    b->left = left;
    b->right = right;

    return b;
}

void deleteExpression(SExpression *b)
{
    if (b == NULL)
        return;

    deleteExpression(b->left);
    deleteExpression(b->right);

    free(b);
}

The tokens needed by the Bison parser will be generated using flex.

%{
 
/*
 * Lexer.l file
 * To generate the lexical analyzer run: "flex Lexer.l"
 */
 
#include "Expression.h"
#include "Parser.h"

#include <stdio.h>
 
%}

%option outfile="Lexer.c" header-file="Lexer.h"
%option warn nodefault
 
%option reentrant noyywrap never-interactive nounistd
%option bison-bridge
 
LPAREN      "("
RPAREN      ")"
PLUS        "+"
MULTIPLY    "*"
 
NUMBER      [0-9]+
WS          [ \r\n\t]*
 
%%
 
{WS}            { /* Skip blanks. */ }
{NUMBER}        { sscanf(yytext, "%d", &yylval->value); return TOKEN_NUMBER; }
 
{MULTIPLY}      { return TOKEN_MULTIPLY; }
{PLUS}          { return TOKEN_PLUS; }
{LPAREN}        { return TOKEN_LPAREN; }
{RPAREN}        { return TOKEN_RPAREN; }
.               {  }
 
%%
 
int yyerror(const char *msg) {
    fprintf(stderr,"Error:%s\n",msg); return 0;
}

Since the tokens are provided by flex we must provide the means to communicate between the parser and the lexer. The data type used for communication, YYSTYPE, is set using Bison's %union declaration.

Since in this sample we use the reentrant version of both flex and yacc we are forced to provide parameters for the yylex function, when called from yyparse. This is done through Bison's %lex-param and %parse-param declarations.

%{
 
/*
 * Parser.y file
 * To generate the parser run: "bison Parser.y"
 */
 
#include "Expression.h"
#include "Parser.h"
#include "Lexer.h"

int yyerror(yyscan_t scanner, SExpression **expression, const char *msg);
 
%}

%output  "Parser.c"
%defines "Parser.h"
 
%define api.pure
%lex-param   { yyscan_t scanner }
%parse-param { SExpression **expression }
%parse-param { yyscan_t scanner }

%union {
    int value;
    SExpression *expression;
}
 
%left '+' TOKEN_PLUS
%left '*' TOKEN_MULTIPLY
 
%token TOKEN_LPAREN
%token TOKEN_RPAREN
%token TOKEN_PLUS
%token TOKEN_MULTIPLY
%token <value> TOKEN_NUMBER

%type <expression> expr
 
%%
 
input
    : expr { *expression = $1; }
    ;
 
expr
    : expr TOKEN_PLUS expr { $$ = createOperation( ePLUS, $1, $3 ); }
    | expr TOKEN_MULTIPLY expr { $$ = createOperation( eMULTIPLY, $1, $3 ); }
    | TOKEN_LPAREN expr TOKEN_RPAREN { $$ = $2; }
    | TOKEN_NUMBER { $$ = createNumber($1); }
    ;
 
%%

The code needed to obtain the syntax tree using the parser generated by Bison and the scanner generated by flex is the following.

#include "Expression.h"
#include "Parser.h"
#include "Lexer.h"
 
#include <stdio.h>
 
int yyparse(SExpression **expression, yyscan_t scanner);
 
SExpression *getAST(const char *expr)
{
    SExpression *expression;
    yyscan_t scanner;
    YY_BUFFER_STATE state;
 
    if (yylex_init(&scanner))
    {
        // couldn't initialize
        return NULL;
    }
 
    state = yy_scan_string(expr, scanner);
 
    if (yyparse(&expression, scanner))
    {
        // error parsing
        return NULL;
    }
 
    yy_delete_buffer(state, scanner);
 
    yylex_destroy(scanner);
 
    return expression;
}
 
int evaluate(SExpression *e)
{
    switch (e->type)
    {
        case eVALUE:
            return e->value;
        case eMULTIPLY:
            return evaluate(e->left) * evaluate(e->right);
        case ePLUS:
            return evaluate(e->left) + evaluate(e->right);
        default:
            // shouldn't be here
            return 0;
    }
}
 
int main(void)
{
    SExpression *e = NULL;
    char test[]=" 4 + 2*10 + 3*( 5 + 1 )";
    int result = 0;
 
    e = getAST(test);
 
    result = evaluate(e);
 
    printf("Result of '%s' is %d\n", test, result);
 
    deleteExpression(e);
 
    return 0;
}

Issues

Reentrancy

Normally, Bison generates a parser which is not reentrant. In order to achieve reentrancy the declaration %define api.pure must be used. More details on Bison reentrancy can be found in the Bison manual.

Using Bison from other languages

Bison can only generate code for C, C++ and Java. For using the Bison generated parser from other languages a language binding tool such as SWIG can be used.

Where is it used?

Here is a non-comprehensive list of software built using Bison:

• The Ruby Programming Language (YARV);
• The PHP Programming Language (Zend Parser);
• GCC started out using Bison, but switched to a hand-written recursive-descent parser for C++ in 2004 (version 3.4), and for C and Objective-C in 2006 (version 4.1);
• The Go Programming Language (GC);
• Bash shell uses a yacc grammar for parsing the command input. It is distributed with Bison-generated files.

Further reading

• Levine, John (August 2009). flex & bison. O'Reilly Media. ISBN 978-0-596-15597-1.

External links

• Website in the GNU Project
  • Manual
• Project home at Savannah
• Entry in the Free Software Directory
• Internals of C parsers generated by GNU Bison
• How to download and install Bison (GNU Parser Generator) on Linux
• Win32 binaries by GnuWin32 (version 2.4.1)