Linux bison Command with Practical Examples

Introduction

In this lab, we will explore the bison command, a parser generator tool used in the development of compilers and interpreters. Bison is a free software implementation of YACC, a powerful tool for generating parsers from a context-free grammar specification. We will start by installing the bison package on our Ubuntu 22.04 environment, then create a simple grammar file and generate a parser using the bison command. Finally, we will learn how to handle syntax errors in the generated parser.

The lab covers the following steps:

• Introduction to bison Command
• Generating Parsers with bison
• Handling Syntax Errors in bison

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Skills Graph

Introduction to bison Command

In this step, we will explore the bison command, which is a parser generator tool used in the development of compilers and interpreters. Bison is a free software implementation of YACC (Yet Another Compiler-Compiler), a powerful tool for generating parsers from a context-free grammar specification.

First, let's install the bison package in our Ubuntu 22.04 environment:

sudo apt-get update
sudo apt-get install -y bison

Example output:

Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following additional packages will be installed:
  m4
Suggested packages:
  bison-doc
The following NEW packages will be installed:
  bison m4
0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 1,141 kB of archives.
After this operation, 4,470 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
...

Now that we have bison installed, let's create a simple grammar file to generate a parser. Create a new file named example.y in the ~/project directory with the following content:

%{
#include <stdio.h>
%}

%token NUM
%%

input:
    | input line
;

line:
    NUM '\n' { printf("Received number: %d\n", $1); }
;

%%

int main() {
    yyparse();
    return 0;
}

This grammar file defines a simple calculator-like language that can recognize and process numbers. The %token NUM line declares a token type for numbers, and the line rule specifies that a valid line consists of a number followed by a newline character.

Generating Parsers with bison

In this step, we will generate a parser using the bison command based on the grammar file we created in the previous step.

First, let's generate the parser source code from the example.y file:

bison -d example.y

This command will generate two files: example.tab.c and example.tab.h. The example.tab.c file contains the parser implementation, while example.tab.h contains the token definitions.

Next, we need to compile the parser source code and link it with a lexer (scanner) to create the final executable. We'll use the flex tool to generate the lexer:

sudo apt-get install -y flex
flex -o example.lex.c example.l
gcc -o example example.tab.c example.lex.c

The flex command generates the example.lex.c file, which contains the lexer implementation. The gcc command compiles the parser and lexer sources and links them to create the final example executable.

Now, let's test our parser by running the example program:

./example
123
Received number: 123
456
Received number: 456

As you can see, the parser correctly recognizes and processes the input numbers.

Handling Syntax Errors in bison

In this step, we will learn how to handle syntax errors in the parser generated by bison.

Let's modify the example.y file to include error handling:

%{
#include <stdio.h>
%}

%token NUM
%error-verbose

%%

input:
    | input line
    | input error '\n' { yyerrok; }
;

line:
    NUM '\n' { printf("Received number: %d\n", $1); }
    | error '\n' { yyerror("Invalid input"); }
;

%%

void yyerror(const char *s) {
    fprintf(stderr, "%s\n", s);
}

int main() {
    yyparse();
    return 0;
}

The key changes are:

1. Added %error-verbose to provide more detailed error messages.
2. Added an error rule in the input and line productions to handle syntax errors.
3. Implemented the yyerror function to print the error message.

Now, let's regenerate the parser and test it:

bison -d example.y
flex -o example.lex.c example.l
gcc -o example example.tab.c example.lex.c

Try running the example program and entering some invalid input:

./example
abc
example.y:12: syntax error, unexpected error, expecting NUM
Invalid input
123
Received number: 123

As you can see, the parser correctly identifies and reports the syntax error when we enter "abc" instead of a number.

Summary

In this lab, we explored the bison command, which is a parser generator tool used in the development of compilers and interpreters. We first learned how to install the bison package on Ubuntu 22.04 and then created a simple grammar file to define a calculator-like language. We then generated the parser source code using the bison command, which produced two files: the parser implementation and the header file. Finally, we learned how to handle syntax errors in the generated parser.

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