Flex, Bison, and C: Looking for a very basic introduction

Question

I am looking for a very short working example of flex and bison with an accompanying Makefile which makes use of the builtin rules. I've tried several google results that were messy, wouldn't build, or were in C++ which isn't acceptable. Good online resources and short sample code is appreciated.

---

ADDITIONAL

     # Makefile example -- scanner and parser.
     # Creates "myprogram" from "scan.l", "parse.y", and "myprogram.c"
     #
     LEX     = flex
     YACC    = bison -y
     YFLAGS  = -d
     objects = scan.o parse.o myprogram.o

     myprogram: $(objects)
     scan.o: scan.l parse.c
     parse.o: parse.y
     myprogram.o: myprogram.c

I would like a Makefile which looks approximately like this with attached source files which do something arbitrarily simple.

Answer 1

The flex project itself comes with a decent set of examples, including make files and bison files.

https://github.com/westes/flex/releases

For an excellent intro to the topic, I suggest lex and yacc 2nd edition:

http://oreilly.com/catalog/9781565920002

Finally, go here for a quick primer:

http://ds9a.nl/lex-yacc/cvs/lex-yacc-howto.html

Edit:

As Bart mentioned, another source is: http://oreilly.com/catalog/9780596155988/

And the following is the skeleton file I use to start a flex project. It uses gnu getopts for parsing command line options and getting the file name. I make no claims as to portability or ease of use! :)

/*
 * This file is part of flex.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 
 * Neither the name of the University nor the names of its contributors
 * may be used to endorse or promote products derived from this software
 * without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE.
 */

    /************************************************** 
        start of definitions section

    ***************************************************/

%{
/* A template scanner file to build "scanner.c". */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <getopt.h>
/*#include "parser.h" */

//put your variables here
char FileName[256];
FILE *outfile;
char **myOut;
char inputName[256];




// flags for command line options
static int specificFile_flag = 0;
static int output_flag = 0;
static int help_flag = 0;

%}


%option 8bit outfile="scanner.c"
%option nounput nomain noyywrap 
%option warn

%x header
%x fileType
%x final

%%
    /************************************************ 
        start of rules section

    *************************************************/


    /* these flex patterns will eat all input */ 
. { }
\n { }


%%
    /**************************************************** 
        start of code section


    *****************************************************/

int main(int argc, char **argv);

int main (argc,argv)
int argc;
char **argv;
{
    /****************************************************
        The main method drives the program. It gets the filename from the
        command line, and opens the initial files to write to. Then it calls the lexer.
        After the lexer returns, the main method finishes out the report file,
        closes all of the open files, and prints out to the command line to let the
        user know it is finished.
    ****************************************************/

    int c;

    // the gnu getopt library is used to parse the command line for flags
    // afterwards, the final option is assumed to be the input file

    while (1) {
        static struct option long_options[] = {
            /* These options set a flag. */
            {"specific-file", no_argument,       &specificFile_flag, 1},
            {"help",   no_argument,     &help_flag, 1},
            /* These options don't set a flag. We distinguish them by their indices. */

            {"debug", no_argument,       0, 'd'},
            {"specificFile", no_argument,       0, 's'},
            {"useStdOut", no_argument,       0, 'o'},
            {0, 0, 0, 0}
        };
           /* getopt_long stores the option index here. */
        int option_index = 0;
        c = getopt_long (argc, argv, "dso",
            long_options, &option_index);

        /* Detect the end of the options. */
        if (c == -1)
            break;

        switch (c) {
            case 0:
               /* If this option set a flag, do nothing else now. */
               if (long_options[option_index].flag != 0)
                 break;
               printf ("option %s", long_options[option_index].name);
               if (optarg)
                 printf (" with arg %s", optarg);
               printf ("\n");
               break;

            case 'd':
                break;

            case 's':
               specificFile_flag = 1;
               break;

            case 'o':
               output_flag = 1;
               break;


            case '?':
               /* getopt_long already printed an error message. */
               break;

            default:
               abort ();
            }
    }

    if (help_flag == 1) {
        printf("proper syntax is: addressGrabber.exe [OPTIONS]... INFILE OUTFILE\n");
        printf("grabs address from prn files\n\n");
        printf("Option list: \n");
        printf("-s    --specific-file   changes INFILE from a prn list to a specific prn\n");
        printf("-d              turns on debug information\n");
        printf("-o                      sets output to stdout\n");
        printf("--help                  print help to screen\n");
        printf("\n");
        printf("list example: addressGrabber.exe list.csv\n");
        printf("prn example: addressGrabber.exe -s 01110500.prn\n\n");
        printf("If infile is left out, then stdin is used for input.\n");
        printf("If outfile is a filename, then that file is used.\n");
        printf("If there is no outfile, then infile-EDIT.tab is used.\n");
        printf("There cannot be an outfile without an infile.\n");
        return 0;
    }

    //get the filename off the command line and redirect it to input
    //if there is no filename or it is a - then use stdin


    if (optind < argc) {
        FILE *file;

        file = fopen(argv[optind], "rb");
        if (!file) {
            fprintf(stderr, "Flex could not open %s\n",argv[optind]);
            exit(1);
        }
        yyin = file;
        strcpy(inputName, argv[optind]);
    }
    else {
        printf("no input file set, using stdin. Press ctrl-c to quit");
        yyin = stdin;
        strcpy(inputName, "\b\b\b\b\bagainst stdin");
    }

    //increment current place in argument list
    optind++;


    /********************************************
        if no input name, then output set to stdout
        if no output name then copy input name and add -EDIT.csv
        if input name is '-' then output set to stdout
        otherwise use output name

    *********************************************/
    if (optind > argc) {
        yyout = stdout;
    }   
    else if (output_flag == 1) {
        yyout = stdout;
    }
    else if (optind < argc){
        outfile = fopen(argv[optind], "wb");
        if (!outfile) {
                fprintf(stderr, "Flex could not open %s\n",FileName);
                exit(1);
            }
        yyout = outfile;
    }
    else {
        strncpy(FileName, argv[optind-1], strlen(argv[optind-1])-4);
        FileName[strlen(argv[optind-1])-4] = '\0';
        strcat(FileName, "-EDIT.tab");
        outfile = fopen(FileName, "wb");
        if (!outfile) {
                fprintf(stderr, "Flex could not open %s\n",FileName);
                exit(1);
            }
        yyout = outfile;
    }


    yylex();
    if (output_flag == 0) {
        fclose(yyout);
    }
    printf("Flex program finished running file %s\n", inputName);
    return 0;
}

Finally, since people keep checking this out, I also have an example lexer and parser with makefile on github.

Answer 2

You could start by looking at the wikipedia bison page. It has a full sample code of reentrant parser written with bison. It uses flex as the lexer and it also has a sample code on how to use it.

If you have any corrections I thank you in advance :)

LATER: The code on wikipedia was tested on linux (gcc) and windows (visual studio) and should work with other compilers also.