String input to flex lexer

Question

I want to create a read-eval-print loop using flex/bison parser. Trouble is, the flex generated lexer wants input of type FILE* and i would like it to be char*. Is there an

Answer 1

here is a small example for using bison / flex as a parser inside your cpp code for parsing string and changing a string value according to it (few parts of the code were removed so there might be irrelevant parts there.) parser.y :

%{
#include "parser.h"
#include "lex.h"
#include  
#include 
#include  
#include 
#include 
using namespace std;
 int yyerror(yyscan_t scanner, string result, const char *s){  
    (void)scanner;
    std::cout << "yyerror : " << *s << " - " << s << std::endl;
    return 1;
  }
    %}

%code requires{
#define YY_TYPEDEF_YY_SCANNER_T 
typedef void * yyscan_t;
#define YYERROR_VERBOSE 0
#define YYMAXDEPTH 65536*1024 
#include  
#include 
#include  
#include 
#include 
}
%output "parser.cpp"
%defines "parser.h"
%define api.pure full
%lex-param{ yyscan_t scanner }
%parse-param{ yyscan_t scanner } {std::string & result}

%union {
  std::string *  sval;
}

%token TOKEN_ID TOKEN_ERROR TOKEN_OB TOKEN_CB TOKEN_AND TOKEN_XOR TOKEN_OR TOKEN_NOT
%type   TOKEN_ID expression unary_expression binary_expression
%left BINARY_PRIO
%left UNARY_PRIO
%%

top:
expression {result = *$1;}
;
expression:
TOKEN_ID  {$$=$1; }
| TOKEN_OB expression TOKEN_CB  {$$=$2;}
| binary_expression  {$$=$1;}
| unary_expression  {$$=$1;}
;

unary_expression:
 TOKEN_NOT expression %prec UNARY_PRIO {result =  " (NOT " + *$2 + " ) " ; $$ = &result;}
;
binary_expression:
expression expression  %prec BINARY_PRIO {result = " ( " + *$1+ " AND " + *$2 + " ) "; $$ = &result;}
| expression TOKEN_AND expression %prec BINARY_PRIO {result = " ( " + *$1+ " AND " + *$3 + " ) "; $$ = &result;} 
| expression TOKEN_OR expression %prec BINARY_PRIO {result = " ( " + *$1 + " OR " + *$3 + " ) "; $$ = &result;} 
| expression TOKEN_XOR expression %prec BINARY_PRIO {result = " ( " + *$1 + " XOR " + *$3 + " ) "; $$ = &result;} 
;

%%

lexer.l : 

%{
#include 
#include "parser.h"

%}
%option outfile="lex.cpp" header-file="lex.h"
%option noyywrap never-interactive
%option reentrant
%option bison-bridge

%top{
/* This code goes at the "top" of the generated file. */
#include 
}

id        ([a-zA-Z][a-zA-Z0-9]*)+
white     [ \t\r]
newline   [\n]

%%
{id}                    {    
    yylval->sval = new std::string(yytext);
    return TOKEN_ID;
}
"(" {return TOKEN_OB;}
")" {return TOKEN_CB;}
"*" {return TOKEN_AND;}
"^" {return TOKEN_XOR;}
"+" {return TOKEN_OR;}
"!" {return TOKEN_NOT;}

{white};  // ignore white spaces
{newline};
. {
return TOKEN_ERROR;
}

%%

usage : 
void parse(std::string& function) {
  string result = "";
  yyscan_t scanner;
  yylex_init_extra(NULL, &scanner);
  YY_BUFFER_STATE state = yy_scan_string(function.c_str() , scanner);
  yyparse(scanner,result);
  yy_delete_buffer(state, scanner);
  yylex_destroy(scanner);
  function = " " + result + " ";  
}

makefile:
parser.h parser.cpp: parser.y
    @ /usr/local/bison/2.7.91/bin/bison -y -d parser.y


lex.h lex.cpp: lexer.l
    @ /usr/local/flex/2.5.39/bin/flex lexer.l

clean:
    - \rm -f *.o parser.h parser.cpp lex.h lex.cpp