Difficulties during the compilation (g++, bison, flex) with yyparse();

Question

I have a problem with compilation of my code:

Flex:

%{
#include "lista4.tab.hpp"
#include <stdlib.h>
extern int yylex();
%}
%%
"=" {return EQ;}
"!="    {return NE;}
"<" {return LT;}
">" {return GT;}
":="    {return ASSIGN;}
";" {return SEMICOLON;}
"IF"    {return IF;}
"THEN"{return THEN;}

"END" {return END;}
[_a-z]+ {yylval.text = strdup(yytext); return IDENTIFIER;}
[ \t]+
[0-9]+          {
                yylval.var = atoi (yytext);
                return NUMBER;
                }
[-+/^*'%'()]    {
                return *yytext;
                }
\n              return RESULT;
%%

Bison:

%{
  extern "C"
  {
    int yyparse();
    int yylex(void);
    void yyerror(char *s){}
    int yywrap(void){return 1;}
  }

  #include <iostream>
  #include <vector>
  #include <string>
  #include <stdlib.h>
  #include <map>

  using namespace std;

  vector <string> instructions;
  map <> vars;
%}

%union{
  char* text;
  int var;
}


%token EQ
%token NE
%token ASSIGN
%token SEMICOLON
%token IF
%token THEN
%token <text> IDENTIFIER
%token <var> NUMBER
%token <var> RESULT

%left '+' '-'
%left '*' '/' '%'
%right '^'

%%

exp: NUMBER
| IDENTIFIER
| IDENTIFIER "+" IDENTIFIER
| IDENTIFIER "-" IDENTIFIER
;
%%

int main(void){
  yyparse();
} 

And bash script:

#!/bin/bash
clear
rm launcher lex.yy.cpp *.tab.cpp *.tab.hpp
bison  -d -o lista4.tab.cpp *.y
flex -o lex.yy.cpp *.l
g++ -o launcher *.cpp -lfl

I posted here only the most important part of code, because rest of them is not necessary here. Anyway, if someone would like to see whole code, I pasted it here: http://pastebin.com/1rS2FBJj. But it's a little bigger and takes more place.

When I tried to compile all to *.c files and then by gcc it was all fine. But when I switched compiler to g++ and files to cpp I get this error:

lista4.tab.cpp: In function ‘int yyparse()’:
lista4.tab.cpp:1397: warning: deprecated conversion from string constant to ‘char*’
lista4.tab.cpp:1540: warning: deprecated conversion from string constant to ‘char*’
/tmp/ccdqpQVx.o: In function `yyparse':
lista4.tab.cpp:(.text+0x252): undefined reference to `yylex'
collect2: ld returned 1 exit status

Could anyone give me a tip how to fix it?

Answer 1

In your Flex file, you declare this:

extern int yylex();

When compiled as C++, that declares a function with C++ linkage. In your Bison file, you declare this:

extern "C" {
  int yylex();
}

That gives it C linkage. They're two different functions. You define the C++ version (or, rather, Flex defines it for you), but you declare the C version, and the C version is the one the compiler thinks you try to call (in the Bison-generated code). The linker sees that the C version is used, but it cannot find the definition.

Pick one linkage and use it consistently. (I'd choose C++, since it allows omission of the "extern" stuff from the code entirely.)