Problem using yyin with yacc file

I am using Yacc and lex to parse a C type language , I have built the data structures using c++. everything works fine but i am not able to read the input file using yyin in main.cpp.

the following is the code : Please help !

#include "parse_tree.h"
#include<iostream>
#include<cstdlib>
#include<stdio.h>
#include"y.tab.h"

extern "C" FILE *yyin;
FILE *fp;

using namespace std;

int main() {



 system("clear");

 yyin=fopen("input_file","r+");
 if(yyin==NULL)
 {
  cout<开发者_JAVA百科;<"\n Error ! \n";
 }
 do{
  cout<<"am parsing !"; 
      yyparse();

 }while(!feof(yyin));
     return 0;


 fp=fopen("outfile","w");

 yyparse();
 }


int yywrap()
{

 return 1;

}

Firstly I don't understand

extern "C" FILE *yyin;

when you can simply write

extern FILE *yyin;

anyway why are you opening the input file for updating (the mode parameter '+') if you are not making any changes to the file then its needless. Just "r" is sufficient. The same with the second statement

fp=fopen("outfile","w");

if needless don't open file in write mode

Also you should probably add an else statement to your code.. without it your error checking becomes useless...

yyin=fopen("input_file","r+");
if(yyin==NULL)
{
     cout<<"\n Error ! \n";
}
else 
{
    cout<<"am parsing !";
    yyparse();
}

Also you are returning from function

return 0;

Again

}while(!feof(yyin));

is useless if you have written your grammar recursively. This statement is only needed when your grammar can parse only one statement at a time.