fstream not working

#include <iostream>
#include <cstdlib>
#include <signal.h>
#include <unistd.h>
#include <pthread.h>
#include <string.h>
#include <fstream>
#define NUM_THREADS    10
using namespace std;

void *print_hello(void *threadid)
{
        char path_file[50];
        long tid;
        pthread_t self;
        self = pthread_self();
        tid = (long)threadid;
        sprintf(path_file,"/disk2/home/arpit/myprog/my_worker_thread_%d",self);
        fstream logfile(path_file,std::ios::in|std::ios::out);
//      cout << "thread number: "<< self << endl;
        for(int i=0;i<20;i++)
        {
                logfile <<  "thread number: "<< self<<"    " << "print hello " << endl;
        }
        return 0;
}

int main()
{
  int i,rc;
  long t=0;
  pthread_t threads[NUM_THREADS];
  int pid_a = fork();
  if(pid_a == 0)
  {
        for(i = 0; i <10000; i++)
        {
                //cout << i << endl;
                if(i==1000)
                {
                        cout << "IN WORKER ENGINE" << endl;
                        sleep(2);
                        pid_t id = getpid();
                        for(int worker = 0;worker < NUM_THREADS ; worker++)
                        {
                                rc = pthread_create(&threads[worker], NULL, print_hello, (void *)t);
                                if (rc)
                                {
                                        cout << "ERROR; return code from pthread_create() is " << rc << "for worker no :" << worker << endl;
                                        exit(-1);
                                }
                        }
                        for(int worker = 0;worker < NUM_THREADS ; worker++)
                        {
                                pthread_join(threads[worker],NULL);
                        }
                        cout << "child process being killed:" <<id<< endl;
                        if(id > 0)
                                kill(id,9);
                }
        }
  }
  else
  {
        pid_t id1 = getpid();
        cout << "IN MASTER ENGINE " << id1 << endl;
        sleep(2);
        cout << "i am in master engine" << endl;
        cout << "i am in master engine" << endl;
  }
  return 0;
}