Error opening files!!! :-(

Hi everyone!
Using Linux with NFS, I run this program and it crashes when trying to open the file "grafo4.grf", a file that exists. The real code is different, I just left the parts where errors occur.
Could anyone give me a pointer to a solution at least?
As always, thanks all replys!
Here is the code:

Code:

#include <iostream>
#include <fstream>

using namespace std;

int main(){
  
  char file_name[10];
  ifstream input;
  int fileNumber = 0;
  bool done = false;

  cout << "Check 1 " << endl;
  while(!done){
    sprintf(file_name, "%s%d%s", "grafo", fileNumber, ".grf");
    input.open(file_name);
    if(!input) done = true;
    else fileNumber++;
    input.close();
  }

  cout << "Check 2 " << endl;
  /* abre o arquivo correto */
  sprintf(file_name, "grafo%d.grf",--fileNumber);
  input.open(file_name);
  if(!input){
    cerr << "Error opening file " << file_name << endl;
    return 1;
  }
  input.close();

  cout << "Check 3 " << endl;
  for(int i=0;i<=fileNumber;i++){
    sprintf(file_name, "grafo%d.grf", i);
    input.open(file_name);
    if( !input ){
      cerr << "Error opening file " << file_name << endl;
      return 1;
    }
    input.close();
  }

  cout << "Check 4 " << endl;
  input.open("corte.crt");
  if(!input){
    cerr << "Error opening file " << file_name << endl;
    return 1;
  }
  input.close();
  cout << "Ok =D" << endl;
  return 0; 

}

> sprintf(file_name, "%s%d%s", "grafo", fileNumber, ".grf");
Well you only allocated 10 chars for filename, and the fixed part of the string is 10 chars before you add any digits to it.