Reading Hex values - how to use 'Unsigned Byte' value instead of signed byte value

Hi there, ive got some code which reads 4 bytes of the Hex values of a file (specified by a command line argument), and prints them out. For some reason, when I read in a value which has a negative signed byte value, the program outputs a small number of F's before displaying the actual hex value. Could anyone help me on this one? The code is here:

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#include <iostream>
#include <stdlib.h>
#include <fstream>
#include <string>
#include <iomanip>

#define NO_BYTES 4

char readbuffer[NO_BYTES];
int no_files;
int i;

using namespace std;

int main(int argc, char *argv[])
{
  ifstream InFile;
  InFile.open(argv[1], ios::in | ios::binary);  

  //print filename to file
  printf("File Name: ");
  printf(argv[1]); //print first argument (filename)
 
  //print filesize to file
  InFile.getline(readbuffer, NO_BYTES+1);
  cout << endl << "Header: ";
 
  //print hex values  
  while (i<NO_BYTES)
  {
  printf("%02X",readbuffer[i]);
  i++;
  }
  cout << endl;
   
  system("PAUSE");      
  return 0;
}
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