#include <iostream.h>
#include <ctype.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{ char c;
      if ( argc != 2 )
         cerr << argv[0] << " takes one argument only\n";
      fstream fio(argv[1], ios::in | ios::out);
      if ( ! fio )
      {   cerr << argv[0] << ':' << " Cannot open"
                          << argv[1] << " for updating\n",
          exit(1);
      }
      while ((c = fio.get()) != EOF) 
          if ( isupper(c) )
          {   fio.seekg(-1, ios::cur);   // position for writing c
              fio.put(tolower(c));
          }
      fio.close();
}

// note could use seekp instead of seekg