Assume a file already exists,
in binary format, non human-readable, contains
multiple records of this format
struct person
{ char title[4]
  char name[20]; // short names padded with spaces
  int ssn;
  int dob;
  double balance; }

want to sort the file so that it's in alph order of name.
the file is too big to store in RAM

int compare(char s[], char t[])
// returns <0 if s<t, ==0 if s==t, >0 if s>t
{ for (int i = 0; i < 20; i += 1)
  { int d = s[i] - t[i];
    if (d != 0)
      return d; }
  return 0; }

fstream f("file.txt");
if (! f.is_open())
{ cerr << "Can't open the file\n";
  exit(1); }
f.seekg(0, ios::end);
int filesize = f.tellg();
int N = filesize / sizeof(person);
// records are numbered 0 to N-1;
while (N > 0)
{ f.seekg(0, ios::beg);
  person biggest;
  int biggestpos = 0;
  f.read(& biggest, sizeof(person));
  for (int i = 1; i < N; i += 1)
  { person curr;
    f.read(& curr, sizeof(person));
    if (compare(curr.name, biggest.name) > 0)
    { biggest = curr;
      biggestpos = i * sizeof(person); } }
  person temp;
  f.seekg((N-1)*sizeof(person), ios::beg);
  f.read(& temp, sizeof(person));
  f.seekp((N-1)*sizeof(person), ios::beg);
  f.write(& biggest, sizeof(person));
  f.seekp(biggestpos * sizeof(person), ios::beg);
  f.write(& temp, sizeof(person));
  N -= 1; }
f.close(); }