abcd
efg
hi
j

abcd
efg
hi
j
5 9 12 14

abcd
efg
hi
j
5 6 7 8

#include <iostream>
#include <fstream>
using namespace std;

int main(int argc, char **argv)
{
  // Prepare file
  fstream f(argv[1], fstream::ate | fstream::in | fstream::out);
  if(!f)
    {
      cerr << "Unable to open file!" << endl;
      return EXIT_FAILURE;
    }

  // Prepare needed data for production
  auto end_mark = f.tellg();
  size_t count{0};
  char c[64];
  // Go back to the beginning
  f.seekg(0, fstream::beg);

  while(f && f.tellg() != end_mark && f.getline(c, 64))
    {
      count += f.gcount();

      auto mark = f.tellg();
      // Go to the end
      f.seekp(0, fstream::end);

      f << count;
      if(mark != end_mark) f << " ";

      // Go back to the last line
      f.seekp(mark);
    }

  f.seekp(0, fstream::end);
  f << 'n';

  system("PAUSE");
  return 1;
}