merge sort text file

[quiz]

Hello. My problem is that I have to implement a sorting algorithm by merging with text files, in the input file (to be sorted) each number is on a separate line.
The solution should have a constant memory complexity (download only a small number of values from files.

How to write a recursive function, which downloads only a small number of values from the file and divides them and merges it

  oid sortowanie(fstream &plik, int start, int endkoniec)
{
    int srodek;
    if(start!=end)
    {
        mid = koniec / 2;
        sortowanie(plik, start, srodek);
        sortowanie(plik, mid + 1, end);
        scalanie(plik, start, mid, ecd);
    }
 
}