The point there is that you don’t need a buffer for whole line, you only have to find the line terminators. Something like:

    printf(“0: “);
    ln = 1;
    while ((ch = getch()) != EOF) {
      putch(ch);
      if (ch == EOL) {
        printf(“%d: “, ln);
        ln++;
      }
    }

And I believe that original unix implementation of nl is mostly this, although with the stdio functions replaced with raw read/write.