[tgv]

I'd never heard of %n, but I use printf's return value (the number of bytes written) for this kind of purpose, so

  n = printf("%s: ", prefix);
  printf("%s\n", line1);
  printf("%*s%s\n", n, " ", line2);

[MertsA]

Yeah but the difference is that you can use %n wherever you need it in the format string. Depending on what arguments come after it figuring out the length of the printed arguments might not be trivial whereas with printf, it already has to keep a count of it during execution in order to return it at the end so it's easy to add support for it.