> what if this code becomes a library and is integrated as part of a long-lived program

Cue one of my favourite comments[1]:

  /*
   * We divy out chunks of memory rather than call malloc each time so
   * we don't have to worry about leaking memory.  It's probably
   * not a big deal if all this memory was wasted but if this ever
   * goes into a library that would probably not be a good idea.
   *
   * XXX - this *is* in a library....
   */

(Of course, this consideration should be appropriately downweighted by YAGNI, as threading memory management through prototype or internal utility code can by itself easily force it into very non-prototype territory wrt effort.)

[1] https://github.com/the-tcpdump-group/libpcap/blob/2180b6e56a...