How can adding a header increase portability? (sys/time.h)

Question

I just noticed this line in the getrusage man page:

Including is not required these days, but increases portability. (Inde

Answer 1

It's because the BSD's require it:

FreeBSD and OpenBSD need sys/time.h, not just time.h, before sys/resource.h.

See this page on UNIX Portability for details.