I just noticed this line in the getrusage man page:
Including
is not required these days, but increases portability. (Inde
In general, it was not uncommon in the early days of C for you to have to manually include header file A before header file B. Maybe an early version of the preprocessor couldn't do nested includes, or maybe it was just stylistic (manpages would often directly include the header files for relevant structures).
"These days", sys/resource.h must either include sys/time.h or repeat the definition of struct timeval, but not every system follows the standard completely.
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.