Including <stddef.h>

[Doug Gwyn]

In article <71 at motto.UUCP> dave at motto.UUCP (dave brown) writes:
>2. Include <stddef.h> at the top of my header

Recommended, since <stddef.h> is an X3J11 invention and therefore
you can expect it to follow the official rules for system headers...

>   What if they have already included it?  Is it guaranteed that it can
>   be included more than once without causing errors?

...which includes the requirement that the system header be includable
multiple times with no effect different from being included just once.