preprocessor directives...

[Wizard]

I noticed that many of header files use the following format:
#if !defined(H_GUARD)
#define H_GUARD
Is this preferable to "#ifndef"? I always thought that the defined()
function was only for more complex tests. Additionally, the defined()
function may not be entirely portable (it might be missing on AIX, but I'm
not positive as I don't have such a system handy).
just curious,
Grant M.