Until at least Microsoft C and gcc are C99-conforming there seems no point
in trying to write portable, C99-dependent code.
I'm not convinced that's the case (though I avoid doing so, myself).
[...]
It doesn't seem unreasonable, then, when certain _portions_ of
functionality specific to the C99 standard reach high levels of
portability, to take advantage of them. Indeed, when certain
extensions that aren't in any standard are widespread, there's nothing
really wrong with using them, either (though they may be offtopic
here). The standard, contrary to some claims, does not define
portability (otherwise we wouldn't even be having this discussion); it
defines a target for implementors to hit if they wish to be
portable. Other targets can co-exist, and sometimes vendors rebel
against the official standard and collectively implement a de facto
one instead.
It seems to me that // comments were widely portable long before C99,
and that one may generally depend on stdint.h and snprintf (thank
God). Inline is widely implemented, but often with significantly
different semantics from C99 (usually based on C++ semantics, I
think), but one can achieve portability with "static inline".
