Re: Portability of preprocessor directives

[Paul Eggert]

"Paul D. Smith" <address@hidden> writes:

> What is the word on portability of preprocessor directives?
> 
> I'm in particular thinking of things like "#if defined()",
> "#elif !defined()", the && and || operators, etc.

They've all been in the C language since the late 1970s and in my
experience are quite portable.  I have heard of one exception: the
Plan 9 builtin preprocessor does not support "#if" at all, because of
Ken Thompson's personal preference.  However, I don't think this is
worth worrying about: anybody porting GNU code to Plan 9 and/or
Inferno will use the ANSI C preprocessor (cpp) to work around this
incompatibility.

Some people prefer "defined (FOO)", but the more typical style in GNU
programs is "defined FOO"; it's just as portable, and it's shorter.

Some compilers, including GCC, optionally issue a diagnostic if you
use "#if FOO" when FOO is not defined.  However, it works correctly
(i.e., FOO evaluates to 0) on all compilers that I know of.  I don't
worry about this issue in my code, but if you want to worry about it
you can write "#if defined FOO && FOO" instead.

You must use "defined" directly within an #if directive.  In other
words, you cannot do this:

  #define FOO (! defined BAR)

and expect FOO to evaluate to 0 when BAR is defined, and 1 otherwise.
Instead, you must do something this:

  #ifdef BAR
  # define defined_BAR 1
  #else
  # define defined_BAR 0
  #endif

  #define FOO (! defined_BAR)

though this doesn't have quite the same semantics, since it determines
the value of FOO at FOO's definition, not at FOO's use.

The original traditional C preprocessor did not support #elif, but
compilers without #elif are long dead and are no longer worth worrying
about.

The original traditional C preprocessor required '#' to be in column
1.  Typical GNU programs still put '#' in column 1, but I think this
is now more of a style thing than a requirement.  If you're worried
about older compilers then it might be best to stick with the style
for a few years more, anyway.