Re: Bogus ifdef guard in sys/null.h

[Jukka Ruohonen <jruohonen%iki.fi@localhost>]

On Sun, Jul 18, 2010 at 03:04:15AM +0000, Valeriy E. Ushakov wrote:
> Why is it unspecified?  You cannot #undef reserved identifiers, like
> _FOO (typical multiple-inclusion guards fall under this) or __foo.
> Anything else - why not?
> 
>        [#2] No other identifiers  are  reserved.   If  the  program
>        declares  or  defines an identifier in a context in which it
>        is reserved (other than as allowed by 7.1.4), or  defines  a
>        reserved  identifier  as  a  macro  name,  the  behavior  is
>        undefined.
> 
> The test doesn't redeclare NULL in between those two #include's that
> both are supposed to define it, so [#2] is not violated.
> 
> NULL doesn't start from the undercore, so [#3] is not violated too.

Some cases in the POSIX standard specifically indicate that the ability to  
undefine and redefine things from the standard headers was an oversight and
may become obsolete in the future revisions.

A good example is stupidity such as:

        #include <stdbool.h>

        #undef true
        #undef false   
        #define false 1
        #define true 0 

Frankly I don't see much point in "#undef NULL" either, no matter what the
standards say.

- Jukka.