misc/56206: src/sys/sys/cdefs.h uses PreProcessor variablesthat aren't defined, causing noisy warnings.

[Bruce Evans]

The following reply was made to PR misc/56206; it has been noted by GNATS.

From: Bruce Evans <bde at zeta.org.au>
To: Garrett Wollman <wollman at khavrinen.lcs.mit.edu>
Cc: freebsd-gnats-submit at freebsd.org
Subject: Re: misc/56206: src/sys/sys/cdefs.h uses PreProcessor variables that
 aren't defined, causing noisy warnings.
Date: Sun, 31 Aug 2003 17:33:09 +1000 (EST)

 On Sat, 30 Aug 2003, Garrett Wollman wrote:
 
 >  <<On Sat, 30 Aug 2003 14:19:14 -0500 (CDT), Larry Rosenman <ler at lerctr.org> said:
 >
 >  > The following patch shuts it up, and is functionally equivalent.
 >
 >  Actually, the patch is not functionally equivalent:
 >
 >  > -#if __STDC_VERSION__ < 199901
 >  > +#if defined(__STDC_VERSION__) && __STDC_VERSION__ < 199901
 >
 >  0 < 199901
 
     #if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 199901
 
 would be functionally equivalent, but is intentionally not used because
 it is harder to read.
 
 The feature of undefined identifiers being 0 in cpp expressions is used
 a lot in FreeBSD headers.  E.g., in <sys/cdefs.h>:
 
     #if __GNUC__ < 2 || __GNUC__ == 2 && __GNUC_MINOR__ < 5
 
 When __GNUC__ is not defined, it evaluates as 0 so this expression is
 true (0 < 2).  There is no warning about this from gcc because gcc
 always defines __GNUC__, at least in the broken versions that warn
 about this.  Compilers that aren't gcc shouldn't define __GNUC__ or
 have the bug.
 
 The bug may be in FreeBSD's version of gcc.  gcc normally suppresses
 warnings in "system" headers but FreeBSD turns this off since it wants
 to check for errors in system headers and at least old versions of gcc
 don't understand which headers are system ones anyway (for buildworld,
 the system headers aren't under /usr/include).
 
 Bruce