Re: Possible alternate definition of CTASSERT to allow its use in header files

On Tuesday 30 September 2008 03:55:26 pm Ryan Stone wrote:
> This was prompted by some recent check-ins removing CTASSERTs from
> header files to prevent spurious errors from popping up.  For example,
> this check-in:
> http://lists.freebsd.org/pipermail/cvs-src/2008-September/095328.html
> 
> I've come up with an alternate definition of CTASSERT that can be used
> in header files.  It works on gcc 3.4.6, 4.0.2 and 4.3.0(the only
> compilers I have quick access to).
> 
> $ cat /tmp/tmp.c
> // New definition
> #define NEWASSERT(x) _NEWASSERT(x, __LINE__)
> #define _NEWASSERT(x, line) __NEWASSERT(x, line)
> #define __NEWASSERT(x, line) extern int __assert_ ## line [ x ? 1 : -1 ];
> 
> //existing BSD implementation
> #define CTASSERT(x)             _CTASSERT(x, __LINE__)
> #define _CTASSERT(x, y)         __CTASSERT(x, y)
> #define __CTASSERT(x, y)        typedef char __assert ## y[(x) ? 1 : -1]
> 
> CTASSERT(1);                                      // line 11
> CTASSERT(0);                                      // line 12
> CTASSERT(1); CTASSERT(0);                // line 13
> 
> 
> NEWASSERT(1);                                   // line 16
> NEWASSERT(0) ;                                  // line 17
> NEWASSERT(1); NEWASSERT(0);        // line 18
> NEWASSERT(1); NEWASSERT(1);        // line 19
> 
> 
> $ gcc -v -c /tmp/tmp.c -Wall -Werror
> /tmp/tmp.c:12: error: size of array `__assert12' is negative
> /tmp/tmp.c:13: error: size of array `__assert13' is negative
> /tmp/tmp.c:13: error: redefinition of typedef '__assert13'
> /tmp/tmp.c:13: error: previous declaration of '__assert13' was here
> /tmp/tmp.c:17: error: size of array `__assert_17' is negative
> /tmp/tmp.c:18: error: size of array `__assert_18' is negative
> $
> 
> Note that the compiler doesn't complain about multiple definitions of
> __assert18 and __assert19 like it does about the multiple definitions
> of __assert13, which is the reason that CTASSERTs can't be used in
> header files.
> 
> Thoughts?  Will this work on compilers other than gcc?

I think this is quite slick actually.  I'm not sure this is standard C though.  
For the kernel it is probably fine so long as icc handles it.

-- 
John Baldwin