Re: [Qemu-devel] error compiling hw/sh7750.c

[M. Warner Losh]

In message: <address@hidden>
            Carlo Marcelo Arenas Belon <address@hidden> writes:
: On Sun, Nov 11, 2007 at 10:06:34AM -0600, Carlo Marcelo Arenas Belon wrote:
: > On Sun, Nov 11, 2007 at 09:30:26AM -0500, Ben Taylor wrote:
: > > So the macro turns the last _INTC_ARRAY(NULL) into
: > > 
: > > "NULL, sizeof(NULL)/sizeof(*NULL)
: > 
: > in my 64bit linux using gcc-4.1.2 it becomes instead :
: > 
: >   ((void *)0), sizeof(((void *)0))/sizeof(*((void *)0))
: > 
: > what version of gcc (gcc -v) are you using in that Solaris 10 box? (HINT: 
not
: > all available versions of gcc compile qemu correctly) :
: > 
: >   http://www.opensolaris.org/os/project/qemu/host/gcc-failures/
: 
: That didn't sound as clear as it should, so I booted an OpenSolaris Nevada 70b
: using the sun provided gcc :
: 
: $ uname -a
: SunOS dell 5.11 snv_70b i86pc i386 i86pc
: $ gcc -v
: Reading specs from /usr/sfw/lib/gcc/i386-pc-solaris2.11/3.4.3/specs
: Configured with: /builds2/sfwnv-gate/usr/src/cmd/gcc/gcc-3.4.3/configure
: --prefix=/usr/sfw --with-as=/usr/sfw/bin/gas --with-gnu-as
: --with-ld=/usr/ccs/bin/ld --without-gnu-ld --enable-languages=c,c++,f77,objc
: --enable-shared
: Thread model: posix
: gcc version 3.4.3 (csl-sol210-3_4-20050802)
: 
: and run a test, the problem is in /usr/include/iso/stdio_iso.h that defined
: NULL as an int and not a pointer as shown by :
: 
: fndef NULL
: #if defined(_LP64)
: #define NULL    0L
: #else
: #define NULL    0
: #endif
: #endif
: 
: contradicting (at least in spirit) the C99 (ISO/IEC 9899:TCS) standard that
: says :
: 
:   "A pointer to void shall have the same representation and alignment 
:    requirements as a pointer to a character type"

NULL isn't a pointer.  It is a integral constant, per section
6.3.2.3.3 of C99.  So *NULL is undefined.  If you want a constant
thats an actual pointer, you need to define one special.

       6.3.2.3  Pointers
...
       [#3] An integer constant expression with  the  value  0,  or
       such  an  expression  cast  to type void *, is called a null
       pointer constant.  If a null pointer constant is assigned
       to or compared for equality to a pointer,  the  constant  is
       converted to a pointer of that type.  Such a pointer, called
       a null pointer,  is  guaranteed  to  compare  unequal  to  a
       pointer to any object or function.

       [#4]  Conversion  of  a null pointer to another pointer type
       yields a null pointer of that type.  Any two  null  pointers
       shall compare equal.

       7.17  Common definitions <stddef.h>
...
       [#3] The macros are
               NULL
       which  expands  to  an  implementation-defined  null pointer
       constant; and
..

The root cause is of this whole problem is *NULL, an undefined,
unportable construct.  That's what should be fixed, not the definition
of NULL if you want this code to be portable.

Warner