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Date: Mon, 19 Oct 2009 22:11:49 +0100
From: Jamie Lokier <jamie@shareable.org>
Message-ID: <20091019211149.GB9954@shareable.org>
References: <20091019191504.GA8157@shareable.org> <m38wf7m8c4.fsf@neno.mitica>
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Subject: [Qemu-devel] Re: [PATCH 00/25] VMState cleanups and conversion of
	network drivers
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To: Juan Quintela <quintela@redhat.com>
Cc: qemu-devel@nongnu.org

Juan Quintela wrote:
> I have good error messages, I don't have good ideas about how to trick
> the compiler.  Error messages are clear.
> 
> Example:
> 
> struct FOO {
>        unt16_t foo[0];
> }
> #define vmstate_offset_pointer(_state, _field, _type)                \
>     (offsetof(_state, _field) +                                      \
>      type_check_pointer(_type, typeof_field(_state, _field)))
> 
> Called with (after doing substitutions)
> type_check_pointer(uint16_t, struct FOO, foo);
> 
> Give that, we try several definitions for type_check_pointer:
> 
> #define type_check_pointer(t1,t2) ((t1**)0 - (t2*)0)
> 
> Gives the following error:
> 
> /scratch/qemu/hw/eeprom93xx.c:144: error: invalid operands to binary -
>                       (have ‘uint16_t **’ and ‘uint16_t (*)[]’)
> 
> Another try:
> 
> #define type_check_pointer(t1,t2) ((t1(*)[])0 - (t2*)0)
> 
> gives
> 
> /scratch/qemu/hw/eeprom93xx.c:148: error: arithmetic on pointer to an incomplete type
> 
> Another one:
> 
> #define type_check_pointer(t1,t2) ((t1(*)[0])0 - (t2*)0)
> 
> gives:
> 
> /scratch/qemu/hw/eeprom93xx.c:151: error: initializer element is not constant
> /scratch/qemu/hw/eeprom93xx.c:151: error: (near initialization for ‘vmstate_eeprom’)

#define type_check_pointer(t1,t2) (0*sizeof((t1(*)[0])0 - (t2*)0))

Or if you want foo[] to work:

#define type_check_pointer(t1,t2) (0*sizeof((t1(**)[])0 - (t2**)0))

Dubiously, but fortunately, the second one works with foo[] and foo[0]
in GCC.

Enjoy :-)
-- Jamie