Re: [Qemu-devel] SLIRP warning messages displayed while compiling

[Philippe Mathieu-Daudé]

Hi Eric,

On 08/12/2017 08:41 AM, Eric Blake wrote:
> Here's an idea:  Instead of using struct ip6 { ... } QEMU_PACKED, use
>
> struct ip6 {
> ...
> };
> QEMU_BUG_ON(sizeof(struct ip6) != 32);
>
> so that the compiler explicitly chokes if it introduced any padding, and
> then we know all our accesses are naturally aligned which shuts up the
> warning.

Following your suggestion I tried:

#define QEMU_BUG_STRUCT_UNALIGNED(s) \
        QEMU_BUILD_BUG_ON(sizeof(s) != sizeof(s QEMU_PACKED))

QEMU_BUG_STRUCT_UNALIGNED(struct ip6);

but this gives:

qemu/slirp/ip6.h:157:1: warning: 'packed' attribute ignored when parsing 
type [-Wignored-attributes]
QEMU_BUG_STRUCT_UNALIGNED(struct ip6);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
qemu/slirp/ip6.h:155:49: note: expanded from macro 
'QEMU_BUG_STRUCT_UNALIGNED'
        QEMU_BUILD_BUG_ON(sizeof(s) != sizeof(s QEMU_PACKED))
                                                ^~~~~~~~~~~
qemu/include/qemu/compiler.h:40:37: note: expanded from macro 'QEMU_PACKED'
# define QEMU_PACKED __attribute__((packed))
                                    ^~~~~~
1 warning generated.