Re: [Qemu-devel] [PATCH] replay: Fix build with -Werror=unused-result

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Sep 21, 2016 at 10:00:23AM +0000, Felipe Franciosi wrote:
> 
> > On 21 Sep 2016, at 07:24, Markus Armbruster <armbru@redhat.com> wrote:
> > 
> > "Pavel Dovgalyuk" <dovgaluk@ispras.ru> writes:
> > 
> >>> From: Felipe Franciosi [mailto:felipe@nutanix.com]
> >>> If compiling with -Werror=unused-result, replay-internal.c won't build
> >>> due to a call to fwrite() where the returned value is ignored. A simple
> >>> cast to (void) is not sufficient on recent GCCs, so this fixes it.
> >>> 
> >>> Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
> >>> ---
> >>> replay/replay-internal.c | 2 +-
> >>> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>> 
> >>> diff --git a/replay/replay-internal.c b/replay/replay-internal.c
> >>> index 5835e8d..6978d76 100644
> >>> --- a/replay/replay-internal.c
> >>> +++ b/replay/replay-internal.c
> >>> @@ -65,7 +65,7 @@ void replay_put_array(const uint8_t *buf, size_t size)
> >>> {
> >>>     if (replay_file) {
> >>>         replay_put_dword(size);
> >>> -        fwrite(buf, 1, size, replay_file);
> >>> +        (void)(fwrite(buf, 1, size, replay_file)+1);
> >>>     }
> >>> }
> >> 
> >> This looks very weird.
> 
> Oh I couldn't agree more. I hate this syntax. It is, however, the simplest way to get around this issue. See:
> http://stackoverflow.com/questions/11888594/ignoring-return-values-in-c

If we want to ignore return value reliably, lets just pull in the
ignore_value macro from gnulib which is known to work across GCC
versions


/* Normally casting an expression to void discards its value, but GCC
   versions 3.4 and newer have __attribute__ ((__warn_unused_result__))
   which may cause unwanted diagnostics in that case.  Use __typeof__
   and __extension__ to work around the problem, if the workaround is
   known to be needed.  */
#if 3 < __GNUC__ + (4 <= __GNUC_MINOR__)
# define ignore_value(x) \
    (__extension__ ({ __typeof__ (x) __x = (x); (void) __x; }))
#else
# define ignore_value(x) ((void) (x))
#endif

GNULIB makes it availavble under LGPLv2.1+

eg used as:

   ignore_value(fwrite(buf, 1, size, replay_file));

Regards,
Daniel
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