Re: [Qemu-devel] [PATCH 2/2] replay: Ignore the return value of fwrite()

[Eric Blake]

On 09/21/2016 10:27 AM, Felipe Franciosi wrote:
> If building with GCC 3.4 or newer (and using -Werror=unused-result),
> replay-internal.c will fail to compile due to a call to fwrite() where
> the return value is not used. Since fwrite() is declared with WUR in
> glibc, callers should check the return value or find other ways to
> ignore it. The error message in this specific case is:
> 
>     replay/replay-internal.c: In function ‘replay_put_array’:
>     replay/replay-internal.c:68:15: error: ignoring return value of
>     ‘fwrite’, declared with attribute warn_unused_result 
> [-Werror=unused-result]
>              fwrite(buf, 1, size, replay_file);
>                    ^
> 
> This commit wraps the fwrite() call with the ignore_value() macro, which
> currently suppresses the error for existing GCC versions.

This explains what you did, but not quite why.  In other words, convince
me that ignoring the error is the right thing to do in the first place...

> 
> Signed-off-by: Felipe Franciosi <address@hidden>
> ---
>  replay/replay-internal.  | 0
>  replay/replay-internal.c | 2 +-
>  2 files changed, 1 insertion(+), 1 deletion(-)
>  create mode 100644 replay/replay-internal.
> 
> diff --git a/replay/replay-internal. b/replay/replay-internal.
> new file mode 100644
> index 0000000..e69de29
> diff --git a/replay/replay-internal.c b/replay/replay-internal.c
> index 5835e8d..61de8f9 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);
> +        ignore_value(fwrite(buf, 1, size, replay_file));

I would be more convinced that this patch is correct if you added a
comment here why fwrite() failures can be ignored in this situation, so
that someone doesn't undo your commit to add in proper error handling.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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