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Re: [Qemu-devel] [PATCH] ui: fix format specifier in vnc_client_io_error

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] ui: fix format specifier in vnc_client_io_error() to avoid break in build.
Date: Mon, 9 Jan 2017 15:35:36 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0

On 01/08/2017 12:28 PM, Rami Rosen wrote:
> When building qemu after setting _VNC_DEBUG to 1 (see ui/vnc.h),
> we get the following error and the build breaks:
> ...
> ui/vnc.c: In function ‘vnc_client_io_error’:
> ui/vnc.c:1262:13: error: format ‘%d’ expects argument of type ‘int’, but 
> argument 3 has type ‘ssize_t’ [-Werror=format=]
>              VNC_DEBUG("Closing down client sock: ret %d (%s)\n",
>              ^
> cc1: all warnings being treated as errors
> make: *** [ui/vnc.o] Error 1
> ...
> This patch solves this issue by fixing the print format specifier
> in vnc_client_io_error() to be %ld, which corresponds to the type
> of the "ret" variable.

NACK.  "ret" is ssize_t, which might be 'long' on some platforms, but is
'int' on others (32-bit platforms come to mind).

> Signed-off-by: Rami Rosen <address@hidden>
> ---
>  ui/vnc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/ui/vnc.c b/ui/vnc.c
> index 2c28a59..4b0a89c 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -1259,7 +1259,7 @@ ssize_t vnc_client_io_error(VncState *vs, ssize_t ret, 
> Error **errp)
>          if (ret == 0) {
>              VNC_DEBUG("Closing down client sock: EOF\n");
>          } else if (ret != QIO_CHANNEL_ERR_BLOCK) {
> -            VNC_DEBUG("Closing down client sock: ret %d (%s)\n",
> +            VNC_DEBUG("Closing down client sock: ret %ld (%s)\n",

%zd is better than %ld.  Note that %zd is technically undefined -
neither C nor POSIX requires that the signed counterpart to 'size_t' be
ssize_t, nor that 'ssize_t' be the same size as 'size_t'; but it is safe
enough in qemu as we already have plenty of existing uses of this idiom
(and any platform that implements 'ssize_t' in a way that doesn't match
'%zd' is stupid when it comes to quality-of-implementation).

Looking forward to v2.

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

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