Re: [Qemu-trivial] [Qemu-devel] [PATCH] virtio-bus: avoid breaking build when open DEBUG switch

[Stefan Hajnoczi]

On Thu, Nov 13, 2014 at 10:39:32AM +0800, address@hidden wrote:
> From: Gonglei <address@hidden>
> 
> Signed-off-by: Gonglei <address@hidden>
> ---
>  hw/virtio/virtio-bus.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
> index eb77019..dfd2d8c 100644
> --- a/hw/virtio/virtio-bus.c
> +++ b/hw/virtio/virtio-bus.c
> @@ -58,7 +58,7 @@ void virtio_bus_reset(VirtioBusState *bus)
>  {
>      VirtIODevice *vdev = virtio_bus_get_device(bus);
>  
> -    DPRINTF("%s: reset device.\n", qbus->name);
> +    DPRINTF("%s: reset device.\n", BUS(bus)->name);

In general the problem with existing DPRINTF() macros is that they use
#ifdef.  This leads to bitrot, such as this instance.

A better approach is:

#define DEBUG_FOO 0
#define DPRINTF(...) \
do { \
    if (DEBUG_FOO) { \
        fprintf(stderr, ...); \
    } \
} while (0)

Now the compiler always checks the DPRINTF() code.

Trace events can be a good solution too.  If you like stderr output,
build QEMU with ./configure --enable-trace-backend=stderr and all
trace_*() calls are turned into fprintf(stderr).

The cool thing about trace events is that they are available in
production too.

Stefan

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