Re: [Qemu-block] [Qemu-devel] [PATCH v2] qemu-img: add the simplest format recognition

[Eric Blake]

On 12/01/2017 04:49 PM, Klim Kireev wrote:
> Now, if you type something like
> 
> qemu-img create disk.qcow2 1G
> or
> qemu-img dd if=/dev/sda of=disk.qcow2
> 
> it creates a raw image and if you need you should
> manually specify an image format with -f qcow2. It would
> be more convenient if it could be detected from an extension.
> 
> This patch adds a simple heuristic to recognize the image format
> for qcow, qcow2, vmdk, vhdx, vdi
> 
> It warns users about guessed format and informs them about '-f' option.
> 
> Signed-off-by: Klim Kireev <address@hidden>
> ---
>  qemu-img.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/qemu-img.c b/qemu-img.c
> index 02a6e27beb..ac1adf1582 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -421,11 +421,22 @@ static int64_t cvtnum(const char *s)
>      return value;
>  }
>  
> +static const char *get_format(const char *filename)
> +{
> +    const char *fmt = strrchr(filename, '.');
> +    if (fmt == NULL || bdrv_find_format(++fmt) == NULL) {
> +        fmt = "raw";
> +    }
> +    printf("!!! %s format was detected.\n"

Assumed, not detected.  (We didn't read the file, because we're creating
it; we're assuming a format based on the file name, not its contents,
and detection tends to imply a contents-based check).

> +           "!!! If you meant another format, specify it with -f.\n", fmt);

Not our usual error reporting style; the !!! is yelling at the user, and
we tend to avoid trailing '.'.  printf() goes to stdout, but the message
should go to stderr(); you really want to use warn_report().

I suggest:

warn_report("No format specified with -f, assuming %s.", fmt);

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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