[PATCH 2/9] virstring: Introduce virStringIsNull()

[Daniel P. Berrangé]

On Fri, Jul 03, 2020 at 12:28:43PM +0200, Michal Privoznik wrote:
> This function will be used to detect zero buffers (which are
> going to be interpreted as hole in virStream later).
> 
> I shamelessly took inspiration from coreutils.
> 
> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> ---
>  src/libvirt_private.syms |  1 +
>  src/util/virstring.c     | 40 ++++++++++++++++++++++++++++++++++++++++
>  src/util/virstring.h     |  2 ++
>  3 files changed, 43 insertions(+)
> 
> diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
> index ae0e253ab9..1d80aeb833 100644
> --- a/src/libvirt_private.syms
> +++ b/src/libvirt_private.syms
> @@ -3197,6 +3197,7 @@ virStringHasChars;
>  virStringHasControlChars;
>  virStringHasSuffix;
>  virStringIsEmpty;
> +virStringIsNull;
>  virStringIsPrintable;
>  virStringListAdd;
>  virStringListAutoFree;
> diff --git a/src/util/virstring.c b/src/util/virstring.c
> index e9e792f3bf..7f6871d5ab 100644
> --- a/src/util/virstring.c
> +++ b/src/util/virstring.c
> @@ -1404,3 +1404,43 @@ int virStringParseYesNo(const char *str, bool *result)
>  
>      return 0;
>  }
> +
> +
> +/**
> + * virStringIsNull:
> + * @buf: buffer to check
> + * @len: the length of the buffer
> + *
> + * For given buffer @buf and its size @len determine whether
> + * it contains only zero bytes or not.
> + *
> + * Returns: true if buffer is full of zero bytes,
> + *          false otherwise.
> + */
> +bool virStringIsNull(const char *buf, size_t len)
> +{
> +    const char *p = buf;
> +
> +    if (!len)
> +        return true;
> +
> +    /* Check up to 16 first bytes. memcmp() below uses block of sizeof(long)
> +     * size (in most cases on 64bits is 8 bytes), doing twice the size just to
> +     * be safe. */

On 32-bit this is only going to do 8 bytes though. I'm not sure
why we need "2 * sizeof(long)" instead of "16". Unless we really
do want this to have different behaviour based on sizeof(long),
in which case the comment could be clearer

> +    for (;;) {
> +        if (*p)
> +            return false;
> +
> +        p++;
> +        len--;
> +
> +        if (!len)
> +            return true;
> +
> +        if ((len & (2 * sizeof(long))) == 0)
> +            break;
> +    }
> +
> +    /* Now we know first 16 bytes are NUL, memcmp with self.  */
> +    return memcmp(buf, p, len) == 0;
> +}
> diff --git a/src/util/virstring.h b/src/util/virstring.h
> index 360c68395c..d0e54358ac 100644
> --- a/src/util/virstring.h
> +++ b/src/util/virstring.h
> @@ -185,6 +185,8 @@ int virStringParsePort(const char *str,
>  int virStringParseYesNo(const char *str,
>                          bool *result)
>      G_GNUC_WARN_UNUSED_RESULT;
> +bool virStringIsNull(const char *buf, size_t len);
> +
>  /**
>   * VIR_AUTOSTRINGLIST:
>   *
> -- 
> 2.26.2
> 

Regards,
Daniel
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