Re: [Qemu-devel] [PATCH v5 07/11] iov: add iov_discard() to remove data

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Dec 05, 2012 at 09:47:06PM +0100, Stefan Hajnoczi wrote:
> The iov_discard() function removes data from the front or back of the
> vector.  This is useful when peeling off header/footer structs.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  iov.c | 41 +++++++++++++++++++++++++++++++++++++++++
>  iov.h | 13 +++++++++++++
>  2 files changed, 54 insertions(+)
> 
> diff --git a/iov.c b/iov.c
> index a81eedc..6eed089 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -354,3 +354,44 @@ size_t qemu_iovec_memset(QEMUIOVector *qiov, size_t offset,
>  {
>      return iov_memset(qiov->iov, qiov->niov, offset, fillc, bytes);
>  }
> +
> +size_t iov_discard(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes)
> +{
> +    size_t total = 0;
> +    struct iovec *cur;
> +    int direction;
> +
> +    if (*iov_cnt == 0) {
> +        return 0;
> +    }
> +
> +    if (bytes < 0) {
> +        bytes = -bytes;
> +        direction = -1;
> +        cur = *iov + (*iov_cnt - 1);
> +    } else {
> +        direction = 1;
> +        cur = *iov;
> +    }
> +
> +    while (*iov_cnt > 0) {
> +        if (cur->iov_len > bytes) {
> +            if (direction > 0) {
> +                cur->iov_base += bytes;
> +            }
> +            cur->iov_len -= bytes;
> +            total += bytes;
> +            break;
> +        }
> +
> +        bytes -= cur->iov_len;
> +        total += cur->iov_len;
> +        cur += direction;
> +        *iov_cnt -= 1;
> +    }
> +
> +    if (direction > 0) {
> +        *iov = cur;
> +    }
> +    return total;
> +}
> diff --git a/iov.h b/iov.h
> index 34c8ec9..d6d1fa6 100644
> --- a/iov.h
> +++ b/iov.h
> @@ -95,3 +95,16 @@ void iov_hexdump(const struct iovec *iov, const unsigned int iov_cnt,
>  unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
>                   const struct iovec *iov, unsigned int iov_cnt,
>                   size_t offset, size_t bytes);
> +
> +/*
> + * Remove a given number of bytes from the front or back of a vector.
> + * This may update iov and/or iov_cnt to exclude iovec elements that are
> + * no longer required.
> + *
> + * Data is discarded from the front of the vector if bytes is positive and
> + * from the back of the vector if bytes is negative.

I think I already commented on this: I think this interface is too tricky,
and use of ssize_t is a bad idea since most of code uses size_t:
you might start getting integer overflow errors if you convert.

Better to have
size_t iov_discard_front(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes);
size_t iov_discard_back(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes);

which explicitly do the right thing.

The fix up users to use size_t everywhere.

> + *
> + * The number of bytes actually discarded is returned.  This number may be
> + * smaller than requested if the vector is too small.
> + */
> +size_t iov_discard(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes);
> -- 
> 1.8.0.1