Il 21/11/2012 19:32, Stefan Hajnoczi ha scritto:
> The iov_get_ptr() data returns a pointer to contiguous data within a
> vector. This allows the caller to manipulate data inside the vector
> without copying in/out using iov_from_buf()/iov_to_buf() when we know
> that data is contiguous within an iovec element.
This works for you because you have a single byte to write. It would
not work for the SG_IO inhdr, which would need iov_to_buf().
What about the following alternative API:
void *iov_get_ptr(struct iovec *iov, unsigned int iov_cnt,
ssize_t offset, size_t *bytes);
which would place the number of valid bytes (i.e. the length of the
remainder of the iovec entry) in *bytes?
Also, I think that offset == iov_size(iov, iov_cnt) should be
acceptable, and it would be the only case in which *bytes == 0.
Otherwise looks good.
Paolo
> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
> ---
> iov.c | 25 +++++++++++++++++++++++++
> iov.h | 11 +++++++++++
> 2 files changed, 36 insertions(+)
>
> diff --git a/iov.c b/iov.c
> index 6eed089..bc78c34 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -395,3 +395,28 @@ size_t iov_discard(struct iovec **iov, unsigned int
> *iov_cnt, ssize_t bytes)
> }
> return total;
> }
> +
> +void *iov_get_ptr(struct iovec *iov, unsigned int iov_cnt,
> + ssize_t offset, size_t bytes)
> +{
> + if (offset < 0) {
> + offset += iov_size(iov, iov_cnt);
> + if (offset < 0) {
> + return NULL; /* offset before beginning of vector */
> + }
> + }
> +
> + while (iov_cnt > 0) {
> + if (offset < iov->iov_len) {
> + if (bytes > iov->iov_len - offset) {
> + return NULL; /* would span iovec elements */
> + }
> + return iov->iov_base + offset;
> + }
> +
> + offset -= iov->iov_len;
> + iov_cnt--;
> + iov++;
> + }
> + return NULL; /* offset beyond end of vector */
> +}
> diff --git a/iov.h b/iov.h
> index d6d1fa6..674dd51 100644
> --- a/iov.h
> +++ b/iov.h
> @@ -108,3 +108,14 @@ unsigned iov_copy(struct iovec *dst_iov, unsigned int
> dst_iov_cnt,
> * smaller than requested if the vector is too small.
> */
> size_t iov_discard(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes);
> +
> +/*
> + * Get a pointer into a vector at offset if the given number of bytes is
> + * contiguous and not split across iovec elements. NULL is returned if
> + * memory would span iovec elements or exceed the length of the vector.
> + *
> + * The offset is ssize_t so that an offset from the end of the vector can
> + * be specified with a negative number.
> + */
> +void *iov_get_ptr(struct iovec *iov, unsigned int iov_cnt, ssize_t offset,
> + size_t bytes);
>
