On Fri, Feb 27, 2026 at 03:03:23PM +0100, Laurent Vivier wrote:
> Add a counterpart to IOV_PEEK_HEADER() that writes header data back
> to an iov_tail after modification. If the header pointer matches the
> original iov buffer location, it only advances the offset. Otherwise,
> it copies the data using iov_from_buf().
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Concept looks good to me.

> ---
>  iov.c | 22 ++++++++++++++++++++++
>  iov.h | 14 +++++++++++++-
>  2 files changed, 35 insertions(+), 1 deletion(-)
> 
> diff --git a/iov.c b/iov.c
> index 8836305fb701..2cf23d284e4a 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -296,6 +296,28 @@ void *iov_peek_header_(struct iov_tail *tail, void *v, size_t len, size_t align)
>  	return v;
>  }
>  
> +/**
> + * iov_put_header_() - Write header back to an IOV tail
> + * @tail:	IOV tail to write header to
> + * @v:		Pointer to header data to write
> + * @len:	Length of header to write, in bytes
> + *
> + * Return: number of bytes written
> + */
> +/* cppcheck-suppress unusedFunction */
> +size_t iov_put_header_(struct iov_tail *tail, const void *v, size_t len)
> +{
> +	size_t l = len;
> +
> +	/* iov_peek_header_() already called iov_check_header() */
> +	if ((char *)tail->iov[0].iov_base + tail->off != v)
> +		l = iov_from_buf(tail->iov, tail->cnt, tail->off, v, len);

IIUC, there's a strong requirement that this only be called after a
matching iov_peek_header_(), yes?  If that's the case then (l != len)
would mean something has already gone badly wrong - so should it be an
ASSERT()?

> +	tail->off += l;
> +
> +	return l;
> +}
> +
>  /**
>   * iov_remove_header_() - Remove a header from an IOV tail
>   * @tail:	IOV tail to remove header from (modified)
> diff --git a/iov.h b/iov.h
> index a7b873d58134..08cf60639358 100644
> --- a/iov.h
> +++ b/iov.h
> @@ -86,6 +86,7 @@ bool iov_tail_prune(struct iov_tail *tail);
>  size_t iov_tail_size(struct iov_tail *tail);
>  bool iov_drop_header(struct iov_tail *tail, size_t len);
>  void *iov_peek_header_(struct iov_tail *tail, void *v, size_t len, size_t align);
> +size_t iov_put_header_(struct iov_tail *tail, const void *v, size_t len);
>  void *iov_remove_header_(struct iov_tail *tail, void *v, size_t len, size_t align);
>  ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
>  		       struct iov_tail *tail);
> @@ -110,6 +111,16 @@ void iov_tail_zero_end(struct iov_tail *tail, size_t size);
>  					       sizeof(var_),		\
>  					       __alignof__(var_))))
>  
> +/**
> + * IOV_PUT_HEADER() - Write header back to an IOV tail
> + * @tail_:	IOV tail to write header to
> + * @var_:	Pointer to a variable containing the header data to write
> + *
> + * Return: number of bytes written
> + */
> +#define IOV_PUT_HEADER(tail_, var_)					\
> +	(iov_put_header_((tail_), (var_), sizeof(*var_)))
> +
>  /**
>   * IOV_REMOVE_HEADER() - Remove and return typed header from an IOV tail
>   * @tail_:	IOV tail to remove header from (modified)
> @@ -128,7 +139,8 @@ void iov_tail_zero_end(struct iov_tail *tail, size_t size);
>  	((__typeof__(var_) *)(iov_remove_header_((tail_), &(var_),	\
>  				    sizeof(var_), __alignof__(var_))))
>  
> -/** IOV_DROP_HEADER() - Remove a typed header from an IOV tail
> +/**
> + * IOV_DROP_HEADER() - Remove a typed header from an IOV tail
>   * @tail_:	IOV tail to remove header from (modified)
>   * @type_:	Data type of the header to remove
>   *
> -- 
> 2.53.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson