[PATCH] node: fix C++ compatibility errors (option 2)

[Nitin Saxena]

On Mon, Jun 30, 2025 at 4:27 PM Bruce Richardson
<bruce.richardson at intel.com> wrote:
>
> C++ does not allow zero-sized unions - they end up being of size 1-byte,
> which leads to C/C++ compatibility issues, flagged by the compiler.
>
>   lib/node/rte_node_mbuf_dynfield.h:78:2: error: union has size 0 in C,
> size 1 in C++ [-Werror,-Wextern-c-compat]
>    78 |         union {
>                  |         ^
>                  1 error generated.
>
> Fix the error by omitting the persistent_data field when it is
> zero-sized, since it's unusable. Any app using the field must already
> specify a size for the persistent data.
>
> Fixes: 746e8736da70 ("node: add global mbuf dynfield")
>
> Signed-off-by: Bruce Richardson <bruce.richardson at intel.com>
> ---
>  lib/node/rte_node_mbuf_dynfield.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/lib/node/rte_node_mbuf_dynfield.h b/lib/node/rte_node_mbuf_dynfield.h
> index b77fdbb94f..09254c585a 100644
> --- a/lib/node/rte_node_mbuf_dynfield.h
> +++ b/lib/node/rte_node_mbuf_dynfield.h
> @@ -69,6 +69,7 @@ typedef struct rte_node_mbuf_overload_fields {
>   * 2. Overloadable fields: Fields which can be repurposed by two adjacent nodes.
>   */
>  typedef struct rte_node_mbuf_dynfield {
> +#if RTE_NODE_MBUF_PERSISTENT_FIELDS_SIZE > 0
>         /**
>          * Persistent mbuf region across nodes in graph walk
>          *
> @@ -78,6 +79,7 @@ typedef struct rte_node_mbuf_dynfield {
>         union {
>                 uint8_t persistent_data[RTE_NODE_MBUF_PERSISTENT_FIELDS_SIZE];
>         };
> +#endif
>         /**
>          * Overloadable mbuf fields across graph walk. Fields which can change.
>          *
> --
> 2.48.1
>

Acked-by: Nitin Saxena <nsaxena at marvell.com>