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

[Thomas Monjalon]

30/06/2025 12:50, Bruce Richardson:
> 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.
[...]
> --- 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

I didn't do a deep analysis but I think I would prefer this option 2.