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

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

[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.

Fixes: 746e8736da70 ("node: add global mbuf dynfield")

Signed-off-by: Bruce Richardson <bruce.richardson@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

Re: [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.

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

[Nitin Saxena]

On Mon, Jun 30, 2025 at 4:27 PM Bruce Richardson
<bruce.richardson@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@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@marvell.com>

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

[Thomas Monjalon]

01/07/2025 16:48, Nitin Saxena:
> On Mon, Jun 30, 2025 at 4:27 PM Bruce Richardson
> <bruce.richardson@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@intel.com>
> 
> Acked-by: Nitin Saxena <nsaxena@marvell.com>

Applied, thanks.