eventdev: fix clang C++ include

[ upstream commit 699155f ]

When compiling on FreeBSD with clang and include checking enabled,
errors are emitted due to differences in how empty structs/unions are
handled in C and C++, as C++ structs cannot have zero size.

lib/eventdev/rte_eventdev.h:992:2: error:
union has size 0 in C, non-zero size in C++

Since the contents of the union are all themselves of zero size,
the actual union wrapper is unnecessary. We therefore remove it for C++
builds - though keep it for C builds for safety and clarity of
understanding the code. The alignment constraint on the union is
unnecessary in the case where the whole struct is aligned on a 16-byte
boundary, so we add that constraint to the overall structure to ensure
it applies for C++ code as well as C.

Fixes: 1cc44d4 ("eventdev: introduce event vector capability")

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

lib/eventdev/rte_eventdev.h

@@ -986,21 +986,31 @@ struct rte_event_vector {
 	};
 	/**< Union to hold common attributes of the vector array. */
 	uint64_t impl_opaque;
+
+/* empty structures do not have zero size in C++ leading to compilation errors
+ * with clang about structure having different sizes in C and C++.
+ * Since these are all zero-sized arrays, we can omit the "union" wrapper for
+ * C++ builds, removing the warning.
+ */
+#ifndef __cplusplus
 	/**< Implementation specific opaque value.
 	 * An implementation may use this field to hold implementation specific
 	 * value to share between dequeue and enqueue operation.
 	 * The application should not modify this field.
 	 */
 	union {
+#endif
 		struct rte_mbuf *mbufs[0];
 		void *ptrs[0];
 		uint64_t *u64s[0];
+#ifndef __cplusplus
 	} __rte_aligned(16);
+#endif
 	/**< Start of the vector array union. Depending upon the event type the
 	 * vector array can be an array of mbufs or pointers or opaque u64
 	 * values.
 	 */
-};
+} __rte_aligned(16);
 
 /* Scheduler type definitions */
 #define RTE_SCHED_TYPE_ORDERED          0