While working on trying to fix undefined behavior for unaligned memory accesses [1], I ran into an issue with the IPC specification [2] which prevents us from ever achieving zero-copy memory mapping and having aligned accesses (i.e. clean UBSan runs).
Flatbuffer metadata needs 8-byte alignment to guarantee aligned accesses. In the IPC format we align each message to 8-byte boundaries. We then write a int32_t integer to to denote the size of flat buffer metadata, followed immediately by the flatbuffer metadata. This means the flatbuffer metadata will never be 8 byte aligned. Do people care? A simple fix would be to use int64_t instead of int32_t for length. However, any fix essentially breaks all previous client library versions or incurs a memory copy. [1] https://github.com/apache/arrow/pull/4757 [2] https://arrow.apache.org/docs/ipc.html
