I completely understand the padding issues that you are dealing with. I was mostly curious why Tom wanted to use asserts to double-check the code in one place, while happily not doing so in what seemed the same kind of situation elsewhere. He has since made the reason for that clear.
On Thursday, January 2, 2014 3:27 PM, Andres Freund <and...@2ndquadrant.com> wrote: On 2014-01-02 15:15:58 -0800, Mark Dilger wrote: > I still don't understand why this case in src/include/pgstat.h > is different from cases elsewhere in the code. Taken from > src/include/access/heapam_xlog.h: > > > typedef struct xl_heap_header > { > uint16 t_infomask2; > uint16 t_infomask; > uint8 t_hoff; > } xl_heap_header; > > #define SizeOfHeapHeader (offsetof(xl_heap_header, t_hoff) + sizeof(uint8)) > > > > Now, if somebody changed t_hoff to be a uint16, that SizeOfHeapHeader > macro would be wrong. Should we put a static assert in the code for that? The reason the various SizeOfHeapHeader are written that way is that we do not want to uselessly store trailing padding in the WAL. E.g. sizeof(xl_heap_header) will be 6bytes, but SizeOfHeapHeader will be 5. I don't see an easy way to guarantee this with asserts and I think you'd notice pretty fast if you got things wrong there because WAL replay will just have incomplete data. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services