Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Tom Lane escribi�:
>> I seem to recall that we've previously found that you have to write
>> MAXALIGN(offsetof(shm_mq, mq_ring[0])) + MAXIMUM_ALIGNOF;
>> to keep MSVC happy with a reference to an array member in offsetof.
> Hmm, this seems to contradict what's documented at the definition of
> FLEXIBLE_ARRAY_MEMBER:
Ah, I thought we had that issue documented somewhere, but failed to find
this comment, or I'd have known that was backwards.
The other possibility I was contemplating is that "export a const
variable" doesn't actually work for some reason. We're not in the habit
of doing that elsewhere, so I don't find that theory outlandish. Perhaps
it could be fixed by adding PGDLLIMPORT to the extern, but on the whole
I'd rather avoid the technique altogether.
The least-unlike-other-Postgres-code approach would be to go ahead and
export the struct so that the size computation could be provided as a
#define in the same header. Robert stated a couple days ago that he
didn't foresee much churn in this struct, so that doesn't seem
unacceptable.
Another possibility is to refactor so that testing an allocation request
against shm_mq_minimum_size is the responsibility of storage/ipc/shm_mq.c,
not some random code in a contrib module. It's not immediately apparent
to me why it's good code modularization to have a contrib module
responsible for checking sizes based on the sizeof a struct it's not
supposed to have any access to.
regards, tom lane