On 01/17/2014 03:15 PM, Tom Lane wrote:

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.

                        


Or maybe we could expose the value via a function rather than a const variable.

cheers

andrew



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